DELPHI: predictions come true
DELPHI is one of the four large collision experiments at the LEP accelerator. The DELPHI detector is located in an underground cave big enough to fit a cathedral. The detector weighs over 3500 tons and has the form of a cylinder with a length and a diameter of 10 meters.
In essence DELPHI is made up of different detectors, placed around the target like onion skins. Direction, impulse and energy of particles can be determined with great accuracy by precisely measuring the particle tracks in the detectors. The inner detector is only 5 centimetres away from the point of collision and registers the tracks of the shortest living particles with a precision of 10 microns. DELPHI is also fitted out with a special detector to identify the rare and interesting kaon particles.
The energy of the particle beams in LEP is around 45 GeV; this energy level was selected to produce many Z0 particles, which are the carriers of the weak force. This is the force responsible for radio active decay and it causes the sun to burn. The weak force is described in the Standard Model, together with the electromagnetic- and the strong force. Besides the carrier particles of these forces, other fundamental particles are described as well: quarks and leptons. Quarks are the building blocks of particles (such as protons) which are governed by the strong force, and can be arranged in three pairs. Leptons (such as electrons) also form three pairs. They are not governed by the strong force.
A lepton- and quark pair form a particle family, of which there are three known. In principle a fourth (or fifth) -not yet observed- particle family could exist. The determination of the number of particle families is an important and real success of the LEP experiments up to now.
When the energy of the particle beams at LEP is varied in the range around 45 GeV, a so called resonance curve develops. In this curve (figure 1) the number of produced Z0 particles is plotted against the sum of particle beam energies. Just like an object will resonate at a characteristic sound frequency, most Z0 particles are produced at the energy comparable with their mass. So, from the place in the curve the Z0 mass can be directly determined.
Remarkably, in this way measured Z0 mass was seen to fluctuate. When a pattern was detected in these small but inexplicable fluctuations, it became clear the moon was responsible. The diameter of LEP changed under the influence of the moon's gravity by about one millimetre. The beam energy changes by about 10 MeV from this effect (figure 2). Only after correction for this 'tidal effect' the Z0 mass could be determined to a five thousandth of a percent accurately.
The determination of the number of particle families is dependant upon the width of the resonance curve. The curve gets wider as the live span of the Z0 particles decreases. And the live span decreases when there are more ways for the Z0 particle to decay; just as a bucket will empty faster when there are more holes in it. Since the Z0 particle decays in -amongst others- a neutrino and an anti-neutrino, the live span of the Z0 is an indication for the number of neutrinos - and so for the number of particle families.
After eight million Z0 events were gathered in the LEP experiments, this number has been set to three, accurate to within one percent. Naturally, only an integer number of neutrinos can exist. Yet, some speculative theories predict exotic particles which do not conform to the laws of the Standard Model. Would the Z0 decay to such exotic particles, the width of the resonance curve would be a little more than 3 neutrinos. The LEP results exclude this.
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