The identification of electrons and positrons is achieved by 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 ρ and in a different rapidity region. Inside this detector, fast charged particles cross the boundaries between materials with different dielectric constants, which leads to the emission of TR photons with energies in the X-ray range. The effect is tiny and the radiator has to provide hundreds of material boundaries to achieve a high enough probability to produce at least one photon.
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 and jets.