Abstract
The performance of the ATLAS Forward Proton Time-of-Flight (ToF) Cherenkov detector is studied using the ATLAS data collected in 2017 during the LHC Run2. The detailed analysis of the results, including detector efficiency and time resolution of the ToF detector, is discussed. The detector construction and its expected performance based on beam test results are briefly summarized first. Operational experiences are presented. They include problems due to attempts to operate Micro-Channel Plate Photomultiplier Tubes (MCP-PMTs) in the vacuum, leading to changes in the detector construction. In addition, a new MCP-PMT design was needed, as a result of the observed gain drop for high rates and its non-recoverability. Also, other hardware changes are presented— the effect of the replacement of glued crystal bars by monolithic ones, as well as 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 document is devoted to the achieved time resolutions, 20 ± 4 ps and 26 ± 5 ps, of two installed ToF detectors. Despite 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. Based on the time resolution, this result characterizes the best ToF detector among those operated by the different LHC experiments in the forward regions during the LHC Run2. Finally, the possibility of reconstructing the z-coordinate of the ATLAS interaction region using the ToF detectors is shown.
Highlights
Based on the series of beam tests performed by the AFP ToF group at the CERN SPS in the years 2014-2018 using the setup configuration shown in Figure 5, the expected time resolution of the ToF detector without the HPTDC and synchronization clock contributions is in the case of one LQBar ∼ 20 ps and in the case of the full train ∼ 14 ps, see Figure 6
The AFP ToF detector was installed during the 2016-2017 LHC winter shutdown and the first timing data were taken in May 2017
Information on the tube serial numbers appears in the legend; dependency of the time resolution in ps on the signal rate and different values of applied High Voltage (HV) for the same two photomultipliers [9]
Summary
The time resolution, using the 2017 detector design, could not reach 10 ps: the main limitations came from the geometry of LQBars (∼ 17 ps/channel) [4, 5], causing the spread in time of arrival of Cherenkov photons to the MCP-PMT due to their different wave lengths, and from the HPTDC (∼ 17-22 ps/channel) [6]. The latter limitation will be minimized by replacing the HPTDC by the picoTDC (< 6 ps/channel), planned in 2020. The hardware limit of the HPTDC, ∼ 8 MHz/segment, imposed by its construction, was not reached in the LHC Run
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