ATLAS Forward Proton Time-of-Flight Detector: LHC Run2 performance and experiences

The performance of the ATLAS Forward Proton Time-of-Flight (ToF) Cherenkov detector is shown using the ATLAS collaboration data collected in the 2017 running period of the LHC Run2. The detailed analysis of the results, including detector efficiency and time resolution of the ToF is performed. The detector construction and its expected performance based on beam tests results are briefly summarized at the beginning of the first part of the talk. Operational experiences, problems caused due to attempts to operate Micro-Channel Plate PhotoMulTipliers (MCP-PMTs) in the vacuum and leading to the change of the detector construction, the request for construction of a new, future, MCP-PMT due to the observed gain drop for high rates and its non-recoverability are described. Also, other tested hardware changes are presented – the effect of the replacement of glued crystal bars by monolithic ones, the change of the MCP-PMT back-end electronics based on simulation to reduce signal cross talk resulting in further detector time resolution improvement. The second part of the talk is devoted to the achieved time resolutions, 20 ± 4 ps and 26 ± 5 ps, of two installed ToF detectors. Despite of their very low efficiencies (below 10\%) in major parts of the analyzed data, this represents a superb time resolution for detectors operating a few millimeters from the LHC beams and making them, from the time resolution point of view, the best ToF detector among those operated by the different LHC experiments in the forward regions during the LHC Run2. At the end the possibility of reconstruction of z-coordinate of the ATLAS interaction region using the ToF detectors is shown.