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Introduction to the DELPHI experiment

Selected experimental results

Figure 1. The Higgs candidate event in DELPHI. The dots are registered signals, and the lines are reconstructed tracks.

During the first three and a half years of running at LEP the main aim has been to study the Z0-particle. The Z0 shows up as a huge resonance in e+e- interactions. LEP has been varying its energy over the resonance in order to determine its parameters. The results of this energy scan is summarized in fig.2 which shows the cross section results of the process e+e--> Z0-> hadrons, and a theoretical fit to the data.

Figure 2. The Z0 peak in e+e- interactions as measured by DELPHI. The green line shows a theoretical prediction fron the Standard Model with three light neutrinos. The red and blue lines show the same with two and four neutrinos respectively.

The mass and total width of the Z0 obtained from this fit is:

From leptonic events one has got the following leptonic partial widths, which confirms the lepton universality hypothesis:

The invisible partial width is a very important measurement. It is determined to be:

This width can be translated into a measure of the number of existing light neutral fermions (i.e. neutrinos), and the number comes out to be:

The fact that this number is consistent with three, leaves no room for any more families of fermions than the three we already know of.

The results above are based on results from all four LEP collaborations.

DELPHI has performed several independent measurements of the strong coupling. The results are:

From analysis of event shape distributions pf hadronic final states:

From scaling violations in fragmentation functions:

From Z0 line shape:

From ratio of hadronic to leptonic -decays:

From b-quark decays:

The fact that these results are consistent indicates that is unique, and independent of, for instance, quark-flavour. Other LEP-collaborations give similar results.

The top quark is too heavy to be seen directly at the energy available at LEP. The top quark was discovered at Fermilab, the mass is determined to be approximately 175 GeV. The searches for the remaining important ingredient of the Standard Model, i.e. the higgs-particles, both neutral and charged, have been negative. The same is true for searches for supersymetric particles.

Ulf Mjörnmark , Oxana Smirnova
1995, 1997

e+e- annihilation as seen by the
  DELPHI detector

e+e- annihilation as seen by the
  DELPHI detector

e+e- annihilation as seen by the
  DELPHI detector

©1997 Particle Physics, Lund University