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.

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:

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Particle Physics Education CD-ROM 1999 CERN