Optimisation of bremsstrahlung recovery in B+ → K+e+e− decays for the LHCb Upgrade II

This document is a preliminary assessment of the electromagnetic calorimeter (ECAL) timing resolution for LHCb upgrade II in preparation for the high luminosity LHC (HL-LHC). LHCb specializes in beauty physics to probe sources of matter-antimatter asymmetry in the universe. A process of interest for this sector is B+ → K+e+e− and it is for the low energy bremsstrahlung emitted during this decay that tuning of ECAL timing resolution is performed. This decay is rare, and while the HL-LHC is intended to illuminate such rare processes, it is also a challenge to mitigate the pile up and radiation damage that comes with such high luminosities. Currently in development for upgrade II is a robust ECAL with increased granularity and new spaghetti calorimeter (SpaCal) technology. All data analyzed in this study comes from a Geant4 Simulation of the upgrade II ECAL. We observe a drastic improvement in timing resolution, O(10) ps, for the new SpaCal technology as compared to the older Shashlik technology. We also show how timing precision improves for the more energetic photon showers and how the precision depends on where in the ECAL the signal is being registered. This study serves as a useful starting point for future bremsstrahlung recovery optimisation studies.