L3: the physics

L3 : the Physics

The main objective of L3 is to understand some of the fundamental laws of particle physics. Particles are the bricks all the Universe is made of. It's therefore important to know what kind of bricks are there, what kind of forces keep them together and, finally, why is it like that? The way of getting some insight on these problems is to take some particles we know, give them enough energy and make them collide to produce new particles and study the process.

The starting point at CERN are electrons and positrons. Electrons are, together with protons and neutrons, the components of atoms, which are the constituents of the matter we are all made of. There are several differences between electrons and protons:

Electrons are negatively charged while protons are positive
Electrons are about 2000 times lighter than protons
Electrons are simple while protons are composed of even smaller particles: the quarks.

But nature is not that simple: for every kind of particle there must be an anti-particle, that's what anti-matter is made of. Therefore we have the anti-electron, also called the positron, which has the same mass (call it weight if you like...) and very similar features as the electron but an opposite charge.

If you think that's enough, than you're wrong...
Physicists have discovered many other particles in the last century: here's a table of the fundamental ones, which are believed not to be made of anything smaller.

QUARKS	up	charm	top
QUARKS	down	strange	bottom
LEPTONS	electron	muon	tau
LEPTONS	e-neutrino	muon-neutrino	tau-neutrino

As you can see, there are two big families: leptons and quarks. The three leptons on the first row (electrons, muons and taus) are similar because they have the same charge (-1) and same type of interactions but they have different masses. The other kind of leptons, namely neutrinos, have no charge (guess what...they are neutral and small: "neutrino" in Italian!) and therefore quite hard to catch in detectors like L3. Then we have quarks: they cannot be seen isolated because they always form composite particles such as protons, neutrons, pions and many others. Particles made up of quarks form the big family of hadrons.

Particles interact, which means that they attract or repulse each other. One of the great discoveries of modern physics is that also interactions (forces) can be described by means of particles. Here they are:

FORCE	PARTICLES
electro-magnetism	photon
weak force	Z
weak force	W⁺, W^-
strong force	gluons
gravitational force	graviton (not seen)

Let us now come to what happens in an experiment at LEP: electrons and positrons are accelerated in the ring of LEP in opposite directions and then they are forced to collide right in the middle of one of the four detectors, in our case L3. Their collision usually leads to annihilation; the two original particles disappear but the energy they were carrying must be conserved: it can therefore be used to produce another pair of particle and anti-particle, provided their mass it's not too big. As we all know from the famous Einstein formula E = mc², energy (E) and mass (m) can be converted into each other. This is why particle physics is also called High Energy Physics: to discover heavier particles one needs to reach higher energies in accelerators.
Therefore after the collision we can have again an electron-positron pair or a muon-anti-muon, tau-anti-tau, neutrino-anti-neutrino, quark-anti-quark...

The new particles produced are often unstable. They can decay in a very short time into other lighter particles and sometimes the decay is so fast that all we see in the detector is the product of the decay.

Collecting and analysing the product of these collisions is the objective of L3: this has been done since 1989 and it has helped physicists to make the picture of particle physics that we've drawn above much clearer than it used to be.

For comments and suggestions: L3 Webmaster
(last update 6 March, 1998)