CMS Experiment – Abstract
(under construction)

Ch. 1 - Introduction
High Energy Physics experiments with beam-beam collider accelerators.

Ch. 2 - Typical (particle) detectors for collider experiments. 
Barrel region, endcaps, layer structure of different task-specific detectors.

Ch. 3 - The CMS (Compact Muon Solenoid) collaboration.

Ch. 4 - The CMS Magnet
A superconducting solenoid of 15m x 21.6m.

Ch. 5 - Tracking Detector(s).
Precise recording of (all) particle tracks (position, direction, curvature) around the (beam-beam) interaction region.

Ch. 6 - Readout electronics for the tracking detector.

Ch. 7 - The Electromagnetic Calorimeter.
It is preceded by a pre-shower detector; lead + Si strip detectors, 300K channels.
 The e/m calorimeter brings to a halt light particles (photons, electrons), measuring their deposited energy and approximate position. Diameter 2.60m x 3.80m. 
Made of PbWO4 crystals looked at by photomultipliers (which?); 120K channels.
Readout electronics built with the Fermi microsystem.

Ch. 8 – The Hadron Calorimeter.
To stop and measure the deposited energy of heavier particles (hadrons). Made of alternate layers of copper and plastic scintillators, coupled via wavelength-shifting optical fibers to photo-multipliers (which?). Diameter 6m x 11.2 m.
The readout electronics is based on the Fermi system (~150Kchannels).

Ch. 9 – The Very Forward Calorimeter
Iron plates + Parallel Plate Gas chambers (PPC).
Four large modules, each with 60 planes of chambers. Each PPC plane = 652 PPC chambers. Total number of PPCs 652 x 60 x 4 x 2 = 320.000 channels.
Readout electronics?

Ch. 10 – The Muon Detector.
The muon detector is placed outside the magnet coil, it has
Diam. 8m to 14.5m; length 2 x 10.60m.
The detector consists of Drift Tubes in the Barrel region, (200 kchannels), an of Cathode Strip chambers in the endcaps region (600 kchannels).
Readout electronics?
The muon electronics also includes track-pattern recognition circuitry used to build-up the Trigger-1 decision.

Ch. 11 – Trigger and Data Acquisition
In addition to the front-end electronics, mounted on the various sub-detectors and including local buffers, local topological trigger circuitry and optical fiber drivers, the R/O system makes use of:

•	a Trigger-1 system, with the task of providing a fast (3usec) deadtime-less data filtering, for the reduction of 10^9 events/sec to a more manageable 10^4 events/sec rate.
•	Readout Units, with the task of formatting and (multi-event) buffering the data belonging to potentially good events 
•	A Switch Farm, to transmit data belonging to each particular event to a tree of several parallel processor farms.
•	A Level-2 (software) decision, executed by a computer farm on a subset of the candidate events data; brings down the event rate to ~10^3.
•	Data storage, monitoring, and control services.

An LHC Experiment (CMS)

Ch. 1 - Introduction - Collider Experiments
High Energy Physics (HEP) studies the constituents of matter and the fundamental forces that control their behavior.
Very few elementary particles appear in our daily environment: the ‘up’ and ‘down’ quark, the electrons surrounding the atomic nuclei; and the electron-neutrinos ne from radioactive decays or from the sun.
The other particles show up only in high energy phenomena, and can be studied only in special conditions:
i) accelerator/collider experiments (HEP experiments) where particles accelerated at very high energies can collide and create heavier particles by energy/matter transformation (E = mc2).
ii) non-accelerator experiments: solar and atmospheric neutrinos, dark matter, proton decay...
iii) cosmological observations: dark matter, matter/energy ratio, galaxies distribution...
A systematic study/search of heavier particles is best done in ‘collider’ experiments, where bunches of accelerated particles  (e+/e- or proton-proton) collide head-on, with equal and opposite speed, at a collision point (See Web Fig. 1.1, in CMS_figs.htm ).
In such collisions, the center-of-mass of the colliding particles is at rest in the laboratory reference system;  the collision products, or secondary particles, are therefore distributed symmetrically around the collision point, with some predominance along the beam axis. 
In order to identify and measure all collision products, and to provide a complete, snap-shot like reconstruction of each event, the volume around the collision point must be fully equipped with suitable types of particle-detecting and particle-stopping devices: they constitute the experiment detector.
The purpose of the detector is to identify the type, direction and energy of each secondary particle resulting from a collision.
To provide full containment of the secondary particles, the detector volume must be tailored to the primary beams energy; at CERN  for instance: 
LEP: 	100=100 GeV e+/e-	detector size: 10m x 15m; 
LHC: 	7+7 TeV protons	detector size: 20-25m x 30m. 
A strong magnetic field is also needed in the measuring volume, to inflect the tracks of charged particles and measure their sagitta (=>momentum) and charge-sign.
There is some relation between the detector volume, the ‘granularity’ (spatial resolution) required for measurement precision, and the number of electronic readout channels; these are of the order of one/mm3 in the inner part of the detector (about 1/3 of the volume), and one/cm3 or less in the outer region.
 
Ch. 2 - Particle detectors
The detection method varies with the kind of particle: fast, light  particles (electrons, photons) are best detected by their electromagnetic interaction with the atomic shells of the detector material - gas ionization, excitation of light emitting molecules, electron-hole pair generation in silicon detectors, etc. 
Heavier particles (hadrons) are better detected by nuclear (‘strong’) interactions with the atomic nuclei of the detector bulk material.
Neutral particles are only detected if they are short-lived and decay into charged products within the experiment detector (e.g. the Zo boson). Light neutral particles like the neutrinos (m  0) do escape from any detector.
Muons (heavy electrons) are a special case; they are long-lived and too heavy, or too light, to fall into the other categories. Hence their tracks are recorded up to the outer edge of the detector (see ‘muon chambers’ below).
Different types of detectors, each one with a specific task, are mounted in successive layers around the collision point: each layer identifies and measures the energy of particles unmeasured by the previous layers.
The sequence of layers is the same in most collider experiments ; getting away from the collision point, one would find (Fig.2.1): 
1 - the beam pipe, a few cm in diameter with locally thin walls not to disturb the secondary particles motion. 
2 - A ‘tracking’ detector, to measure the position and sagitta of particle trajectories (in the magnetic field), and provide a topological event reconstruction. The lowest possible amount of materials is used, not to alter particles tracks or energy. 
The inner part uses a few layers of silicon detectors to measure track points with micron precision. The outer part, up to about one meter  radius, provides additional track measuring layers, with sub-mm measuring accuracy.

3 - An Electromagnetic Calorimeter (ECAL) with the purpose of stopping all electrons and photons (e±, g’s), and measure their total energy and impact point. The e/m calorimeter relies on high Z (atomic number) materials to obtain the maximum e/m interaction of light particles with the atomic shells.
4 - The magnet coil (in CMS it is built outside the Hadron calorimeter below).
5 - A Hadron Calorimeter (H-Cal), to stop and measure the impact energy of hadrons (quark-made heavy particles). Lot of bulk, heavy material - copper, iron or uranium - is used to stop even the most energetic particles by nuclear (‘strong’) interactions.
6 - The muon (m±) detector, often called ‘muon chambers’; in view of the large surface of this detector (thousands square meters in CMS), gas chambers are commonly used for their surface/price*performance good ratio. They 
measure the tracks of muons which go through the previous layers.

The complete detector could consist of concentric cubes, or spheres; but for construction reasons, e.g. to get a homogeneous magnetic field, the preferred structure is one of concentric cylinders (called barrel), closed at each end by ‘end-caps‘ made of  several layers of disk-shaped detectors. The end-caps portion near the beam axis, called the ‘forward’ region, is often extended by additional detector layers called the ‘very-forward’ (VF) region.
After this tedious explanation, the detectors of some experiments, for instance Aleph and CMS (Web Fig. 2.2, 2.3, 2.4) should look more familiar! 

Ch. 3 - The CMS (Compact Muon Solenoid) Collaboration 
The first presentation of the detector project by a proto-collaboration dates from 1990. In  1992 a Letter of Intent was sent to the LHC Committee (LHCC). 
In  1994 the Technical Proposal was submitted. At that time, 273 USA Physicists from 37 Institutes, and 250 Russian Physicists joined the collaboration, bringing in more financial resources.
Today, the collaboration consists of ~ 1300 Physicists from 132 Institutions.
The experiment derives its name from the magnet, a superconducting solenoid of F6m x 13m, with an axial field of 4 Tesla, and from the fact that muon detection (tracking) has been carefully optimized.
The solenoid contains a high resolution central tracker, a precise electromagnetic calorimeter and the hadron calorimeter.
The detector can measure Higgs bosons (if found) with mass = 95-150 GeV by their two-photon decay, or any other particle decaying in four leptons (e or m) with a mass between 135 and 525 GeV.
The estimated cost of the detector is 460 MCHF - not including  manpower. The year of completion is ~ 2004.

Ch. 4 - The CMS magnet
The clue of the CMS detector is the huge superconducting solenoid magnet (Web Fig. 4.1).
The coil
A strong and uniform axial field (4 Tesla, or 40.000Gauss) is obtained by circulating 20.000A in the coil; the latter consists of 2112 turns of 62x22mm2 Al profile with an embedded flat superconducting cable - 40 strands of 1.8mm NbTi, current density 2700A/mm2, 43 km total length.
The coil must be operated at 4.5oK, the critical temperature (at which superconductivity is lost) being 6.5 oK. The whole coil is housed in a double walled, 240 tons stainless steel vessel, with the outer part (F=7.3m) cooled at 60oK. 
The cooling media are 5000 liters of liquid He, 200m3 of pressurized He gas in a surface building, and a 550 l/hr liquid nitrogen booster for the cool-down periods. 
The cooling time from room temperature is 40 days, and 12 days after a fast ‘quench’ (i.e. total superconductivity loss, the coil temperature increasing to 85 oK).
The superconducting coil, built in 4 separate sections, will be assembled in 16 months in a dedicated building.
The joke
The solenoid magnetic flux is returned via a 1.8m thick, 11.500 tons, saturated iron joke - F14.6m x 21.6m - outside the superconducting coil.
The joke is split in 5 separate wheels or rings, each 2.6m long, which can be slid and pressed together. Each ring is built in a 12-sided structure.
The central ring supports the superconducting coil vessel (500 tons), and the barrel detectors inside the solenoid (1500 tons).
The whole joke consists of three iron layers, 300+600+600mm, instrumented inside with muon chambers.
In the same way, the joke end-caps are made of three separate iron disks with F14m and thickness 300+600+600mm.
NMR (Nuclear Magnetic Resonance) and Hall probes will be mounted in the central region and in the joke to monitor the magnetic field.

Ch. 5 - The tracking Detector
The purpose of the tracking detector is to reconstruct tracks left by charged particles. The requirements are:
•		precise particle momentum measurement 
(at least 1012 points/track);
•		high granularity, to reduce the cell occupancy (particle signals per event and cell) at high collision rates.
The need to keep the cell occupancy below a few percent (Table 5.1) leads to cells of a few square cm size, and to a large number of readout channels (~10^7).
The tracker has a hollow cylindrical shape, with r = 7130cm and length = 600cm. It consists of a ‘barrel’ and two ‘forward’ region, and uses three types of detectors (Web Fig. 5.1):
		
Inner tracker (r  40cm)	| Silicon pixel detectors (2 layers)
				| Silicon strips, single or double-sided 				  (3 layers)
Outer tracker (r = 40130cm)	| Microstrip Gas Chambers 
 				  (MSGCs, 12 layers)
The pixel detectors (125m pitch) provide three dimensional space points for the high track density close to the primary vertex 
(Web Fig. 5.2).
Microstrip detectors (50m pitch) have been chosen for the layers following the pixels, for their high spatial precision and time resolution, combined with adequate radiation hardness.
The outer tracking layers consist of low thickness Micro Strip Gas Chambers, MSGCs (Web Fig.5.3). The fast charge collection from the thin gas volume ensures good time resolution. Spatial resolution is coarser (200 m pitch) but adequate for the less congested outer region.
MSGCs are sealed gas chambers, with etched strip electrodes on a glass substrate: 9m anode and 70m cathodes, with 200m repetition pitch.
The strip length varies with the distance from the beam, from 5cm to 15cm or 25cm. A few kV are applied between the anode and cathode strips.
The whole tracking detector has been extensively simulated and optimized.
All layers are built with tile-like modules of a few cm square, mounted in spirals with some overlap between adjacent modules, for complete angular coverage and to give a passageway for the cables and the cooling water pipes.
The individual detectors are aligned at better than 15m.
The total power consumption of the tracking detectors is 40kW. Carefully designed water cooling will be used to maintain the Si detectors and some elements of the readout electronics (viz. the opto-transceivers) to low and uniform temperatures (Table 5.3).
Other characteristics of the detector are shown in the tables below.
The coordinate system for the detector, is as follows:
z	(neg. or positive) measures the distance from the center (collision) point along the beam axis
r	( 0 ) measures the radial distance from the beam axis
	(azimuth,  0-360o) measures the angle around the beam axis

Table 5.1 Resolution and number of layers:
Detector Type 	Pitch	no of layers
(barrel)	no of layers
(endcaps)	cell occupancy	
Si pixels	125 	2	3	< 1%	
Si strips	50 	3	3		
MSGC	200/400 	12	5 + 9	< 4%	

Table 5.2 Readout channels:
Detector Type 	Readout channels (barrel)	Readout channels (endcaps)	
Si pixels	5.5 107	2.2 107	
Si strips	2.8 106	1 106	
MSGC	3.3 106	7.9 106	

Table 5.3 Total dimensions and temperatures:
Detector Type 	r (barrel)	r (endcaps)	|z| (endcaps)	Temperature	
Si pixels	7.7-11.7 cm	15-30 cm	39-69 cm	0 1C	
Si strips	20.5-40 cm	18.8-49 cm	125-169 cm	0 1C	
MSGC	40-130 cm	up to 130 cm	164-290 cm	10 2C	

Tracker Assembly
The system is split in subassemblies called ‘wheels’, due to their shape. There are 9 wheels in the barrel part, and 28 wheels in the forward regions.

The precision of the mounting and alignment system should be compatible with the track measurement precision , i.e. 15m.
Light sources, mirrors and CCD cameras mounted within the detector will allow permanent survey of the mechanical tolerances.
Also, the position of the tracker as a whole will be monitored from the external muon detector, by using muon tracks.
The project must also take into account the cabling and services (gas, liquids, low and high voltages), the assembly and disassembly procedures (for maintenance), and the minimization of materials on the particles path before the calorimeters.

Ch. 6  - The tracker readout electronics
For the readout, analogue signal transmission has been preferred, to preserve the signal amplitude and timing information. For instance, the spatial resolution of the pixel detector (125m pitch) can be enhanced to 10-15m by interpolation of  adjacent pixel signals (Web Fig. 5.2).
The readout system can be split in three functional parts 
(Web Fig. 6.1):
- the front-end electronics, mounted on the detector; 
- the optical transmitters, the optical cables (70-90m), and the receivers
- the VME receiver modules
Only the front end depends on the detector (pixels or micro-strips), together with some local, detector mounted, readout control chips (ROC) used to organize sparse readout, time-stamp the data, and handle the control and calibration signals.

Front-end - Si Pixel detector
The basic pixel module has a sensitive area of 1.6 x 6.4cm.Two rows of eight readout chips, 8x10mm, each accommodating 64 pixels, are bump bonded to the detector unit (Web Fig. 5.2).
The detector equipped with chips is fixed onto a hybrid with bond pads and input/output buses.
The local readout chips provide 8-deep, 8 bit buffers to store successive hits and their time stamp, waiting for the  first level trigger decision (3.2ms later).
The signals read out after a trigger are brought via kapton cables to the edge of the detector and to the optical transceivers (described below).
In total, 19.000 chips will require some 300 8-fiber ribbon cables.

Front-end - Si microstrip and MSGCs readout
Both gas and silicon microstrips use the same front end electronics (Web Fig. 6.2), with minor preamplifier modifications.
The readout is based on a proven front-end chip, to be upgraded to 128 channels/chip.
The strip signals are AC coupled to charge sensitive preamplifiers with ~50ns time constant; the output voltage is sampled at the beam crossing rate of 40 MHz, stored in an analog pipeline for 128 beam crossings, then, following level 1 trigger, de-convoluted by an analog signal processor, and multiplexed to 8-bit words for the optical fiber drivers.

The optical transceivers
A laser/photodiode system allows to transmit both analogue and digital data through 8-fold optical fiber ribbons over a distance of 70-90m, to the barracks adjacent the CMS detector cavern.

The VME receiver modules 
The receiver module (Web Fig. 6.3), triggered by level 1 decision, contains the opto-transceiver end (for 8 x 8-fiber cables), flash ADCs to digitize the analogue data, zero suppression and simple cluster finding logic, and memory to store the data for the needs of higher level data acquisition. 
The CMS receiver modules also provide also the calibration and control signals for the front end detector.
By using 64 optical channel/receiver module a system of ~ 100 VME crates is foreseen.

Ch. 7 - The Electromagnetic Calorimeter (ECAL)
In an e/m calorimeter fast, light  particles (electrons, photons), undergo repeated electromagnetic interaction with the atomic shells of the (high Z) detector material; having a very short free path, one particle produces two particles, which produce four particles, etc. and so on until the energy of the resulting particles is lower than the ionization energy (a few eV) of the medium. 
In other words, incident particles  produce ‘showers’ of similar particles (Web Fig. 7.1). 
The purpose of the calorimeter is to measure the total energy of all particles in a shower, as well as the direction and impact point of the particle originating the shower.
This role is specially important in the CMS detector, because some undiscovered particles (e.g. the Higgs boson, ~150GeV) have decay modes into two leptons (e.g. electrons with ~70 GeV each).
For good spatial reconstruction, the e/m calorimeter is made of cells, each one supplying an accurate measurement of the energy deposited therein.
After considering various options, the solution adopted is to use prismatic blocks of PbWO4 (lead tungstate) crystal. The latter is a new scintillating material (8.28 gr/cm3 and refractive index 2.16) producing light at 440-530nm, with fast decay time (10-25nsec) and good radiation resistance.
Mass production of  PbWO4 crystals of adequate size must be organized. The mounting method must be adapted to the fragility of the material. Also, the light yield is temperature-dependent, and much better at low temperatures; hence the crystals temperature has to be stabilized and monitored.
The expected calorimeter performance is better than 5% energy resolution, and 40mRad angular resolution.
The calorimeter (Web Fig. 7.2) is made of PbWO4 crystals, optically isolated from one another, mounted side by side and roughly oriented towards the interaction point. At the rear of each crystal, a solid state photodetector with a shaping amplifier, a temperature regulator, and a spring loading mechanism are used; a calibration optical fiber is also integrated. The crystals are grouped in ‘baskets’, corresponding to DF = 20o, suspended to rails on the inner side of the Hadron calorimeter.
The barrel part (r = 131-186cm; z  =   340cm) will use 93.300 crystals of 20.5x20.5x23cm, for a total 9.75m3 and 81 tons.
The end-cap parts will use crystals of slightly different shapes and sizes; 16.128 crystals, 2.38m3 and 9 tons in total.
The front end of the readout electronics (Fermi system, below), up until the optical transceivers, is also mounted on the detector.

The pre-shower detector
To enhance the rejection of po’s,  which do also decay into two photons, and increase the definition of the shower vertex, a pre-shower detector is added in front of the e/m calorimeter, in the barrel region r = 132-141cm, z =  150cm, and in the corresponding endcap regions (Web Fig. 7.3).
The pre-shower itself consists of 18 mm of lead, to start the decay of neutral particles, followed by Si-strip detectors, with pitch 2mm; 6x6cm wafers, 300m thick, with 32 strips each, are glued onto a water cooled ceramic substrate ( ~5oC). The strips from two adjacent wafers in f will be bonded to obtain 12 cm long strips. The readout electronics boards are mounted behind the Si wafers.
7200 Si wafers are used in the barrel region, and 2496 in each endcap side (where Si wafers are mounted in two layers).
The total number of channels in the barrel will be 115200, and approximately 160000 in the two endcaps .
A 4cm thick honeycomb structure supports the pre-shower detector (4 tons/ half barrel); the honeycomb structure is filled with neutron moderator material to reduce the radiation dose for the pre-shower (1013-1014 n/cm2 in 10 years) .
The readout system for the pre-shower Si detectors is the same as for the Si strips of the Tracking Detector (Sect. 6); 32 channels are connected, via a short Kapton flat cable which is bonded to the strips, to the front-end ICs which contains the preamplifiers, 128-deep analogue memories and the control electronics. 
The charge of each input channel is stores at a frequency of 40 MHz in 128 memory slices. If no Level-1 trigger is received, a given memory position is overwritten after 128 ¥ 0.025 = 3.2 ms. If a trigger is received, the time slice in the memory corresponding to the appropriate beam crossing (and eventually the preceding and the following ones) is frozen and can be read out asynchronously.

The photodetectors and the Electronics chain
The ECAL readout chain consists of a photodetector, the very front-end electronics (VFE) to capture the photodetector signal and the front-end electronics (FE) to digitize it, as well as the interfaces to the Trigger and Data Acquisition (DAQ) systems. 
The VFE consists of a preamplifier, shaping and analog compression electronics and a line driver to the FE which includes the ADC.
The signal acquisition and digitization are based on the FERMI system, described later. The electronic chain is organized as shown  below. 

Fig. 7.4 - The schematic organization of the ECAL electronics. 
The geographic location of the various elements is also indicated.

The photodetector
The photodetector must operate in the 4 T magnetic field, have 
a good quantum efficiency for the scintillator emission wavelength (400 < l < 600 nm), and be radiation tolerant (4 ¥ 1014 neutrons/cm2 in  ten years).
The above requirements can be satisfied by silicon avalanche photodiodes (APDs, for instance EG&G or Hamamatsu). The gain mechanism of these devices, however, depends strongly on the applied electric field and the device temperature, which shall be stabilized and monitored.

The FERMI Microsystem
The analogue signals, after shaping, enter the FERMI compressor and ADC. FERMI is a multichannel data acquisition and signal-processing module implemented with Si-on-Si chips. The base chip, realized as a  large silicon multilayer substrate with four metal layers, supports ASICs, components, interconnections and transmission lines.

Fig 7.5 - Overview of FERMI.

Input signals, with a dynamic range of 16 bits, are compressed and digitized at sampling frequencies of up to 80 MHz. The10-bit 70 MHz ADC is under revision to provide a true 11-bit 80 MHz converter for the final version. The resulting data are linearised and expanded to a 16-bit representation by means of a look-up table, which adds baseline and individual calibration corrections. The expanded data are discriminated, with independently programmable thresholds, then summed and processed in order to provide transverse energy and time information to the Level-1 trigger processor. 
Data are also stored in a dual port memory until the decision from the Level-1 trigger becomes available. 
A number of FERMI microsystems will be mounted on a board, with additional circuitry providing various control functions and some trigger combination logic, as well as the optical link drivers to the remote DAQ system.
The FERMI boards are installed directly behind the calorimeter in a harsh environment with limited accessibility. 
The hardware fault tolerance solutions implemented in the FERMI architecture use error correction coding, redundancy and reconfigurability.

8 - The Hadron Calorimeter
Requirements
The hadron calorimeter (HCAL) surrounds the electromagnetic calorimeter and acts in conjunction with it to measure the energies and directions of particle jets.
  The active elements of the barrel and endcap hadron calorimeter  (Web Fig. 8.1) consist of plastic scintillator tiles with wavelength-shifting (WLS) fiber readout (see optical system below). Layers of these tiles alternate with layers of copper absorber to form the sampling calorimeter structure. The tiles are arranged in projective towers with fine granularity to provide good di-jet separation and mass resolution (Web Fig. 8.2).
The hadron calorimeter must have good hermeticity, good transverse granularity, moderate energy resolution and sufficient depth for hadron shower containment. 
In the central region the barrel hadron calorimeter within the solenoid is too thin for complete shower containment. Further scintillator sampling layers with the same tower granularity are therefore added outside the coil and cryostat.
 The calorimeter readout must have a dynamic range from 20 MeV to 2 TeV to allow the observation of single muons in a calorimeter tower while maintaining adequate response for the highest energy hadronic showers. The muon signal will be used for calibration and assist in muon identification.
Since the ECAL and HCAL calorimeters are located inside the CMS solenoid and cryostat, in the 4 T field, the response of all devices in high magnetic fields must be measured and understood. In addition, the absorber must be non-magnetic and the mechanical structure must be able to withstand the forces generated in the unlikely event of fast discharge of the superconducting solenoid magnet. 
The 25 ns time interval between beam crossings sets the scale for the time resolution needed in the calorimeter. At LHC design beam beam intensity there are approximately twenty interactions per bunch crossing. The calorimeter must help distinguish the rare interesting events from this background (which is strongly suppressed in the calorimeter by the 4 T field) and must have the granularity and time resolution to suppress multi-bunch crossing pile up.

Structure
The hadron calorimeter consists of a barrel (HB) and two endcap 
(HF = Hadron Forward) sections inside the solenoid and a tail catcher outside the solenoid (Web Fig. 8.1). 
The barrel (HB) is divided into two equal cylindrical sections which will be bolted together and inserted from one end of the coil vacuum tank. Each of these half-barrels consists of eighteen identical wedges weighing 27 tonnes each, constructed out of flat absorber plates parallel to the beam axis. The body of the calorimeter is copper but the inner and outer plates are stainless steel for structural strength. 
Despite the use of non-magnetic materials, a fast discharge of the superconducting solenoid can produce large transient electrical and magnetic forces due to eddy currents in metal parts. To decrease the effect of these forces, the wedges will be insulated electrically from one another on a large fraction of their contact surfaces.

Optical System 
The (blue) light from each scintillator tile is collected by a (green) Wave Length Shifting fiber placed in a machined groove in the scintillator. After exiting the scintillator the WLS fiber is spliced to a clear fiber which transports the light to the endcap region where the photodetectors will be placed. Bundles of fibers terminates into custom developed optical connectors. 
Well controlled scintillator thickness and fiber diameters are critical for the optimal performance of the calorimeter. Attenuation lengths of the fibers also must be well controlled.

Production Issues
Several technology developments were required to realize the tile/fiber system. These developments include fiber splicing, mirroring, optical connectors and cables, and fundamental measurements of the tile-fiber optical system. 
Fibers are spliced together by controlled melting of the ends inside a restricting tube (thermal fusion). This technique insures long-term mechanical stability, strength to withstand repeated flexing, high optical transmission and very small variation in transmission for different splices (transmission is 91% 1.8%).
Multifiber optical connectors made via precision injection molding were developed. These connectors allow the optical signals to be treated in a similar way as electrical signals. The scintillator tile trays can be quickly connected to and disconnected from multifiber optical cables that look strikingly like multiconductor electrical cables. The reproducibility of the optical connector transmission is 83%  0.6% for a single fiber.
Photodetectors 
The HCAL photodetectors which convert the optical signals from the ~ 10 mm2 fiber bundles are required to have a linear dynamic range of 105 and operate in a uniform 4 T magnetic field. For calibration purposes, the detectors must have the capability of measuring the signal generated by a radioactive source as a DC current to a precision of 3%. 
The photodetectors being located inside the detector, where service access is infrequent, their MTBF and lifetime must correspond to the operation conditions.
Two types of photodetectors that can operate in magnetic fields and still provide gain have been considered. These are the proximity focused hybrid photodiode (PFHPD) and the semiconductor avalanche photodiode (APD).

Proximity Focused Hybrid Photodiode
The proximity focused hybrid photodiode is an image intensifier operated in the electron bombardment mode. Photoelectrons emitted from the photocathode are accelerated by an electric field and stopped in a silicon diode target where electron-hole pairs are produced generating the signal. In the device under consideration, the 10 kV electric potential is uniform and the acceleration gap is only 1.5 mm to minimize magnetic field effects. Commercial devices are presently available in standard 18 mm or 25 mm diameter single channel versions. Prototypes have been made in which the diode is subdivided into pixels to make a cost effective multichannel device suitable for reading out fiber bundles corresponding to a number of calorimeter towers. High gain prototype pixel devices using avalanche photodiode targets are also under evaluation.

Semiconductor Avalanche Photodiode
Large area APDs are under intensive study for readout of the ECAL crystals. For HCAL, these photodetectors could meet the requirements of large dynamic range, operation in a 4 T magnetic field, and precision DC current capability. The device gain is lower than the hybrid photodiode, being only 50 - 100, but this is compensated in part by the higher quantum efficiency. 

Front-End Electronics and Services 
The electronic readout system of the HCAL is based on the CERN FERMI system already seen for the ECAL. The dynamic range requirement (20 MeV to 2 TeV) is similar to that of the ECAL. 
Electronics boxes containing the optical cable connectors, the photodetectors and associated HV supplies, as well as their preamplifiers and their low voltage distribution, will be distributed in the available space between the barrel to endcap, close to the HCAL detector itself. The FERMI system will also reside in this region. They will be attached to either the barrel or the endcap and will be able to move along with their own sub-detector.

Calibration and Monitoring - 137Cs Radioactive Sources
All layers of the hadron calorimeter will be equipped with thin stainless tubes (f = 1 mm) that will route 137Cs radioactive sources throughout the system. A wire with a point-like Cs source will be pushed through these tubes by a remotely controlled system of drivers. The DC current induced by the source traversing one tile of a tower will provide an accurate measurement of the response of the entire measuring chain. 

Laser Light Calibration System
A laser light system will be used to monitor the stability of the photodetectors and the associated electronics. In addition, it will be used to monitor the linearity of the pulse height measurement chain and provide control of the timing of each channel. It will consist of a triggerable nitrogen laser, a system of neutral density filters covering an adequate dynamic range and a light distribution system delivering the UV light to scintillating tiles via quartz fibers. The intensity of the laser pulses will be monitored by directing a part of the light to a block of scintillator and measuring the resulting light pulses with a PIN diode. 
The rest of the light will go through a system of attenuating neutral density filters covering a dynamic range of four orders of magnitude to a cascade of distributor/commutator boxes. To achieve 1% calibration some 10000 photoelectrons must be detected. 
The total laser power requirement is ~ 1.4 x 10–3 J taking into account the total number of towers, photodetector efficiency and allowing for reasonable losses of light in the distribution process.

Ch. 9 - THE VERY FORWARD CALORIMETER
Introduction
The VF improves the measurement of missing transverse energy and enables very forward jets to be identified. These (particles) jets are a distinguishing characteristic of several important physics processes.
The VF is required to have moderate energy resolution, sufficiently fine granularity to tag forward jets, a very fast response time and  adequate radiation hardness.
This chapter describes two options for the VF detector. The first is based on parallel plate chambers (PPCs). A second option based on quartz fibers (QFs) is being considered, referred to as the VF/QF option. 

The VF/PPC option
The VF/PPC is a modular, iron/gas sampling calorimeter based on PPCs. The VF/PPC is intrinsically radiation hard, since the active medium is a circulating gas. The structural elements, mainly ceramics and iron, are radiation resistant. The chambers have a fast signal response.
The PPC is a gaseous detector with planar electrodes working in the avalanche mode. The gap is filled with a mixture of CO2 and CF4 at atmospheric pressure. 
A PPC cell consists of two metallized ceramic plates that serve as electrodes, separated by a ceramic frame whose thickness (1.5 mm) determines the gas gap. The gas gap can be controlled to within 4m during large-scale PPC production. Flatness and parallelism can be controlled to within 5m. This ensures that local gain variations will be less than 7%.
A high voltage is applied across the electrodes to provide a high uniform electric field of 40 - 50 kV/cm.
The PPC signal is generated in less than 1 ns. The chambers can work in high charged particle fluxes . Gas gains ranging from 102 to 105 are possible. At low gains (~500), the spark probability is estimated to be of order 10–14 per charged particle crossing a gap.
The VF/PPC consists of four large modules of transverse size 165x135cm2, arranged as shown in Web Fig. 9.1, on each side of the interaction point. A module comprises 60 planes of parallel plate chambers interleaved with plates of 35 mm thick iron absorber.
Each PPC plane in a module contains ~652 PPC cells and is ~2 cm thick. This granularity is adequate for jet finding and keeping the cell occupancy down to the requisite level. 
The total depth of the VF/PPC is about 3.3 m. Each iron plate weighs 570 kg, and the VF/PPC on each side of the interaction point weighs about 140 tons.

The Front-End Electronics
The front-end preamplifier is designed to have a dynamic range up to  an input charge of ~2 pC. A gain of about 5 mV/mA matches this requirement. Due to the high neutron flux in the very forward region, the preamplifiers of the VF/PPC will be located at the sides of the detector, requiring cable lengths of about 2 m.

Calibration and Monitoring
Several techniques for the relative calibration of the VF/PPC modules and PPC cells are being investigated (radioactive sources, radioactive gases, and cosmic rays). 
A number of VF/PPC parameters will be monitored including the gas density (via the temperature and pressure), the gas composition and purity. Voltages, currents and temperatures will be monitored throughout the detector and the associated electronics.

The Quartz Fiber VF
An alternative detector technique for the VF is based on the use of quartz fibers, embedded in copper or iron absorber material, in a manner similar to that used in scintillating-fiber calorimeters (Web Fig. 9.2). This type of calorimeter offers some specific advantages.
High-purity quartz (suprasil) is one of the most radiation hard substances known. Such radiation hardness is more than adequate for the VF exposed to the design luminosity for ten years. 
Light in quartz fibers is generated by the Cherenkov effect. Therefore, the VF/QF is only sensitive to relativistic charged particles. This has some important consequences. Only electrons and positrons produced in e.m. showers give rise to an appreciable shower signal. Hadron showers predominantly register through their e.m. shower core. Measurements done with prototype calorimeters have confirmed this feature.
As a consequence, the VF/QF is insensitive to low-energy (MeV) neutrons which will traverse it in large numbers. Similarly, the calorimeter is largely insensitive to the effects of induced radioactivity. The VF/QF is also extremely fast. The production of Cherenkov light is an instantaneous process and timing tails resulting from shower thermalization are absent; all the light produced in a hadron shower is collected in ~12 ns. 
Fibers with a core of fused silica and a cladding of fluorinated fused silica will be sufficiently radiation hard to be used in the 'hottest' region of the experiment; for the less demanding region cheaper plastic-clad quartz fibers should be adequately resistant.

Design of a Quartz Fiber VF
The detector consists of iron or copper absorber filled with 200m plastic-clad quartz fibers on a hexagonal grid with a spacing of 1.5 mm (1.5% packing fraction). 
The fibers are parallel to the beam and are sandwiched between layers of absorber to form wedge-shaped modules. In the first two segments, the fibers are grouped to form a tower structure The granularity of the readout channels is dictated by the physics requirements. Air light pipes guide the Cherenkov light to an array of photomultiplier tubes.

An LHC Experiment (CMS) - Part 2

Ch. 10 - The Muon Detector

Introduction
Muons from p-p collisions are expected to provide clean signatures for a wide range of new physics processes. Some of these processes are expected to be rare and will require the highest beam intensity. The task of the muon detector is to identify these muons and provide a precise measurement of their momenta from a few GeV up to a few TeV.
Muon identification is, in principle, easy at any energy; all that is required is a sufficiently thick hadron absorber surrounding the interaction region. 
The muon detector is placed outside the calorimeters and the magnet coil, interleaved with the iron return yoke plates, within 40 cm wide air gaps. It consists of four muon ‘stations’, MS1-MS4 in the barrel region, and MF1-MF4 in the Endcap regions (Web Fig. 10.1). The magnetic field in the iron plates bends the muons trajectories, allowing an independent muon momentum measurement.
In the barrel region the low particle rate (< 10 Hz/cm2) makes it possible to discriminate high energy muons from low energy ones (which constitute the main background), by fat track curvature recognition. It is therefore possible to build a trigger for events with one, two or three high energy muons, independently of the amount of accompanying hadron activity, using the information of the muon detector alone.
Several different muon detection technologies were considered to provide the required position determination (100m). In the barrel where the expected occupancies and rates are low, a system of drift tubes (DTs), slower but accurate, is the natural candidate. 
In the endcap region cathode strip chambers (CSCs) have been chosen because of their capability of functioning in a high non-uniform magnetic field and at high rates. 
Both barrel drift tubes and cathode strip chambers in the endcap region are expected to be able to identify the parent bunch crossing and to provide spatial information for the Level‑1 trigger. 
A few planes of fast dedicated trigger detectors with excellent timing capability and reasonable position resolution - RPCs, resistive plate chambers - have been added both in the barrel and endcap regions.
DTs, CSCs and RPCs are described in this chapter.

The barrel m detector
In the muon barrel region (low particle rate), a precise momentum measurement can be achieved by:
•	using a slower but more precise type of detector (drift chambers)
•	having a redundant number of muon stations, as well as  chamber planes per station, providing  many distributed measured points per track.
•	a good lever arm (~ 20cm) between the outer chambers of each station.
•	using custom readout electronics to reconstruct the pattern of track segments, and determine precisely each segment position (~100u) and angle (~ 1mrad).
Each muon station MS1-MS4, about 25cm thick, consists of a structural sandwich of (increasing r):
•	4 planes of drift tubes parallel to the beam 
•	4 planes of drift tubes perpendicular to the beam
•	a 10cm thick Al honeycomb to give the required stiffness
•	4 additional layers of tubes parallel to the beam
Since the solenoidal field bending tracks in the r-f plane, the muon momentum measurement is essentially based on the layers of tubes parallel to the beam, for which the best lever arm is ~20cm.
For better coverage, adjacent layers are staggered by half a cell (Web Fig. 10.2a), and the different stations are slightly overlapped (Fig. 10.2b). 

At any given azimuthal angle f a muon track intersects at least three out of four muon stations, taking into account all dead zones.
Each station includes two layers of RPCs (Resistive Plate Chambers) for prompt trigger purpose.

The drift chambers (Drift Tubes, DTs) consist of 4 cm x 1.1 cm rectangular tubes (see chamber construction). The tube length has been limited to 2.56 m to match the longitudinal segmentation of the iron yoke; the chamber width increases from 2 m in the inner station to 4 m in the outer station.
The barrel part of the muon detector is 15m long, and is divided in five wheels of twelve 30o sectors each.
The barrel detector comprises 240 chambers, for an area of 1730 m2.  Access to the chambers is obtained by separating the 5 barrel wheels on the rail system. The total number of electronic channels is 195.000. 

Chamber Construction (barrel)
The drift tubes have a rectangular section (drift cell) of 4 x 1.1cm (Web Fig. 10.3), and a length of 256 cm. Stainless steel wires of 50 mm diameter are strung in the drift tubes. Each anode wire is read out from the system.
The maximum drift distance of ~2cm corresponds to a maximum drift time of 320-400nsec, depending on the proportions of the Ar-CO2 gas mixture (~50 mm/ns). The maximum drift time is compatible with the expected cell occupancy (10 Hz/cm2).
The drift tubes are made of aluminum to ensure good mechanical precision and stiffness. The chambers consist of 2 mm thick aluminum ground planes, separated by 1.2 mm thick aluminum I‑beams to form an 11 mm gap. The I‑beams act as cathodes and are insulated from the ground planes by 500 mm plastic strips. 
The I-beams of a plane are held in position with a precise jig to be glued to the aluminum planes, together with additional field shaping electrodes.
The anode wires are stretched between endplugs at each tube end. Then another chamber is glued on top of the first, and so on.
The electrostatic cell layout is as shown in Fig 10.3. The central anode wire is at +HV (about 2.1 kV), while the C-shaped cathodes are at -HV (about ‑ 2.3 kV).
The resolution of a single chamber is 200m. It's planned to operate the chamber in proportional mode with a non-flammable gas mixture.

Chamber Electronics (barrel)
No analogue information is carried by the DT wire signals, except the time of arrival of a pulse signal with respect to the p-p bunch crossing time. The anode wires are read out with a preamplifier, discriminator and line driver, mounted on PC boards close to the wire ends inside the gas volume.
The wire signals are then sent to both TDCs (Time to Digital Converters) and Meantimers (MT), as shown in Fig 10.4. 

Fig 10.4 - Block Diagram of the CSC electronics, 
a) anode electronics, b) cathode electronics.

The TDCs, the meantimers, the correlators and the readout electronics will sit in a more accessible area, outside the iron yoke. The electronics are located on boards grouping 128 channels each and housed in local readout crates (LRC). 

TDC and Readout 
A TDC chip will be developed at CERN; it will operate with a time bin of 0.8 ns and consist of 32 channels. Time digitizing is done with respect to the previous beam crossing and thus only a 25 ns full range is needed. The number of the bunch crossing is registered, which allows a large time range to be covered. A 10 ns double pulse resolution and a 256-hit deep event buffer is foreseen. A trigger matching function can select events related to a particular trigger, e.g. two good muons.                

Meantimer technique
With a maximum drift time (~ 400 ns for 2 cm) greater than the bunch crossing separation (25 ns), the measure of drift time raises a problem: the time of arrival to of a particle (i.e. the particle origin, neglecting the flight time) is determined with an uncertainty of several times 25 ns.
The meantimer technique (Fig. 10.5) allows to unambiguously determine at which bunch crossing a muon particle must be ascribed. For the purpose, drift cells in adjacent layers are staggered by ½ cell.
The technique provides the bunch crossing identification and the two drift-times without any necessity for a to determination and no left-right ambiguity.

Fig. 10.5 - Particles crossing an array of staggered drift tubes at normal incidence hit two cells a, b. The meantimer signals from the two wires come at fixed time tk=ta+tb after the particle passage through the cells. For inclined tracks, the drift‑time sum tc+td is shorter than for normal incidence, while the sum td+te is larger. But by taking the mean-time of the two meantimers outputs, one obtains the bunch-crossing time delayed by some fixed delay.

Correlator
The correlation between the positions measured in the different stations is automatically taken into account by predefined sets of valid patterns and electronics-incorporated pattern filters.
The decision time for the first-level muon trigger is about 1 msec. Correlators combine the meantimers’ information and sort all tracks by angle value.

Electronics Layout
The local readout crates (LRC) will also house the readout servers (ROS) and trigger server (TS) units, for data collection and transmission to subsequent levels. All LRC electronics will be interfaced to the slow control for monitoring and programming of operation, and to the fast timing trigger and control distribution system.
A system has been developed to feed programmable patterns of test pulses onto the anode wires, through capacitive coupling to the H.V. system of the chamber. 

Independent Muon Trigger
The Level-1 trigger requires muon track angle and position, together with bunch crossing identification, from each chamber. A dedicated meantimer circuit suitable for the structure of the chambers has been studied that is able to identify the bunch crossing and to measure track parameters. The device searches for tracks with at least three out of four hits in each group of chambers. A fixed relation between position and track angle is required by the meantimers in order to trigger only on tracks pointing to the vertex (~ collision point). 

The Endcap Muon Detector
In the endcap region the forward muon detector comprises 4 layers of muon stations (MF1 to MF4) interleaved with the 3 iron disks making up the yoke endcaps.
The muon stations are subdivided azimuthally into 9 or 12 sectors, and radially in two rings, to keep the chamber size reasonable (Web Fig 10.6).
The Endcap detector uses cathode strip chambers (CSCs), capable of providing precise spatial and timing information in the presence of high magnetic field and high particle rates. 
Each station consists of six layers of Cathode Strip Chambers (CSCs, Web Fig. 10.7a).
CSCs are multiwire proportional chambers with thin anode wires, and cathode planes containing some configuration of strips. For the endcaps, a suitable geometry is one of trapezoidal chamber with radial cathode strips and roughly circumferential anode wires.
The ability to measure the charge induced on some adjacent strips gives these chambers a high intrinsic precision, while the short drift path gives a fast time response. 
A CSC consists of a sandwich of rigid 20mm Al honeycomb plates, with a copper plane on one side and copper strips on the other side; 2.5 mm spacers between the honeycomb plates provide the gap where the anode wires are stretched (Fig 10.7b).
A gas mixture of Ar-CO2-CF4 is used. A muon track provides at least three readings per muon station, with a resolution of 40-70u per layer.
(Fast) signals from anode wires are used to measure the radial coordinate and provide bunch-crossing identification; groups of 10-20 wires are grouped together for this purpose.
Radial strips provide the azimuth coordinate f, to measure the muons momentum.
The individual chamber size ranges from 150x44cm to 330x140cm.
There are 345.600 cathode strips and 235.000 anode readout channels in the present system, corresponding to a total of 648 CSCs, for a total area of 1328 m2.

Endcap Muon Electronics
Charge and timing information from the CSCs strips is stored for approximately 128 bunch crossings or 3.2 µs. This time is required to build the trigger decision in the central trigger logic, and to return it to the front-end circuitry. Following a trigger, data are transferred to the data acquisition system. Front-end pattern recognition circuitry identifies and measures local track segments quickly to provide input to the muon trigger logic. All components must be able to withstand the expected radiation level.
Sixteen cathode strip channels from each of the six layers in a station are connected to a front-end strip card mounted on the chamber. The cards carry preamplifiers, storage chips and basic trigger pattern-recognition circuitry. The anode channels are arranged similarly, except that no analogue measurement is done. The anode cards incorporate bunch timing and trigger pattern recognition circuitry. All data from both strip and wire cards, whether for data acquisition or for triggering, pass through a single motherboard mounted on the face of each chamber.
The spatial coordinate across the direction of the strips is measured by charge interpolation of the signals read from adjacent strips. The resolution of this interpolation is proportional to the percentage error of the charge measurement.
To achieve the best possible spatial resolution, the cathode charge measurement must be accurate to within 1%, achieved with a 12-bit ADC.
Alternative designs are evaluated for front-end data acquisition and storage. Full wave sampling and storage using switched capacitor arrays (SCA) before the ADC, or data driven track-and-hold amplifiers and flash ADCs with digital pipelines. 
For pattern recognition on anode signals, the segmentation is coarser than that for the strips. The patterns are used mainly to identify tracks for trigger purposes and to establish the bunch crossing.

Resistive Plate Chambers
A fast dedicated muon detector is essential to unambiguously identify muon tracks associated with bunch crossings in the presence of high rates and background expected at LHC. Signals from such detector directly provide the time and position of a muon hit with the accuracy required for the trigger purposes, up to rates of 103 Hz/cm2. 
The detectors chosen for this purpose are the resistive plate chambers (RPC, Web Fig. 10.8). 
Each RPC consists of a gas-filled gap, obtained with thin spacers between two graphite-coated bakelite plates. For efficiency and reliability two RPCs are combined to form a double gap RPC, as shown in Fig. 10.8. A single readout plane, segmented in strips, is placed between the two chambers to pick up the signal from both gas volumes. 
The outer faces of the bakelite plates are coated with graphite. An electric field is generated by applying a voltage difference between the two graphite-coated planes. 
Stiffening frames are added to the RPC module to make it self-supporting. The RPCs are mounted to match the active area of the tracking chambers. 
In the barrel, the width of each RPC module is 0.86 m. The length of the RPC module varies from 2 m to 4 m according to the muon station. The strips run parallel to the beam direction, and are read out at the end of each single module. The strip width in the bending plane is from 2 to 4 cm for stations MS1 to MS4.
In the endcap region the RPCs are required to have a trapezoidal geometry to cover the CSC chambers. The strips run radially and have a maximum length of 1 m; the strip width is between 0.7 cm and 3 cm. 
The front-end electronics and the cooling system are housed locally on the module surface.
The assignment of the bunch crossing requires an intrinsic detector time resolution of a few nanoseconds and a signal propagation time along the readout strips also of a few ns, i.e. the strip should not be longer than 1 - 1.5 m.
The RPCs have a total surface close to 3400 m2. The number of channels is 192000 (118000 channels in the barrel and 74000 in the forward). This detector has fast response time, relatively low cost and is easily mass produced. 

Ch. 11 - Trigger and data acquisition

At the nominal LHC beam intensity, the expected p-p interaction rate will produce ~109 events/sec.
The task of the CMS trigger and readout system (Fig. 11.1) is 
a) to filter these events according to precise physics requirements  (see Level-1 details), and therefore 
b) reduce the 109 event rate to about 100 Hz, i.e. the maximum acceptable for data storage and off-line analysis.
This is achieved by successive decision levels:
•	Level-1 A hardware-based trigger system, based on topological information supplied by the detector electronics. 
Trigger-1 is programmed to reduce the event rate from 1GHz to less than 100kHz, i.e. to the bandwidth of the subsequent readout system.
On a level-1 accept, event data are transferred from the Front-End electronics to the Readout Units.
•	Level-2 A 'software trigger'. A subset of event data, coming from detectors like the calorimeter and muon systems, is sent from the Readout Units to a part of the Event Filter Units (computer farms). 
This avoids reading out the large volume of tracking data for events to be rejected. 
The Level-2 filter will reduce the event rate by at least one order of magnitude. 
•	Level-3 On a Level-2 accept, the remainder of the full event data are transferred from the Readout Units, through a large (1000x1000) Switch Fabric, to the computer farms.
A final Level-3 algorithm is then applied to the complete event. The output (storage) rate is ~ 100Hz.

The CMS data acquisition system can be seen as consisting of several layers:
•	The Trigger-1 and Front-End electronics (FE)
•	The Readout Units (RU)
•	A Switch Fabric
•	The Event Filter Units (EFU, parallel computer farms)
•	The Monitoring, Control and Data Storage services
 The flow of event data is controlled by an Event Manager system.

Fig. 11.1: Data acquisition basic structure.	

Front End Electronics
The front-end electronics is specific to each sub-detector and mounted onto it. The FE consists mainly of custom designed VLSI circuits, to cope with the large number of channels, the space constraints, the low power requirements, and for the inner detectors also radiation resistance. 
From the point of view of the DAQ system, however, the FE functionality is common to all the sub-detectors (Fig 11.2).
For each channel, there are a preamplifier, shaper, digitizer, and the buffers to hold the data for the duration of the level-1 trigger decision (3 msec, or 120 bunch crossings). 
The FE also includes local readout devices for pipelining, derandomizing, zero-suppression and signal processing. The FE readout is driven by the 40.08 MHz machine clock, Level-1 trigger accept and bunch counter synchronization signals.

Fig. 11.2: Detector front-end logical model.

For low and medium occupancy detectors (inner trackers and calorimeters, occ. ~ 1-10%) the buffer behaves as a synchronous circular buffer. For very low occupancy detectors (e.g. muon) only discriminated hits data are saved and labeled with the time tag given by a local bunch-crossing counter. For each event a programmable number of bunch-crossing data are read out from the pipeline. This number also represents the dead time related to Trigger-1. 
An industry standard serial bus is interfaced to all FE for system initialization and slow control
Following a Trigger-1, the data corresponding to each selected event must be moved from about 1000 front-end derandomizer/buffers all around the apparatus, to a single location, for physics analysis and further event filtering. This represents about 106 event data fragments per event.
Analogue or digital data are then transmitted from the front-end buffers, over optical data links, to the Front-End Drivers (FED) and Readout Units relative to each sub-detector or sub-system and located in readout crates (see underground counting room, in table 11.3).

Level-1 Trigger
The Level-1 trigger is based on the pattern recognition circuitry built into the front-end electronics of detectors like the calorimeter and muons (the Tracker information is too big and too fragmented to be used for a fast decision, see table 11.1).
The trigger electronics on the detectors consists of region specific hardware processors with a pipelined architecture, which permits a topological decision with no deadtime. The information of the local triggers is sent through optical links to a Global Trigger system (GT), located in the nearest electronics barrack.
 The GT aligns the local results by bunch crossing number, and generates the decision according to (per channel) programmed acceptance criteria. 
Due to the high speed and dataflow requirements, also the global trigger runs in a pipelined mode, and take a yes/no decision every 25 ns; the criteria are programmable, to accommodate unforeseen background or physics rates, and perform monitoring and control tasks.
The GT is based on specialized hardware processors with a fixed latency (basically a programmable, synchronous data filtering system at 40.08 MHz).
The acceptance rate should not exceed 105 Hz.  The trigger ‘accept’ decision is re-distributed to all the front-end by a Timing and Control optical-fiber unit (TC).
The trigger granularity is about 4000 channels for the calorimeters and about 105 channels for the muon detectors. 

Readout Units
The task of the readout system is to suppress null information and compact and assemble all the data fragments in a complete event record to be fed into a processor memory.
For each sub-detector, or part of it, there is a set of Readout Units.
A Readout Unit (RU) consists of one or more detector-specific Front-End Driver (FED) modules, transferring data from groups of front-end elements to a multi-event dual port memory (RDPM). 
The FEDs provide the formatting and local buffering of data fragments and handle the interface protocols for the control and readout buses. A readout controller (ROC) supervises the data flow; each FED-ROC group acts as a stand-alone data acquisition system.
Each FED has two access ports, in addition to the optical inputs. A standard VMEbus interface provides for overall control and monitoring from the ROC, while a dedicated link to the RDPM is required to accomplish a throughput of nominally 100 MByte/s, up to 400 MByte/s, depending on the event-building switch-matrix configuration. The RDPM link is sufficiently buffered to allow for fluctuations in the Level-1 trigger rate. 
The number of front-end channels read by a FED can handle an average of 104 bits per event, with a data bandwidth of a few 100Mbit/s. 
Each RDPM will be attached to between one and six FEDs, depending on the local event data sizes and the event builder switch input rate. 
The complete readout consists of about 800 FEDs, allowing a throughput of 104-105 events, with ~1 MByte event size. 
Each RDPM must be capable of buffering up to 100000 event fragments.
As shown in Fig. 11.3, a physical readout crate contains several readout units and a readout crate controller and communication module (ROC). The ROC comprises a readout crate supervisor (RCS) and a readout flow controller (RFC). The specific tasks of the ROC are: to ensure the correct initialization of the RUs; to monitor their operation during data taking; to handle errors, exceptions and interrupts; and to handle status communication to the central system control.

Fig. 11.3: Readout crate components.	

Switching network
A large switching network with a total throughput of approximately 500 Gbit/s forms the interconnection between the RDPM sources and the EFU destinations.
The implementation of this network is based on switching technologies like the ATM, Fiber Channel and SCI standards, depending on their commercial availability.
Commercial Fiber Channel switches are now available with up to 64 ´ 64 ports, and switches with 256 ´ 256 ports are being designed. Eight of these switches require to form a 1024 ´ 1024 network, assuming that the input and output channels in each port are capable of operating independently. Commercial ATM switches expandable up to 512 ports (at 622 Mbit/s) are available and future versions foreseen for the years 2000 will support up to 4096 ports. 

Event Filter. 
The event filter has three tasks: 
1)	To perform a fast decision on the quality of an event, based on data from a limited number of readout units. This is the virtual Level-2 trigger, i.e. the step that is expected to provide a reduction factor ~10 with respect to the Level-1 trigger.
2)	To complete the event-building process, after a Level-2 accept, by reading in the rest of the data from the readout units.
3)	To execute physics algorithms that utilize all of the detector information, with the goal of reducing the event rate to 10 - 100 Hz for output to mass storage.

High-Level Filters, Algorithms and Rates.
The presence of general-purpose CPUs at the farm level provides considerable flexibility both in the design of the physics algorithms and in the choice of data used by these algorithms. The Level-2 trigger can thus access in full detail any sub-detector information required for a chosen algorithm. In practice, the Level-2 trigger selects a subset of this data, based on Level-1 trigger information, to perform quick algorithms that verify the physics objects found by the Level-1 trigger.
The simplest model for the Level-2 algorithm is to reapply the Level-1 trigger algorithms on the calorimeter data. The increased resolution and detector segmentation should result in a further rejection factor.

Event Management
Data flow is heavily pipelined or buffered, allowing asynchronous operation of the various sub-systems in different layers of the DAQ system. The individual readout units direct front-end readout operations. The farm processors (Level-2 and -3) perform the high level event filtering. To maintain coherency between these two stages, the event manager will coordinate the assignment of complete, or partially complete, events from the readout units to the event filter units.
All events will be referenced by their event number. The inherent latency in building events from 1000 RDPMs through the Switch Fabric requires that multiple events be built in parallel. Typically up to 100 events of 1 MByte each will be handled simultaneously.

Timing, Trigger and Control (TTC) Distribution
The system is based on the use of a few relatively high power laser sources, which distribute the signals via a hierarchy of passive optical tree couplers to several thousand destinations each.
The TTC system compensates for time-of-flight, electronic and signal propagation delays, and delivers properly synchronized bunch crossing and event numbers to the controllers together with each trigger decision.

Global Control and Monitoring.
The GCMS supervises the operation of the entire trigger and data acquisition system. The following functionality will be provided:
•	configuration of all sub-systems, including initialization and dynamic reconfiguration,
•	launching and termination of the data acquisition tasks,
•	accumulation of statistical information about data taking and detector performance,
•	recording of run conditions, configuration parameters and calibration constants in a database for later processing and evaluation,
•	checking of data integrity,
•	accumulation and presentation of physics performance histograms and event display,
•	detection, recording and analysis of error conditions. Serious conditions will generate informative alarms for the operators.
The system will be designed in a modular and scaleable way, to be easily adaptable to different configurations. The global system communicates with the distributed systems for the different sub-detectors in a standardized way and an object-oriented approach to the design will be adopted. This should make it possible to develop and use such a system in test-beam setups. Workstations with a standard Graphical User Interface (GUI), connected via local or wide area networks, will be used to interact with the different tasks and to present status information about the sub-detectors and DAQ system.

Operating Systems
Given the tasks that the event filter has to perform the choice of the operating system is of paramount importance. Under the control of the event manager, it should allow for the execution of the Level-2 and Level‑3 filters in real-time with very fast context switching and low latency. At the same time, it must be possible to execute tasks with lower priority to allow for proper monitoring and control. Dynamic reconfiguration should allow the configuration of the event filter to be adapted to changing running conditions. Drivers for service network interfaces will permit access to the processing elements for a client/server computing model. Finally a software environment similar to the offline one will ensure that reconstruction programs can be run essentially unmodified.
Current developments suggest that a micro-kernel based operating system architecture is the most promising. All the basic facilities are implemented in a well-structured way and can therefore be configured easily. This approach has several additional advantages:
•	better OS engineering,
•	improved real-time performance,
•	support for parallel and distributed systems,
•	portability and standardization.

APPENDIX
The average data volume generated by the CMS detector at each crossing is expected to be about 1 MByte. Table 11.1 shows the breakdown by sub-detector. 
Table 11.1 - Average sub-detector event size at nominal luminosity
Sub-detector	No. channels	Occupancy %	Event size (Kbytes)	
Pixel	80 000 000	0.01	100	
Inner Tracker	16 000 000	3	700	
Preshower	512 000	10	100	
Calorimeters	250 000	10	50	
Muons	1 000 000	0.1	10	
Trigger	10 000	100	10	

Table 11.2 - CMS data acquisition parameters.
Average event size 	~1 MByte	
Level-1 trigger rate	100 kHz	
No. of readout modules (200-5000 Byte/event) 	1000	
Event builder (1000 ? 1000 switch) bandwidth	~500 Gbit/s 	
Event filter computing power	~5 106 MIPS	
Data production	~TByte/day	
No. of readout crates	~300	
No. of electronics boards	~10000	

Table 11.3 – Location of the electronic systems
Front End Electronics	On the detector	
Level 1 Trigger	Local (underground) Counting Room	
Global Trigger	ibidem	
Readout Crates	ibidem	
Event Manager	Surface Control Room	
Event Builder	ibidem	
Switch Farm Interface	ibidem	
Event Filter Farms	ibidem	
Final Computing Farms	Main Control Room	
Storage Devices	ibidem	
Global Control	ibidem	

Table 11.4 – Technology evolution assumptions 
Unit processing power increase, at constant cost	An order of magnitude every five years	
Unit memory density increase, at constant cost	Factor of four every two years	
Developments within the telecommunications industry	Will continue during the next decade at least at the current rate.	

Table 11.5 – Glossary of terminology.
FE	Detector Front-End electronics read out by analogue/digital data links	
LV1 	LeVel-1 regional trigger units (calorimeter and muon)	
GTS 	Global Trigger System	
RC	Readout Crate. Each crate houses one or more autonomous data acquisition units (a total of 1000) with multievent buffering capability	
Event Builder 	Multiport (1000•1000) switch network	
EVM	EVent Manager. Event scheduling and filter task control	
SFI	Switch Farm Interface. Event data assembly into processor memory. The system includes the farm status communication with the EVM and the control.	
CPU 	Event filter processing unit. It may be a single workstation board or a unit in a multiprocessor server.	
Fast Control	Trigger and EVM signal broadcasting to readout crates. Status collection from readout modules and feedback to the trigger processor.	
Control	System test, initialization, monitoring etc. Supervision of operations associated with the data flow main steps.	
Services 	Control room consoles for analysis, display and monitoring consoles; network connections; mass storage and data archives.