Using RICH Particle Identification in the LHCb High Level Trigger Selection of $B_s \to D_s h$

The use of RICH particle identification (PID) in the High Level Trigger (HLT) selection of $B_s \to D_s h$, where $h = pi$ or $K$, has been explored. The online RICH PID is found to be a more efficient cut to reject minimum bias background events than others presently used. The application of RICH PID therefore allows other cuts to be relaxed and the trigger efficiency to be increased. In addition, the control of the background is sufficient to allow a small fraction of additional events to be triggered without requiring all the tracks be reconstructed right the way through the LHCb detector. A track segment is used to determine the direction of one of the particles in a candidate decay and then the $D_s$ mass is "forced" to determine the momentum of this segment. This allows the recovery of events lost owing to the tracking inefficiency in the HLT. The $D_s$ mass-forcing requires a number of cuts to be tightened in order to reduce the additional background generated by the loss of the $D_s$ mass constraint. The HLT efficiency is 0.70 with the RICH and modified cuts and 0.75 including the additional $D_s$ mass-forced events. This is to be compared to 0.60 without the use of online RICH information and using the standard cuts.