Part B: Measuring the Time-Of-Flight (TOF) and Transition Radiation (TRD)
In the last part of this article we introduced the two concepts of identifying particles: Difference in interaction and mass determination. Muon identification in the ALICE muon arm served as an example for the first, while ionization measurements in TPC and ITS were described as examples for the latter. In this part of the article we continue with two more techniques that are employed in ALICE in order to identify particles, where both of them are based on mass determination.
Using a stop watch
Time-of-flight (TOF) measurements yield the velocity of a charged particle by measuring the flight time over a given distance along the track trajectory. Provided that the momentum is also known, the mass of the particle can then be derived from these measurements. The method is utilized in ALICE by using the data from the TOF detector, which is a large-area detector based on multi-gap resistive plate chambers (MRPCs). It covers a cylindrical surface of 141 m2 with an inner radius of 3.7 m. In principle the MRPCs are simple parallel-plate detectors. It is interesting to see that they are built of thin sheets of standard window glass separated using fishing lines to provide the desired spacing. This creates narrow gas gaps with high electric fields; ten such gaps per MRPC are needed to arrive at a detection efficiency close to 100 %.
The simplicity of the construction has allowed a large system to be built with an overall TOF resolution of 80 ps, but at a relatively low cost. This performance allows the separation of kaons, pions and protons up to momenta of a few GeV/c. As Fig. 4 shows also electrons and deuterons can be detected. Combining such a measurement with the PID information from the ALICE TPC has proved to be very useful in improving the separation between the different particle types, as Fig. 5 shows for a particular momentum range.
Figure 4: Particle velocity beta = v/c as measured with the ALICE TOF detector as a function of the particle momentum p for a data sample taken with heavy-ion collisions provided by the LHC in the year 2011. The bands for electrons, pions, kaons, protons and deuterons are clearly visible. Particles outside those bands are tracks wrongly associated with a TOF signal. No data is available for momenta smaller than about 300 MeV/c, since these particles do not reach the detector due to the curvature of their tracks in the magnetic field.
Figure 5: In ALICE different PID measurements can be combined to achieve better separation of particle species. Here, for a certain momentum window, the Time-Of-Flight information from the TOF detector is combined with ionization measurements (dE/dx) in the TPC.
Detecting additional photons
The identification of electrons and positrons in ALICE is achieved using a Transition Radiation Detector (TRD). In a similar manner to the muon spectrometer, this system enables detailed studies of the production of vector-meson resonances, but with extended coverage down to the light vector-meson rho and in a different rapidity region. Below 1 GeV/c electrons can be identified via a combination of PID measurements in the TPC and TOF. In the momentum range from 1 to 10 GeV/c the fact that electrons may create Transition Radiation (TR) when passing a dedicated “radiator” can be exploited. Inside such a radiator, fast charged particles cross the boundaries between materials with different dielectric constants, which can lead to the emission of TR photons with energies in the X-ray range. The effect is tiny, and the radiator has to provide many hundred material boundaries in order to achieve a high enough probability to produce at least one photon. In the ALICE TRD, the TR photons are detected just behind the radiator in MWPCs filled with a xenon-based gas mixture, where they deposit their energy on top of the ionization signals from the particle’s track (see Fig. 6).
Figure 6: Schematic illustration of the TRD principle showing a projection in the plane perpendicular to the anode and cathode wires of the drift chamber. The radiator is not to scale. Electrons produced by ionization energy loss (dE/dx) and by TR absorption drift along the field lines towards the anode wires. TR photons are absorbed preferentially at the beginning of the drift volume, which leads to signals with large amplitudes at late times (towards the end of the drift time).
The ALICE TRD was designed to derive a fast trigger for charged particles with high momentum and can significantly enhance the recorded yields of vector mesons. For this purpose 1/4 million CPUs are installed right on the detector in order to identify candidates for high-momentum tracks and analyse the energy deposition associated with them as quickly as possible (while the signals are still being created in the detector). This information is sent to a global tracking unit, which combines all of the information to search for electron-positron track pairs within only 6 us.
The third part of the article will be published in the next issue of ALICE MATTERS, focusing on the remaining particle identification techniques that are employed in ALICE.