Primary vertex time reconstruction using the LHCb ringimaging Cherenkov detectors

¹ European Organization for Nuclear Research (CERN), Geneva, Switzerland
² Università degli Studi di Milano-Bicocca, Milano, Italy

Published online: 7 October 2025

Abstract

The introduction of picosecond time information in the LHCb ringimaging Cherenkov (RICH) detectors during LHC Run 4 will allow the primary vertex time (PV t₀) to be estimated using only RICH detector information. The presented method can be integrated into the RICH reconstruction algorithm to determine the PV t₀. This PV t₀ is a necessary input for the application of a time gate around the predicted photon time-of-arrival for each track, which reduces out-of-time photon background from other tracks and improves particle identification (PID) performance. In the RICH reconstruction, each photon object (PO), which is a possible association of a photon detector hit to a particle track, has a PV assigned to it. However, only a fraction of POs is correct and the resolution of the RICH PV t₀ strongly depends on the fraction of correct POs for the PV, called the PV purity. To improve the PV purity while maintaining sufficient photon statistics, two PO selection criteria are introduced: the PO signal amplitude and the selection of POs uniquely associated with a pixel hit. The resulting increase in PV purity is 34 % for each selection criteria, while maintaining more than 20 POs for 98 % and 95 % of the PVs in the sample for the first and second selection respectively.