The aerogel Čerenkov detector (ACD) serves to reliably discriminate pions against protons, and particularly improves the K±- identification substantially.
Results of Monte-Carlo simulations are shown in fig. 1 and 2. They are carried out with realistic relative proton, charged pion and kaon yields within the spectrometer acceptance. For momenta p < 1300MeV/c the number of events is plotted as a function of flight-time deviation from kaons, Δt = tK(p) - tmeasured(p). Hence Δt = 0 corresponds to kaons. For lighter particles (pions) which are faster than kaons a positive deviation is obtained, heavier ones (protons) are slower and produce a negative deviation. The upper part of fig. 2 is for all events using the momentum reconstruction of the spectrometer and the time-of-flight from the TOF detector. The lower part is obtained if a veto signal from the ACD is required.
The ACD consists of 5cm thick aerogel tiles arranged in a wall of 46 x 46cm2 front surface within a light collecting box. The inner box surface has a highly diffuse-reflecting paint. The Čerenkov light is detected through 12 photomultiplier. For high efficiency it is crucial that small numbers of Čerenkov photons can be detected. The capability of the photomultiplier to detect single photons was confirmed in tests using white LEDs .
Measurements of the efficiency, εACD, were carried out using electrons behind the (horizontal deflecting) beamline E tagger and with a 1.1GeV/c pion beam at the HADES cave of the GSI. Both methods yield in very good agreement εACD = 99.5%. The result is illustrated in Figure fig. 3 for the pion measurement . In combination with the tagger a good timing resolution of τFWHM ≈ 6ns is achieved (see fig. 4).
 S. Friedrich, diplome thesis, Giessen (2008)