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 Z^{0}particle. The Z^{0} 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^{}> Z^{0}> hadrons, and a theoretical
fit to the data.
Figure 2. The Z^{0} 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 Z^{0} 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 Z^{0} line shape:
From ratio of hadronic to leptonic decays:
From bquark decays:
The fact that these results are consistent indicates that is unique, and
independent of, for instance, quarkflavour. Other LEPcollaborations 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
higgsparticles, both neutral and charged, have been negative. The same is true
for searches for supersymetric particles.
Ulf Mjörnmark , Oxana Smirnova
1995, 1997
