DELPHI in depth
The DELPHI experiment
DELPHI is one of the four experiments around the LEP accelerator and it is the one
that has registered the collisions on this CD-ROM.
In 1999, the DELPHI collaboration consisted of about 550 physicists from 56 universities
and institutes in 22 countries.
The DELPHI detector is an instrument about 10 metres in diameter and 10 metres long
totalling about 3500 tons. Design and construction of the experiment took seven years.
It is divided into three mobile sections, a barrel and
two end-caps closing in on either side of the barrel to be able to detect particles
going in any direction. The detector surrounds the LEP accelerator beam pipe at one
of the places where electrons and positrons collide. The collisions take place in
the centre of the barrel section. The three sections consist of many different
subdetectors, 19 in total, based on various technologies.
The aim with the whole set up is to :
Particle identification is a job for all the subdetectors working together
since each gives different hints about the identity of the particles that pass through it.
The momenta of the charged particles are calculated by measuring the curvature of the
paths the particles move along in the magnetic field, as seen by the tracking subdetectors.
The orientation of the curvature with respect to the magnetic field also indicates
whether a particle is positively charged or negatively charged.
- get a precise three-dimensional view of the reaction through tracking,
- measure the energies of the particles using devices called calorimeters,
- measure the momenta of the charged particles using a magnetic field,
- and find out what kind of particles are produced by using particle identification
You will not need to use all the different subdetectors of DELPHI to
perform the analyses on this CD-ROM, so we'll restrict ourselves to
just the ones that you will need.
Tracking in DELPHI is mainly performed by the exotically-named Time Projection
Chamber (TPC), assisted by a very precise tracking detector close to the collision
point called the Vertex Detector (VD), the Inner Detector (ID), chambers in the end-caps
called Forward Chamber A and B (FCA, FCB), and the Muon system (MUB, MUF, MUS). The TPC is a
cylindrical volume filled with gas in which the charged particles ionize the gas along their
trajectories. An electric field drifts the band of ionization towards one side of the
volume where on the wall the ionization is detected. The wall is divided in a way that
a two-dimensional picture of the ionization bands, corresponding to the tracks of
passing particles, is produced. The accuracy on the two coordinates thus obtained is
around a quarter of a millimetre. The third coordinate is extracted by measuring the
arrival time of the ionization and projecting back to work out where the particle must
have passed, hence the name Time Projection Chamber. Since the drift speed of the
ionization is known accurately, this gives the third coordinate to a precision of
just under a millimetre. In this way a three-dimensional picture of the event can be
reconstructed in the TPC.
Calorimetry, energy measurement, in DELPHI is performed by two types of
calorimeters: electromagnetic and hadronic. The electromagnetic calorimeters measure the
energies of particles interacting via the electromagnetic force: electrons, positrons
and photons. The hadronic calorimeters measure the energies of particles interacting
mainly via the strong force, that is particles containing quarks, collectively
known as hadrons. Both types of calorimeters are devised in such a way that these
particles interact with a dense medium and give rise to a cascade of secondary
particles. The dense medium is thick enough to stop the particles with the highest
possible energies at LEP, with the exception of muons and neutrinos, which we will come
back to later. The dense medium is interleaved with an active medium where the shower
of particles deposits a fraction of its energy. By measuring the total energy deposited
in the active medium the energy of the initial particle can be calculated. Since the
electromagnetic force is very different from the strong force the best material for
the dense medium is quite different for the two different kinds of calorimeter. Lead
makes a good dense medium for building an electromagnetic calorimeter. It can
absorb electromagnetic particles in a relatively compact volume whilst strongly
interacting particles punch their way through to the hadronic calorimeter. In DELPHI,
iron is chosen for the dense medium of the hadronic calorimeter.
DELPHI's electromagnetic calorimeters are called the High-density Projection Chamber,
HPC, and the Forward ElectroMagnetic Calorimeter, FEMC. The HAdron Calorimeter is
simply known as the HAC.
Particle identification is performed by combining information from several
subdetectors. To identify, for instance:
- A photon: If a big energy deposit is seen in the electromagnetic calorimeter but no track
pointing to the deposit is seen in the tracking system we know it's a neutral particle.
Since it stops in the electromagnetic calorimeter we can infer it's a photon.
- An electron or a positron: If a large energy deposit is seen in the electromagnetic
calorimeter and there is a track pointing to it in the tracking system we know it's a
charged particle. Since it stops in the electromagnetic calorimeter we can this time
infer that it's an electron or a positron. The orientation of the curvature of the
track tells us which.
- A hadron (particle containing quarks): If there is a big energy deposit in
the hadron calorimeter we know it's a hadron and that the collision involved the
production of quarks. As before, the question of charge can be answered by looking
at the tracking system. Hadrons sometimes leave energy behind in the electromagnetic
calorimeter, so both calorimeters will register energy deposits.
- A muon: Muons interact weakly with matter. They can quite easily pass through
the dense materials of the calorimeters. If a track is seen in the tracking system
and the muon detectors signal that a particle has passed, then it has to be a muon.
Muons do not lose much energy in the calorimeters, so if there are energy deposits
in the calorimeters along their paths, they will be small.
- A neutrino: Neutrinos interact extremely weakly. They can penetrate the whole
earth without ever interacting which makes them very difficult to detect.
However, DELPHI is not entirely blind to them when they are produced with other,
detectable, particles. To find out whether a neutrino has passed through DELPHI we use
the law of conservation of energy. If, when summing the total energy in an event, a
fraction is missing with respect to the energy that was initially put in, it's a sign
that some particles have escaped undetected. It's a fair bet that they were neutrinos.
- Short-lived particles: Short-lived particles are special cases. If
you have done, or plan to do, the "Z branching ratios" project, you will
encounter tau particles. These are short-lived particles that quickly decay
into other particles. Tau-antitau events can be easily identified because
they generally consist of two back-to-back sprays of particles. They look
a bit like quark-antiquark two-jet events but the number of particles in the
jets is much smaller for taus than it is for quark jets.
- Very short-lived particles: Some particles do not even live long enough to
reach the detector closest to the interaction point. However, when they decay, the
particle they decay into will appear to originate from a point which is some distance
away from the interaction point. With the very accurate tracking of the Vertex
Detector the distance between the interaction point and the decay of the very
short-lived particle can be measured, as in this event, giving an indication of
its lifetime. You will not need to identify such short lived particles to do any
of the analyses on this CD-ROM.
| Particle Physics Education CD-ROM ©1999 CERN