The LHCb trigger

LHCb will study CP violation and other rare phenomena in B-decays with a forward spectrometer at the LHC. The LHCb trigger has to efficiently select a few Hz of interesting B-decays from a non- elastic pp interaction cross-section of 80 mb, which corresponds to 16 MHz of interactions at the preferred LHCb luminosity of 2 * 10/sup 32/ cm/sup -2/s/sup -1/. The first trigger level reduces the rate to 1 MHz using large E/sub T/ triggers. At 1 MHz all data is digitised, and a subset of tracking information is used to reduce the rate to 40 kHz, at which rate the full event building is performed. Having access to all detector information, the rate is subsequently reduced to around 200 Hz and written to storage. This paper emphasises the data available at the various trigger stages and the algorithms employed to select B-decay candidates, rather than describing the implementation of the trigger. (12 refs).