Type of interactions
In the DELPHI detector high energy electrons and positrons (the
antiparticle of the electron) collide. During the first phase of LEP,
the energy of the colliding particles were set to 46.5 GeV each in
order to produce the Z0 particle which has a mass of 91
GeV. During the second phase of LEP, which started in 1996, the energy
of the colliding particles are set such that a pair of W particles can
be produced. The mass of W is around 80 GeV, so the energy of each
colliding particle has to be at least 80 GeV to be able to produce two
W particles.
The single Z0 event is easier to start with. The events
with two W or Z0 particles are more complicated because of
the different way the two particles can decay and the many possible
combinations. The Z and W particles are produced almost at rest as
almost all the collision energy goes into producing the particles. The
collision energy is almost entirely transformed into mass. When a
particle at rest decays, the decay products go back to back.
In each event many different type of particles can be produced. Some
of the most important particles to identify are:
- The electron - a charged particle that deposits its energy in the
electromagnetic calorimeter
- The muon - a charged particle going through all the detectors
- The quark and the gluon - give rise to sprays with 10 to 30
particles.
Production and decay of
Z0
The Z0 can decay to almost any pair of quarks or leptons.
The exception is the decay into a pair of top quarks as the top quark
is too heavy.
Decay into leptons
- decay to electrons; two high energy tracks back to back stopping
in one of the electromagnetic calorimeters (HPC or FEMC)
- decay to muons; two high energy tracks back to back going through
the complete DELPHI detector
- decay to taus; the tau is very short-lived - its decay products,
one or three charged particles will be observed
- decay to neutrinos is invisible
Decay into quarks
- free quarks can not escape from the collision point due to the
strong force between the quarks. The separating, outgoing quarks will
give rise to a spray, a jet of particles. Decay to quarks will most
of the time give rise to two jets of particles with many particles in
each jet.
Short-lived particles - beauty and
charm particles
The particles that contain the heavy b or c quark are very
short-lived, typically 10-13 to 10-12 s. They
will therefore decay very close to the collision point. With the high
resolution detector (VD) it is possible to observe that some of the
charged particles do not come from the collision point but from a
point a small distance away. This is the "signature" for the decay of
a very short-lived particle like a beauty or a charm particle.
Strange particles
Many particles containing an s quark have lifetimes of the order of
10-10 s. They can decay in many of the DELPHI detectors. It
is relatively easy to observe these decays when they occur in the
large volume drift chamber TPC.