In addition to the information given by ITS and TPC, more specialized detectors are needed: the TOF measures, with a precision better than a tenth of a billionth of a second, the time that each particle takes to travel from the vertex to reach it, so that one can measure its speed. The high momentum particle identification detector (HMPID) measures the faint light patterns generated by fast particles and the TRD measures the special radiation very fast particles emit when crossing different materials, thus allowing to identify electrons. Muons are measured by exploiting the fact that they penetrate matter more easily than most other particles: in the forward region a very thick and complex absorber stops all other particles and muons are measured by a dedicated set of detectors: the muon spectrometer.