Part 1: Introduction, muon ID and ionization measurements (TPC and ITS)
Under extreme conditions of temperature and/or density, hadronic matter “melts” into a plasma of free quarks and gluons, the so-called QGP. To create these conditions in the laboratory, heavy ions (e.g. Pb nuclei) can be accelerated and made to collide head on, as was done at the LHC for two dedicated periods in 2010 and 2011. A key design consideration of ALICE is the ability to study QCD and quark (de)confinement under these extreme conditions. This is done by using particles, created inside the hot volume as it expands and cools down, that live long enough to reach the sensitive detector layers situated around the interaction region. ALICE’s physics programme relies on being able to identify all of them, i.e. to determine if they are electrons, photons, pions, etc. and to determine their charge. This involves making the most of the (sometimes slightly) different ways that particles interact with matter. This article gives an overview of the methods used for particle identification (PID) and their implementations in ALICE and describes how new technologies were used to push the state of the art.
In a “traditional” particle physics experiment particles are identified (electrons and
muons, their antiparticles and photons), or at least assigned to families (charged or neutral hadrons), by the characteristic signatures they leave in the detector. The experiment is divided into a few main components, as shown in Fig. 1, where each component tests for a specific set of particle properties. These components are stacked in layers and the particles go through the layers sequentially from the collision point outwards: first a tracking system, then an electromagnetic (EM) and a hadronic calorimeter and a muon system. All layers are embedded in a magnetic field in order to bend the tracks of charged particles for momentum and charge sign determination.
This method for PID works well only for certain particles, and is used for example by the large LHC experiments ATLAS and CMS. However, for example distinguishing the different charged hadrons using such a technique is not possible.
Figure 1: Components of a “traditional” particle physics experiment. Each particle type has its own signature in the detector. For example, if a particle is detected only in the electromagnetic calorimeter, it is fairly certain that it is a photon.
Muons may be identified using the just described technique by using the fact that they are the only charged particles able to pass almost undisturbed through any material. This behaviour is connected to the fact that muons with momenta below a few hundred GeV/c do not suffer from radiative energy losses and so do not produce electromagnetic showers. Also, because they are leptons, they are not subject to strong interactions with the nuclei of the material they traverse. This behaviour is exploited in muon spectrometers in high-energy physics experiments by installing muon detectors behind the calorimeter systems or behind thick absorber materials. All charged particles other than muons are completely stopped, producing electromagnetic (and hadronic) showers.
The muon spectrometer in the forward region of ALICE features a very thick and complex front absorber and an additional muon filter comprised of an iron wall 1.2 m thick. Muon candidates selected from tracks penetrating these absorbers are measured precisely in a dedicated set of tracking detectors. Pairs of muons are used to collect the spectrum of heavy-quark vector-meson resonances (J/Psi, ...). Their production rates can be analysed as a function of transverse momentum and collision centrality in order to investigate dissociation due to colour screening.
Hadron identification can be crucial for heavy-ion physics. Examples are open charm and open beauty, which allow the investigation of the mechanisms for the production, propagation and hadronization of heavy quarks in the hot and dense medium formed in the heavy-ion collisions. The most promising channel is the process D0 --> K- π+, which requires very efficient hadron identification owing to the small signal-to-background ratio.
Charged hadrons (in fact, all stable charged particles) are unambiguously identified if their mass and charge are determined. The mass can be deduced from measurements of the momentum and of the velocity. Momentum and the sign of the charge are obtained by measuring the curvature of the particle’s track in a magnetic field. To obtain the particle velocity there exist four methods based on measurements of time-of-flight and ionization, and on detection of transition radiation and Cherenkov radiation. Each of these methods works well in different momentum ranges or for specific types of particle. In ALICE all of these methods may be combined in order to measure, for instance, particle spectra. Fig. 2 shows as an example the abundance of pions in lead-lead collisions as a function of transverse momentum and collision centrality.
Figure 2: Pion yield from lead-lead collisions at centre-of-mass energy √sNN = 2.76TeV for different collision centralities (0-5% corresponds to the most central) measured in ALICE. To cover the whole momentum range, data from different ALICE sub-detectors are combined - in this plot the PID information comes from ionization (ITS and TPC) and time-of flight (TOF) measurements.
Kicking electrons from atoms
The characteristics of the ionization process caused by fast charged particles passing through a medium can be used for particle identification. The velocity dependence of the ionization strength is connected to the well-known Bethe-Bloch formula, which describes the average energy loss of charged particles through inelastic Coulomb collisions with the atomic electrons of the medium. Multiwire proportional counters or solid-state counters are often used as detection medium, because they provide signals with pulse heights proportional to the ionization strength. Since energy-loss fluctuations can be considerable, in general many pulse-height measurements are performed along the particle track in order to optimize the resolution of the ionization measurement.
In ALICE this technique is used for particle identification in the large time projection chamber (TPC), and also in four layers of the silicon inner tracking system (ITS). A TPC is a large volume filled with a gas as detection medium. Almost all of this volume is sensitive to the traversing charged particles, but it features a minimum material budget. The straightforward pattern recognition (continuous tracks) make TPCs the perfect choice for high-multiplicity environments, such as in heavy-ion collisions, where thousands of particles have to be tracked simultaneously. Inside the ALICE TPC, the ionization strength of all tracks is sampled up to 159 times, resulting in a resolution of the ionization measurement as good as 5%. Fig. 3 shows the TPC ionization signal as a function of the particle rigidity for negative particles, indicating the different characteristic bands for various particle types. 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, in particular for low momenta up to several hundred MeV/c.
Figure 3: Measured ionization signals in the ALICE TPC as a function of the particle momentum for Pb-Pb collisions at √sNN = 2.76TeV. An offline trigger was applied in order to enhance track samples with charges z < ?1. The charged (anti-)hadrons are well separated, in particular at low momentum. Electrons, anti-deuterons, anti-tritons and anti-3He nuclei are visible as well. The dashed lines are parameterizations of the Bethe-Bloch curve.
The second part of the article will be published in the next issue of ALICE MATTERS - Focusing on: particle identification using the Time-Of-Flight measurements and the detection of Transition Radiation.