LHCb RICH Fast-timing photon detection at the SPS charged particle beam

The use of a new optoelectronic readout chain with O(100)ps time resolution is a major milestone in the development of RICH detectors for environments with a high number of particle interactions per bunch crossing, such as the High-Luminosity LHC. A prototype chain, based on MAPMT and SiPM photon sensors coupled to readout electronics integrating the FastIC ASIC and a TDC-in-FPGA, is presented. The FastIC is an 8-channel ASIC for analogue-to-digital conversion with 25 ps time resolution. This ASIC is the predecessor of the FastRICH ASIC for the LHCb RICH upgrades. The signals from the photon detectors are timestamped in a custom multi-channel TDC-in-FPGA designed using a multi-phase clock sampling architecture with an average bin width of 150 ps. The prototype optoelectronic readout chain was tested in a charged particle beam of 180 GeV/c hadrons at the CERN SPS facility. The reference time-of-arrival of the particle tracks was measured with about 100 ps time resolution and the number of tracks per event was registered using a particle tracking system. The single-photon time resolution of the MAPMT sensors is extracted using a binned method in order to correct for the calibrated TDC bin widths and detector time-walk. The resulting best estimate of σ = 182 ± 24 ps is in agreement with pulsed-laser measurements in the lab and consistent with the expectations from the manufacturer.