The anticoincidence (AC) system consists of 4 scintillators covering the sides of the magnet (CAS), one larger scintillator (CAT) covering the top of the magnet, with a hole in correspondence of the acceptance of the tracker and a set of four anticounters (CARD) covering the four lateral sides of the volume included between the S1 and S2 planes of the Time of Flight system.
Information from the anticoincidence system will be used to reject particles not cleanly entering the acceptance of the experiment, but giving rise to coincidence in the trigger scintillators. This happens when particles interact with high-density materials in the instrument (in particular, the magnet or the calorimeter) and give rise to showers that induce a trigger signal (false trigger).
The anticoincidence data are analyzes only off line. A second level trigger based on the signal from the AC system and from the calorimeter has been implemented, but at present it is not used.