Part C: Cherenkov Ring Imaging and Calorimetry
In the first part of this article we had introduced the two concepts of identifying particles: Difference in interaction and mass determination. In this final part of the article we finish by describing two more techniques that are employed in ALICE in order to identify particles. The RICH technique is based on mass determination, while calorimetry reveals the different ways different particles interact.
Measuring an angle
Cherenkov radiation is a shock wave resulting from charged particles moving through a material faster than the velocity of light in that material. The radiation propagates with a characteristic angle with respect to the particle track, which depends on the particle velocity. Cherenkov detectors make use of this effect and in general consist of two main elements: a radiator in which Cherenkov radiation is produced and a photon detector. Ring imaging Cherenkov (RICH) detectors resolve the ring-shaped image of the focused Cherenkov radiation, enabling a measurement of the Cherenkov angle and thus the particle velocity. This in turn is sufficient to determine the mass of the charged particle.
Figure 7: The left image shows a schematic layout of an ALICE HMPID module, showing the radiator where Cherenkov photons are produced, the expansion gap, the photon detector (MWPCs with CsI deposited on the cathode plane) and the frontend electronics. The right image shows an example ring as seen by the HMPID. The ring has a radius of the order of 10 cm, but its shape is distorted due to the impact angle of the particle.
If a dense medium (large refractive index) is used, only a thin radiator layer of the order of a few centimetres is required to emit a sufficient number of Cherenkov photons. The photon detector is then located at some distance (usually about 10 cm) behind the radiator, allowing the cone of light to expand and form the characteristic ring-shaped image. Such a proximity-focusing RICH is installed in the ALICE experiment. The High-Momentum Particle IDentificaton (HMPID) detector (See Fig. 7) is a single-arm array that has a reduced geometrical acceptance. Similar to the ALICE TOF it is able to identify individual charged hadrons up to momenta of a few GeV/c, but with slightly higher precision. Fig. 8 shows the dependence of the Cherenkov angle as measured by the ALICE HMPID on the particle momentum, indicating the different characteristic bands for three different hadrons. A particle is identified when the corresponding point in the diagram can be associated with only one such band within the measurement errors. The method works very well for momenta up to a few GeV/c.
Figure 8: Dependence of the Cherenkov angle measured by the ALICE HMPID on the particle momentum. The lines are the theoretical curves. No data is available for momenta below about 600 MeV/c, because only particles with higher momenta reach the detector.
Calorimeters detect neutral particles, measure the energy of particles, and determine whether they have electromagnetic or hadronic interactions. PID in a calorimeter is a destructive measurement. All particles except muons and neutrinos deposit all their energy in the calorimeter system by production of electromagnetic or hadronic showers. Photons, electrons and positrons deposit all their energy in an electromagnetic calorimeter. Their showers are indistinguishable, but a photon can be identified by the non-existence of a track in the tracking system that is associated to the shower.
A high-resolution electromagnetic calorimeter, the PHOS, is installed in ALICE to provide data to test the thermal and dynamical properties of the initial phase of the collision. This is done by measuring photons emerging directly from the collision. PHOS is a homogeneous EM calorimeter, which covers a limited acceptance domain at central rapidity. It is based on lead tungstate crystals, similar to the ones used by CMS, read out using Avalanche Photodiodes (APD). The crystals are kept at a temperature of 248 K, which helps to minimize the deterioration of the energy resolution due to noise and to optimize the response for low energies.
Completing the picture
Finally, a pre-shower detector, the PMD, studies the multiplicity and spatial distribution of such photons in the forward region. It utilizes as a first layer a veto detector to reject charged particles. Photons on the other hand pass through a converter, initiating an electromagnetic shower in a second detector layer where they produce large signals on several cells of its sensitive volume. Hadrons on the other hand normally affect only one cell and produce a signal representing minimum-ionizing particles.
Each of the methods described in this article provides a different piece of information. However, only by combining them in the analysis of the data produced by ALICE, can the particles produced in the collisions be measured in the most complete way possible, so that they can reveal the whole picture of what happens in the collisions.<
For more about PID in general see: C Lippmann 2012 Nucl. Instr. Meth. A 666 148.
The ALICE experiment is described in detail in: ALICE collaboration 2008 JINST 3 S08002.