Accelerators, Particles and Experiments
Foreword

This is a very simplified, although ambitious, description of:

1) 	Particle Accelerators
2) 	High Energy Physics (HEP) experiments
3) 	Particle Detectors
4) 	Elementary Particles
5) 	The Physics Quest

The purpose is not to explain things in depth (it would take books), but to provide a quick understanding of HEP activities and the related ‘jargon’.
 
HEP experiments aim at the detection and study of subatomic constituents of matter. 
The dimensions to be investigated have become smaller and smaller over the last hundred years; from the atomic scale (1-2 Angstrom, 1Å = 10-8cm), to the nuclear size (1 Fermi, or10-13cm); then further down to nucleus constituents (‘nucleons’), the proton and neutron; and down again for nucleon constituents, quarks and gluons. 

A method to look at objects that are too small to be seen in other ways, consists in shooting to them small and fast projectiles (accelerated particles). From the projectile deviations, or from the fragments of a ‘hard’ collision, it is possible to infer some properties of the ‘invisible’ objects.

The smaller the ‘objects’ to be examined, the more difficult it is to get near to them, and the faster and better collimated must be the projectiles.
In a similar way, silicon lithography, or biological microscopy, must use shorter light wavelength for higher resolution.

High energy particle accelerators are machines which supply ‘beams’ of high energy particles to produce the kind of (particle) collisions required to advance a little further the frontier of experimental and theoretical physics. 

In addition to accelerators, very sophisticate particle detectors are needed to detect and record the products of collision products and their properties.

Providing the accelerators for (sub) nuclear collisions, and the detectors to study  the collision products, are the two main domains of activity of a HEP laboratory like CERN.

See also the attached Various_figs.htm file. 

Ch. 1 - Particle Accelerators

At CERN, 	(Web Fig. 1) machines of different kinds accelerate and transfer charged particles - typically electrons or protons, but also anti-electrons or anti-protons, and more recently heavy ions - to various energies. 

Accelerators 	can be linear, or straight, called then ‘Linacs’, or circular, e.g. synchrotrons and storage rings or colliders. 
Actually, circular accelerators also have straight sections to install RF cavities (below) and other equipment. 

Since the 1980's 	circular accelerators have evolved to 'storage rings' or ‘colliders’; that is, instead of ejecting the particles at the end of a short accele-rating cycle, counter-circulating beams of accelerated particles can be stored for days, and made to collide each-other. This evolution has been allowed by substantial refinements in the ‘beam optics’, vacuum quality, and beam stability control.

The CERN PS	or Proton Synchrotron belongs to the first generation (extracted beams), while the SPS, designed as a Super Proton Synchrotron, has been initially changed to a proton/anti-proton collider, following a proposal by C. Rubbia. At present, it is again a source of extracted beams for fixed target experiments, as well as an injector for the LEP e+/e- collider. 
Another change in large accelerators was to build them in underground ‘tunnels’, for cost and environment considerations. See the Appendix for some data on the PS, SPS, LEP, LHC machines at CERN.

Accelerated particles 	can be made to collide either onto fixed targets (metal, liquid hydrogen, etc.), or against the particles of another beam circulating in the opposite direction (colliding beams).

Accelerators are built 	with some essential components, which repeat themselves regularly in the modules or ‘cells’ of a periodic structure. These components are:

The vacuum pipe 	typically an inox tube, a few cm in diameter and up to several km long, which provides a free ‘path’ for accelerated particles. Vacuum, or high vacuum (10-9 to 10-11 torr, 1 torr = 1mmHg) is maintained in the pipe to allow accelerated particles to circulate long time without hitting residual gas molecules. 
Vacuum techniques include pre-heating the pipe to extract the gas molecules imbedded in the metal, using special gaskets and soldering materials, several types of vacuum pumps, monitoring the vacuum level, detecting or protecting against ‘leaks’, etc. Each accelerator has its Vacuum group of specialists.
 Radio-Frequency cavities. 	These are copper cavities of about spherical shape, mounted around the beam pipe, and built to resonate at a radio frequency  multiple of the beam circulating period. Beam particles must traverse the RF cavity at a time when the internal electric field has the right direction and phase for particle acceleration. 
The final acceleration is the result of several accelerating steps, due to the high number of  RF cavities and the high number of revolutions around a ring. In other words, a final energy of Mega, Giga or Tera electronvolt (MeV, GeV, TeV), is obtained by multiple kicks to the charged particles - electrons, protons, or other. 
The accelerated beams have a 'bunch' structure because the oscillating electric field in a RF cavity has the right direction and phase only during a small part of the total period. 
Modern RF cavities are ‘superconducting’, i.e. coated with superconductor materials and kept to cryogenic temperature, to improve both the power efficiency and the attainable electric field.
In LHC, superconducting RF cavities will produce a bunch frequency of 40MHz, and megavolt RF electric field.

Bending magnets 	(for circular accelerators). The trajectory of charged particles can be bent into a circular orbit in a horizontal plane by means of a vertical magnetic field with the right intensity; this field is supplied by electromagnets (‘dipoles’) installed onto the vacuum pipe all around the ‘ring’. 
An upper limit  for the bending action is set either by the highest magnetic field practically obtainable (in proton accelerators), or by synchrotron radiation losses in electron/positron accelerators (LEP).
For instance, the LHC shall use about 1200 superconducting magnets, each 15m long, with a field of 8.7 Tesla.

 Focusing/defocusing 	magnetic elements (quadrupoles, sextupoles, etc.) are needed to maintain the particles in a well collimated beam; particles would otherwise get naturally dispersed by several actions - acceleration, deflection, mutual repulsion, stray fields, etc.

 Beam monitors 	are used to precisely monitor the position, dimensions, intensity and timing of beam bunches; beam feedback systems are needed to control beam position and beam instabilities.

Control systems	for all the accelerator equipment; magnet power supplies, cooling installations, control rooms, etc. ‘Machine operators’ ensure the operation of an accelerator day and night, most of the time. There are shut-down or maintenance periods.

Ch. 2 - Experiments

High Energy Physics 	(HEP) experiments with accelerated beams consist as already said in sending particles (electrons, protons), accelerated to the maximum possible speed/energy, to collide with other particles,  either contained in the material of a fixed target (metal, liquid hydrogen), or, in collider rings, belonging to a counter-rotating beam moving in the opposite direction of the primary particles.

There are therefore 'fixed target' experiments, usually in surface buildings like the CERN West Area and North Area halls - and colliding beam experiments, usually underground, like at LEP (electron-positron) or LHC (proton-proton).

In a fixed target 	experiment the collision products are ejected mainly in a region opposite to that of the incident particle.
 
With colliding beams, 	instead, the center-of-mass of the colliding particles is at rest in the laboratory; the products of collision are ejected all around the interaction point, with more products along the (two) beams axis, and occasionally in transversal ‘jets’ of particles.

The aim 	of an experiment is to detect and measure the 'products' of a collision, often looking for the appearance of new or undetected particles, or verifying predictions of particles’ behavior. 
To this purpose, suitable ‘particle detectors’ must equip the full volume around the interaction point, i.e. the volume into which the produced ‘secondary’ particles can fly and/or decay. 
This volume, named in short ‘the detector’, is preferably immersed in a strong magnetic field, produced by a large dedicated magnet (the ‘experiment magnet’). The magnetic field bends or curls the trajectories of  secondary particles, allowing to determine the charge sign, ‘momentum’ and energy of each particle.

With increasing accelerator energy , the ‘detector’ volume, needed to ‘contain’ all produced particles, grows as well: from about 10m x 10m x 10m for LEP, to some 15m x15m x 20m for the LHC. 

For proper identification 	of all collision products, one needs a complete, snapshot  ‘event reconstruction’ (Web Fig. 2). An ‘event’ is a hard collision which produces interesting types of fragments or ‘secondaries’. For the reconstruction, both the collision point and the secondaries’ trajectories must be measured with sub-millimeter accuracy over a length of several meters, in the detector volume.
Particularly, a precise measure of all the charged particles tracks is the only way of measuring also the energy and momentum of neutral particles, which may be ‘invisible’ (because they do not interact with the detector devices), when they do not decay into charged particles within the detector.

A precise measure 	of trajectories requires that about every cubic mm in the region near the collision point (about 1/3 of the detector volume), and roughly every cubic cm in the rest of the detector, be equipped with some  device capable of recording the passage of one or several particles (see Particle Detectors, next chapter). This gives a hint about  number of ‘readout’ channels. The measuring volume of the CMS detector for LHC, for instance, is about 3000 m3.

Other aspects 	of a HEP experiment concern:
- building a ‘trigger’, i.e. a prompt signal determining whether an ‘event’ is of the required type or not, worth or not being recorded; 
- data acquisition, filtering and recording at a rate high enough to capture all potentially ‘good’ events; 
- on-line event reconstruction; calibration, mechanical alignment (within microns) of the detector elements; 
- test and maintenance.
- the scientific and financial set-up of a collaboration gathering typically more than 1000  physicists and engineers from many countries and institutes.

Ch. 3 - Particle Detectors

Old detectors	We’ll pass over some historical types of particle detectors: photographic emulsions, cloud and bubble chambers, spark or streamer chambers. They contributed a lot to particles identification, but are nowadays museum or demonstration material. They had the advantage of showing the particles, i.e. providing tracks visible either by eye or by photographic cameras. An ongoing activity in many Physics Institutes was, not too long ago, the analysis of thousands of film rolls.

Modern detectors	are ‘electronic’, i.e. they detect a particle with the help of electronic amplification devices; the particle tracks can be seen only on computer screens after some mathematical ‘reconstruction’.
The main advantage over past detectors is the speed of response, which then determines the ‘rate’ at which particles can be recorded. The response time of modern detectors is of the order of nanoseconds (10-9 sec), or microseconds (10-6sec) for the slower ones. Electronics and computers are obviously faster than cameras.

Another benefit 	of fast detection is the so called ‘multi-track capability’, i.e. the capability of a detector to record the passage of several particles at about the same time, provided there is a minimum spatial separation between multiple tracks.

A common characteristic 	of detectors is that they exploit the (electromagnetic) interaction of charged particles with the material of the detector itself, gas, liquid or solid. This interaction produces a small signal, optical or electrical, which is amplified, filtered, converted to digital, and recorded by custom designed electronic devices, all in very short time.

Neutral particles 	can’t be detected as such; the only way, for instance with neutrinos, is to put so much material (e.g. tons of iron) on their path as to enhance the probability of a collision producing some detectable (charged) products.

Particle detectors can be grouped either by their purpose, or by their operating principle. 

By the purpose, there are: 

'Tracking' detectors	 which measure with high accuracy the track (part of the trajectory) of a particle. 

'Calorimeters’ 	(electron calorimeters or hadron calorimeters), which do also measure roughly the particle position, but have the main task of ‘stopping’ the  particle(s) by built-in heavy material (uranium, iron, lead), and thereby measure the 'total particle deposited energy'.  ‘Sampling’ calorimeters use layers of heavy materials (U, Pb, Fe) alternated with planes of detectors. ‘Crystal’ calorimeters use scintillating crystals (see below), in small modules to ensure adequate volume ‘granularity’. 

‘Trigger’ detectors 	where accuracy can be traded off for a fast and clear-cut (yes/no) signal of a particle passage. These signals provide both a precise time label for the event and a ‘trigger’ or ‘decision’ to read out all the other more refined detectors.

By the operating principle: 

'Scintillation' detectors	made of plastic plates or fibers doped with organic scintillating substances, which emit tiny flashes of light when traversed by energetic particles. A light flash is transformed into an electric pulse by a photodiode or photomultiplier device.
Some types of crystals are also good scintillators; they are used mostly in calorimeters, where they provide both the bulk material and the scintillating properties. 
Cherenkov counters, where fast particles produce a cone of light, are other optical detectors.

'Wire chambers' 	also called ‘proportional wire chambers’, are detectors which use patterns of thin, parallel metal wires, stretched in a gaseous chamber, and maintained at a high voltage. The electric charges (electron-ion pairs) produced in the gaseous mixture by energetic crossing particles are collected by the nearest wire (‘hit wire’), that then determines the particle track coordinate.

‘Drift chambers’  	and ‘drift tubes’ work on a principle similar to that of wire chambers, but with wires spaced by centimeters, instead of millimeters. It is then the charge migration time, from the track position to an anode wire, which measures the track coordinate with good accuracy.

‘Silicon detectors’ 	use silicon wafers, each of a few cm2, with pixels or patterns of miniature parallel metal strips at the surface (pitch 20-200m). Charge pairs produced in the silicon bulk are collected by the nearest electrodes and determine the track coordinate. 
By alternating silicon layers with X-Y strip orientation, one can measure both coordinates of incident particles.

Like any measuring device, particle detectors are characterized by the ‘resolution’ (minimum detectable variation of track position), ‘accuracy’ or spatial precision, ‘stability’ (constant response over weeks or months periods), and in the case of accelerators like the LHC, producing very high flows of particles, also by the ‘radiation resistance’. 
Calibration before and during use, and continuous maintenance, are the guarantee of constant performance.

Ch. 4 - Elementary Particles
	
This chapter must be undertaken with some dose of patience; after all, we don’t know what is a particle, even less elementary.

There are two traditional approaches to the subject: describe the evolution which has brought through the years to recent ideas in this domain; or start from the present knowledge, and possibly append some historical considerations; we take the second approach.
____________

In the so called ‘standard model’, there are 12 known/expected ‘elementary’ particles, divided in two groups of six particles each: leptons and quarks:
Table 1a: Elementary Particles
	Leptons	Quarks	
[unstable]	t        	tau	nt    	t-neutrino	t	top	b	bottom	
[unstable]	m        	muon	nm    	m-neutrino	c	charm	s	strange	
stable     	e	electron	ne     	e-neutrino	u	up	d	down	
					
electric charge	-1	0	2/3	-1/3	
spin	1/2	1/2	1/2	1/2	

... and the corresponding anti-particles	

Table 1b: Particle masses (in GeV)
Leptons	Quarks	
	Mass 		Mass 		Mass 		Mass 	
t        	1.7771	nt    	< 0.03	t	180	b	4.7	
m        	0.106	nm    	< 0.0003	c	1.5	s	0.2	
e	0.000511	ne     	< 7 x 109	u	0.005	d	0.01	

Roughly speaking, the leptons are light and small, like the electron, which makes up the shells around the atomic nucleus. Quarks (heavier) are the building blocks of protons and neutrons in the atomic nucleus: a proton p+ is made of two ‘up’ and one ‘down’ quark (p = uud); a neutron n is a combination udd. 

This simple picture has some dark corners though.

•	Only the components of the lower layer, shown by an arrow in Table 1a, are stable and found in ‘everyday’ matter; that is, for the world that we see, we would need only the electron, the electron-neutrino and the ‘up’ and ‘down’ quarks.

•	The particles of the upper layers are heavier, unstable, and decay rather quickly into the lower layer (stable) members - except neutrinos. For instance the muon m has all the properties of an electron, but is 206 times heavier, and decays in about 2msec; the tau t is 1784 times heavier and even more unstable.

•	It is not clear why the heavier leptons and quarks are needed, or if they are ‘excited states’ of some other structure; it seems proved though that there aren’t any heavier particles to be added on top of the table.

•	It is not known why there are different types of neutrinos, if they have a mass, and if one type of neutrino can turn into another.

•	Quarks have never been seen in isolation, nor has any ‘fractional’ electrical charge (1/3 or 2/3) ever been measured. Various theories of ‘confinement’ have been developed to explain why quarks cannot escape as such from a nucleon. 

•	There is however indirect evidence of three point-like objects within a proton or neutron when they are hit by high energy electrons. The same technique was used long ago to verify that the atom had a very small nucleus (the famous “fly in a cathedral”).

•	Quarks have an additional quantum property, similar to electric charge but which can take three different values: this quantum number has been named ‘color’, and its three values ‘red’, ‘green’, ‘blue’, by joking physicists. Therefore quarks obey the laws of  “quantum chromodynamics”.

•	For each ‘elementary’ particle, lepton or quark, there is the corresponding anti-particle (made of anti-matter), with identical properties but opposite quantum numbers. Anti-particles, however, are only produced in HEP experiments or by cosmic rays.

To continue with particles’ nomenclature, it should be said that the elementary particles of Table 1 are all ‘fermions’. This name is related to the statistical behavior of these particles, and also to the ‘spin’ quantum number; which measures the particles’ angular momentum (self-rotation or spinning, in a way).

Spin 	is quantized, i.e. it always comes in half integer or integer multiples of the quantity  = h/2. The spin also determines the particles ‘statistic’ behavior. It is well known that particles with half-integer spin (1/2 , 3/2 ,...) obey the Pauli exclusion principle, or Fermi’s statistics: no two equal particles in the same quantum state, which for instance compels electrons to occupy different energy layers around an atomic nucleus. So, particles with half-integer spin, including the proton and neutron, are called ‘fermions’.

Bosons	There are other particles, with integer spin values (0, , 2,...), which are not subject to the exclusion principle: they obey the Bose statistics (any number of particles in the same status, like Japanese in the subway), and are therefore called ‘bosons’. 
Bosons are as important as fermions, to the extent of making up a third family of elementary ‘particles’ (in addition to leptons and quarks): they are the force carriers, or field quanta, in interactions  between elementary particles.

Which is the role of boson ‘force carriers’:

on one side, all the various particles would have no meaning if they would not ‘interact’ with each other. 
On the other side, in the quantum world there is no ‘action at a distance’ type of force: all interactions happen by the exchange of ‘messenger’ particles, also called ‘field quanta’ or ‘vector bosons’.

The number of force carriers is determined by the different types of force.
There are four fundamental forces in nature, i.e. different types of interaction between particles, each one with its own field quanta:
Table 2: Fundamental Forces
	Gravitation	Electromagnetic
 Force	Strong (nuclear) Force	Weak force	
Source	mass	electric charge	color charge	weak charge	
Range
	infinite	infinite	10-15m	10-17m	
Relative strength	10-39	1	102	10-12	
Field quanta/ boson	graviton ()	photon	gluons (n=8)	W±, Zo   ()	
Boson mass	zero	zero	zero	~100 GeV	
Boson spin	2	1	...	...	
Interaction time	?	10-15	10-23	10-8 	

... and the corresponding anti-particles	

Out of the four forces, only gravitation and the electromagnetic force play a role in our earth environment. Each of the four forces has however a special character and role, and all are needed to explain for instance the sun fusion processes.
 
1 - Gravitational 	interaction, being 10-39 times weaker than the electromagnetic forces, is negligible at the atomic scale, i.e. between single particles. But it is the dominant force on the earth and in the universe, for two reasons: a) because it is the only force which is always ‘additive’ (electric charge, in contrast, tends to cancel out - negative charges are almost always  balanced by positive charges). b) Gravitational attraction spans through stars and galaxies; it is said to have an ‘infinite’ range of action, as compared to the 10-15-10-17m range of atomic forces.

2 - The electromagnetic 	(e/m) force needs no presentation; it acts on all electrically charged particles and bodies. It holds nuclei and electrons together to form atoms; the atoms together to form molecules; molecules together to form liquid or solid compounds (water, iron, diamond). It works in all electric or electronic devices - phone, TV, computers, lamps.

3 - The weak force 	acts on both quarks and leptons; it can change a type of lepton (quark) into another lepton (quark), for instance in radioactive decays, which unveiled this force. 
The lepton (literally: light) particles are subject only to the weak force (and to the e/m, if electrically charged) whereas quarks and their derivates - see ‘hadrons’ below - are also subject to the ‘strong’ force.

4 - The Strong force 	is the one which holds quarks together in nucleons (proton and  neutron), and holds the protons and neutrons together in the atomic nucleus: here the quark-quark interaction, 100 times stronger than the electromagnetic force, overcomes the high electric repulsion between very close, positively charged protons. The quark-quark forces are carried by ‘gluons’ (field quanta), which are eight because they reflect the 3x3color combinations. 

Particles which interact through the strong force, and which involve quarks, have got the traditional name of ‘hadrons’ (strong). They can be of two types:

1) 	3-quark assemblies (QQQ); the best examples being the proton (uud) and neutron (udd), already seen. These particles have also got the historical name of ‘baryons’ (heavy), and have half-integer spins (algebraic sum of three half integers).

2) 	 Quark/anti-quark assemblies (QQ); the hadrons of this family are called ‘mesons’ (half-heavy), and have zero or integer spin. A good example is the p pion in Table 4.

Note that of all the hadrons (baryons or mesons), only the proton is stable; at least, its lifetime has been checked to be longer than 1033 years.

The pattern of particles and forces described hitherto is summarized in Table 3.
Table 3: Particles and Interactions
	Leptons
	Quarks
	
	Electrons	Neutrinos	Hadrons
	
			Baryons	Mesons	
Strong Force	No	No	Yes	Yes	
Weak Force	Yes	Yes	Yes	Yes	
e/m Interaction	if charged	No	if charged	if charged	
Gravitation	Yes	No (if m = 0)	Yes	Yes	

By playing all the possible combinations of two or three quarks or anti-quarks, one can obtain the dozens of particles (baryons, mesons, and some very heavy ‘hyperons’) which have been observed at accelerator experiments and have puzzled physicists during the last fifty years. 
Originally, they appeared to form families or ‘groups’ of particles with similar properties, somewhat like the chemical elements in Mendeleev’s table; a fact today explained by the recurrent presence of one or another type of quark.

For instance in the following table:
Table 4: A family of hadrons
h        	eta	L         	lambda	W        	omega	
K     	k-meson or kaon	n 	neutron	X        	Xi	
p    	pi meson or pion	p         	proton	S        	sigma	

which used to describe apparently different particles, the p+ meson would consist of a (ud), the Ko  of an (sd), the W of three strange quarks (sss), and so on.

Ch. 5 - The Physics Quest
Past
The idea, or hope, that all observable substances could be made out of a few, non fractionable components is as old as the first Greek philosophers, who devised the famous air-earth-fire-water scheme, or the very idea of ‘atom’. But it has taken then 25 centuries for real research at the atomic scale to begin, as a separate branch of physics departed from chemistry or alchemy, astronomy, theology or even superstition. 

At the beginning of this century, it was well known that matter was made of elements and atoms, and many (most) natural phenomena were explained by the Newton theory of gravitation and Maxwell theory of electromagnetism. 
It was a remarkably good explanation: theories based on a few, elegant mathematical ideas, whose deductions and consequences (fundamental laws and forces) were consistent with the observed phenomena of the real world. A world was made of known elementary constituents obeying these laws.

There were some problems though: i) the two theories were completely disconnected (no relation between the two fundamental forces) - ii) space and time were regarded as simply ‘being there’, and so was matter, without any further explanation - iii) new phenomena were observed which could not be explained by the classical theories.

The discovery of  the electron and of radioactivity, and the inception of Einstein theory of relativity put some shadow on the ‘classical’ theories. By 1930, their place had been taken by new theoretical schemes: quantum mechanics, the general theory of relativity - the two still disconnected - and a more elaborate scheme of the atom.

Although many details remained obscure, physics seemed once more to be returning to a relatively simple set of principles. It was possible to depict deep matter as being composed of a small number of elementary particles (electrons, protons, neutrons), subject to the laws of relativity and quantum mechanics.
Then came the need to postulate neutrinos, (so elusive that they were found quite later); the discovery of the positron (i.e. anti-matter) in cosmic rays, the muon (a ‘useless’ heavy electron), and the clear manifestation of additional forces associated with the atomic nucleus. Further on, experimental work with accelerators unveiled a plethora of other particles, produced as subatomic fragments, and an intriguing tangle of forces.

Physics went again into a cycle of observations and explanations, because new theories were needed to explain the new observed phenomena. It took another fifty years for physicists to identify the deeper level of structure (standard model) that underlies the subatomic richness, and to try to unify the weak, electromagnetic and strong forces.

Present
There are currently questions of great relevance waiting for an answer:

•	The standard model, described in Chapter 4, does not explain the relative values of elementary particles masses (both leptons and quarks), or the lack of regularity in these values. In physics, arbitrary is synonymous of unexplained.

•	 Is the neutrinos’ mass different from zero.

•	About 9/10th of the universe mass, as calculated from astronomical observation, is invisible! Quite a few people would like to know where or ‘what’ it is.

•	Matter and anti-matter particles behave symmetrically, yet our universe seems to be made only of matter; at least there seems to be a large predominance of matter over antimatter, a fact that needs to be explained.

•	Within the standard model, the Higgs mechanism suggests that particles acquire their mass by interacting with a force field (Higgs field) present everywhere. The Higgs boson could have a mass as high as 1000GeV (1 TeV); it could be detected in LHC experiments starting in 2005, with some luck.

•	Some GUT (Grand Unified Theories) require the existence of magnetic monopoles, and/or the proton decay. Experiments are run worldwide on these subjects, yet with no result. 

Future
Most physic laws can be derived by assuming intuitive symmetry properties of space-time. Symmetries in physics divide into geometrical symmetries (rotations and reflections) and abstract symmetries such as gauge symmetries (conservation of electric charge).

Theoretical physicists are trying to derive more unified fundamental laws, if not a TOE, by more elaborate ‘super-symmetries’, with alternate success.

Some of these super-symmetries involve ‘string’ objects, called therefore ‘superstrings’ (open, or closed like a ring); the vibrational states of superstrings should generate, like violin notes,  all elementary particles, fermions  and bosons, and make all fundamental forces descend from a unique interaction mechanism.

Problem is that there is no simple mathematical or intuitive assumption at the base of present superstring theory; hence there are thousands of possible theories. Worse, these new objects would have dimensions of the order of 10-33cm; this apparently innocent figure is 10-20 times smaller than an atomic nucleus, i.e. far beyond any possibility of observation - the energies needed to ‘look’ at this scale being far beyond our experimental reach.
Moreover, space-time in superstring theories would have 10 dimensions, of which we only notice four because the other six are ‘curled-up’, to a size of 10-30cm, instead of linear.

Many theoretical physicists are busily working at superstrings theories, while others do not believe them at all .

____________

Another fact commonly taken for granted is not trivial; namely the fact that mathematics, an abstract creation of human mind, based only on some canons of elegance and economy, provides us with answers about the physical phenomena.

Not only are mathematical patterns reproduced in the structure of the world; mathematics is  so much at the root of our understanding of natural laws, that it provides the very possibility of science. Would there be a ‘relation’ between the structure of our mind (which creates mathematics) and the structure of the world?

And then the “big” question: do we know 1% or 90% of what there is to be known? Many scientists in the past would have chosen the second answer, and gone wrong.

Appendix: CERN accelerators

The original Proton Synchrotron PS, with 2km diameter, was built in 1959 at the ground level, to provide 28 GeV accelerated proton beams. It is still used for test/secondary beams and as an injector for other machines. It has become so flexible as to accelerate protons, electrons, positrons and even heavy ions in interlaced mode.

The Super Proton Synchrotron SPS, with 7km circumference, was built underground (at an average depth of 40m) in 1971-77 to provide 450 GeV proton beams (1013 protons/sec). It had about 1000 magnets, a cost of 1.150 MSFr, and was initially converted to a proton-antiproton storage ring, thanks to the high quality of beam optics and vacuum. Today it provides multiple experimental beams and is used as LEP injector.

LEIR Low Energy Ion Ring, has replaced LEAR Low Energy Antiproton Ring.

LEP, the Large Electron-Positron collider, was built in 1983-88.
LEP phase1, with 50 + 50GeV e+/e- beams, has produced 8 million Zo (m = 91.18 GeV).
LEP phase2 aims at a 100 + 100 GeV energy; it has reached in 1997 91.5 GeV/beam, and it is producing pairs of W+ W- bosons (compared to single W’s in the SPS).
The upgrade from LEP1 to LEP2 is carried on by replacing the original RF cavities with superconducting cavities, which allow a higher accelerating field.	
Recently it has been decided to add 48 more superconducting RF cavities (36 MSFr and 
+ 9 GeV center of mass energy) to go from 183 to 192 GeV.
LEP in numbers
Commissioning	1989
Circumference of the accelerator:	27 km (26.659m)
Place:	between Geneva airport and Jura foothills
Depth of the tunnel:	50 to 175 m
Number of dipole bending magnets:	3368
Bending field:	0.1 tesla (low field to reduce synchrotron radiation)
Number of focusing quadrupoles:	816
Number of focusing sextupoles:	504
Number of correcting magnets:	700
Inclination:	1.4%
Standard cells:	8 arcs of 31 cells, of 79m each
Alignment accuracy:	Tunnel 1cm; LEP parts 0.1 mm
Average vacuum:	10-9 torr
Beam lifetime:	12 hour with 1012 particles/beam
Speed of circulating beams:	c (speed of light) - (0.3 10-10) % 

Upgrade of RF cavities:	upgrade to LEP phase 2, see table

Date	Copper RF cavities	Supercond. cavities	Energy/beam GeV	
1990	128	0	45	
1995	120	60	70	
1996	120	172	88	
1997	86	240	94	
1998	52	272	96	

Beam cross-section:	10m x 250m at intersection points
Beam Position Monitors:	504
Radio Frequency:	16 x 1MW Klystrons; 352.21 MHz
Synchrotron Radiation:	100 W/m => 2KW/m in phase 2 (requiring water 
	cooling of vacuum pipe)
Controls:	160 computers, in 24 buildings, surface/underground 
Magnet Power Supplies:	750
Injectors:	200 MeV Linac, 600 MeV Linac, 600 MeV EPA, the PS at 3.5GeV for e+/e-, the SPS at 20GeV for e+/e-
Cooling:	10 water-cooling towers, 10MW/each (using the lake of Geneva water)
Access pits:	18 (1.4 Mm3)
Tunnel excavation:	3 tunneling machines (25 m/day) + 3.5 km blasting
Surface Buildings:	71 buildings, floor area 51,000m2

The LHC 

Program
The LEP tunnel was originally built to house a second accelerator (superposed to the first) if needed; today it seems simpler/cheaper to dismantle LEP, before installing the LHC components. This may put a premature end to the LEP program, which has just attained its full energy (a pity, IMHO).

In 1985 the ECFA committee stated that a 1 TeV accelerator, built in the LEP tunnel, would be the most viable option after LEP exploitation. In 1990 a preliminary study was started and presented in Dec 1991. In 1993 the study had been extended to the detectors and experiments: the total estimated cost was 2,660 MSFr, incompatible with CERN budget and a completion date of 2002 (missing about 500 MSFr).
The decision to build the LHC was taken in Dec. 1994. Commissioning is foreseen in 2005. If needed, full payment can be delayed till  2008.
 
Construction
The LHC will have about 1000 superconducting magnets, each 15m long, to be installed in the existing 27 Km LEP tunnel. These magnets have twin apertures, each with opposite magnetic field; two vacuum chambers allow counter-rotating proton beams to be housed in a single cryogenic magnet assembly. A 8.65 Tesla magnetic field is obtained by circulating 26000 A through very thin niobium-titanium conductors, kept at 1.8oK (for comparison, the temperature of outer space is 2.7oK)
The LHC test string, comprising one quadrupole and two 10m dipoles (magnets), cooled down to 1.8oK, was operated successfully in April 94, contributing  to LHC approval.

Beams
The LHC collision energy will be 7 + 7 TeV, which corresponds to the conditions of matter 10-12 sec after the big bang (a temperature of 1016 K, or a universe size of 0.3 mm).
The LHC can also collide Lead208 ions, 27 times charged, carrying a total energy of 1150 TeV, causing local transition from nuclear matter to quark-gluon plasma at collision points.
In proton-proton collisions, the events with interesting products correspond to head-on quark-quark collisions, which are very rare (only 1 out of 1012 interactions will be a hard quark collision). 
To make these rare events observable, the LHC needs to provide a very high collision rate -also called ‘luminosity’: the proton bunches of each beam (1.7*1011 protons/bunch) will be only 25 nsec apart (40 MHz collision rate), and each bunch-bunch crossing will produce about 20 interactions of any kind.
The experiments apparatus, placed 100-150m underground at four fixed (experimental) beam crossing points, will have the hard task of observing all these events, filtering out the "good" ones. 
The data acquisition and filtering rates, and the radiation level, put unprecedented requirements on the detectors and electronics. A Detector research and Development Committee is promoting since 1990 new technologies to fulfil these requirements.

____________
Fig 1 - The CERN complex of accelerators
The area shown is about 30 km wide by 60 km high 
(from http://www.cern.ch/CERN/Divisions/PS/ PSComplex/CERNAcc.html ) .

[see attached file CERNacc.gif]

Fig 2 - Two ‘event’ (particle collision) reconstructions. 
The upper one at LEP (electron-positron collider, the lower one at LHC, proton-proton collider 
(http://www.cern.ch/CERN/Microcosm/today.gif), (http://www.cern.ch/CERN/Microcosm/lhc.gif).

[see attached files today.gif and lhc.gif]