The DELPHI Detector

DELPHI stands for DEtector for Lepton, Photon, and Hadron Identification, which gives a pretty good idea of what the detector does. Lepton is the collective noun for electrons, muons, taus and neutrinos. It comes from a Greek word meaning 'small'. Photons are the particles that carry the electromagnetic interaction, and hadrons are all particles that feel the strong interaction. It's origin is also a Greek word, this time meaning 'thick'.

The DELPHI detector measured about 10 metres in diameter and was 10 metres long, it weighed about 3500 tons. DELPHI's design and construction took seven years, and the detector collected data for over a decade. DELPHI consisted of three parts, a cylindrical barrel and two so-called endcaps closing the ends of the cylinder, each of these was mounted on rails so that it could be moved in and out for maintenance.

The DELPHI detector surrounded the LEP accelerator's beam pipe at one of the places where electrons and positrons collided. Collisions took place in the centre of the barrel section, and whatever direction particles emerged, they had to pass through either the barrel or an endcap. Each of the three sections consisted of many different parts, called subdetectors, of which there were 19 in total.

The aim with the whole DELPHI set-up was to :

• get a precise three-dimensional view of the event by tracking the emerging particles,
• measure the energies of the particles using called calorimeters,
• measure the momenta of the charged particles using a magnetic field,
• and find out what kind of particles are produced by using information from several subdetectors.

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 direction of the curvature with respect to the magnetic field also indicates whether a particle is positively charged or negatively charged.

A schematic diagram of the DELPHI detector. The detector components named in blue are components that you will use in the projects.

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) also take part in the tracking.

The TPC is a cylindrical volume filled with gas in which the charged particles ionize the gas along their trajectories. An electric field caused the liberated electrons to drift towards one end of the volume where they are detected. The end wall is divided in such a way that a two-dimensional picture of the ionization, corresponding to the tracks of passing particles, can be reconstructed. 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 called electromagnetic and hadronic. The electromagnetic calorimeters measure the energies of particles interacting via the electromagnetic interaction: electrons, positrons and photons. The hadronic calorimeters measure the energies of particles interacting mainly via the strong interaction. These are particles made up of quarks, collectively known as hadrons.

Both types of calorimeters rely on the same basic principle: particles interact with a dense medium and give rise to a cascade of secondary particles. The dense medium is thick enough to stop even particles with the highest possible energies at LEP, with the exception of muons and neutrinos, which we will come back to later. In both calorimeters, 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 interaction is very different from the strong interaction 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 was 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 with no track pointing it, then we know the deposit must have been left by a neutral particle. Since the particle stops in the electromagnetic calorimeter and does not punch its way through to the hadronic calorimeter, we can infer that it must have been 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 detectors we know the particle was charged. Since it stops in the electromagnetic calorimeter we can this time infer that it's an electron or a positron. The direction 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 detectors. 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 pass quite easily 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 pass through 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 the total energy of the particles emerging from a collision is less than the combined energy of the colliding electron and positron, then some particles must have escaped undetected. It's a fair bet that they were neutrinos.

The imprints of different particle types in the different layers of a particle detector.

• Short-lived particles: Short-lived particles are special cases. If you 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 identified because they generally consist of a few particles emerging back-to-back.

• Very short-lived particles: Some particles do not even live long enough to reach the detector closest to the collision point. However, when they decay, the particle they decay into will appear to originate from a point which is some distance away from the collision point. With the very accurate tracking of the Vertex Detector the distance between the collision point and the decay point of the very short-lived particle can be measured.

The DELPHI Vertex Detector allows the decay point of very short-lived particles to be pinpointed, giving an indication of the particle's lifetime.

Particle Physics Education CD-ROM ©2001 CERN