L3 started taking measurements in 1989 and since then it has given many valuable contributions to particle physics, together with the other three experiments at LEP.

The most important breakthrough was the measurement of the so called lineshape of the Z0 resonance. The Z0 is one of the particles which carry the weak force and measuring its mass and the various ways it can decay in lighter particles it's possible to determine how many families of leptons exist in nature. This number has been fixed to be three.

The picture shows the Z⁰ lineshape with the experimental coloured symbols and the three lines corresponding to 2, 3 and 4 families of leptons: the central value is evidently the one in agreement with experiment.

Other important results are those related to universality: the particles in the three families above behave all in the same way under the action of the weak force. What makes the difference between electron, muon and tau is therefore only their different masses: the muon is about 200 times heavier than the electron while the tau has a mass which is 3554 times the electron mass. But now, think about it the other way round: why should these particle which have the same physical behaviour present such a different range of masses? Similar patterns are present also in the quark family. Explaining the mystery of mass and of its generation is maybe the most important goal that particle physics is trying to achieve today. At L3 some contributions have been given also in this field. Some limits have been put to the value of the mass of the Higgs boson : this is the particle that according to the present theory should explain the problem of mass generation. And this is the particle almost everybody would like to see first...

The rest of the work being done at L3 is also very important. Many physical quantities have been measured with an unprecedented precision: this makes the standard model of high energy physics one of the great achievements of twentieth century physics.

This work is still going on. Since the end of 1995 LEP has entered its second phase, called LEP2, in which the energy of the beam of electrons and positrons has been raised to explore new regions and study new particles. Accurate measurements have been made of the properties of the W particles, close relatives of the Z⁰, starting from their mass and their decay channels.
Another interesting topic is the search for new particles. Theorists predict the existence of new families of particles, such as those required by supersymmetry. But no theory is good without an experimental confirmation and none of these supersymmetric particles has yet been found. L3 participates to this search and it has set some limits to the possible range of masses that still needs to be explored.

Click HERE for a complete list of L3 published papers.

For comments and suggestions: L3 Webmaster
(last update 8 May, 1998)