A Real-Time Transverse Momentum Discriminator for the BABAR Level 1 Trigger System1

The Transverse Momentum Discriminator Module (PTDM) is one of the three main modules used in the Level 1 Charged Particle Trigger System of the BABAR Detector at PEP-II. It provides trigger decisions for charged particles with a transverse momentum, Pt, greater than a configurable threshold. The transverse momentum discrimination algorithm works by evaluating the curvature of the charged tracks in the 1.5 T axial magnetic field. The capabilities of the PTDM are key to a stable and efficient operation of the BABAR experiment even under severe background conditions. A sharp threshold behavior with an efficiency rise from 10% to 90% within 30 MeV=c at 600 MeV=c is achieved by using track segment information with very good spatial resolution (0.7 mm). This resolution is obtained by using the drift time information from the drift chamber. In this paper the design and implementation of the PTDM are described.