Performance of the ATLAS Forward Proton Time-of-Flight Detector in 2017

The performance of the ATLAS Forward Proton Time-of-Flight (ToF) detector is studied using the ATLAS LHC data collected in the $2017$ running period of LHC Run2. A study of efficiency and time resolution of the ToF is performed. Good time resolutions of individual ToF channels, ranging between $20$~ps to $50$~ps are found, although the efficiency observed is well below $10\%$ in major parts of the analysed data. The events from ATLAS physics runs at moderate pile-up taken at the end of $2017$ are selected with signals in the ToF stations at both sides of the ATLAS interaction region. The overall time resolution of each ToF detector based on resolutions of the individual channels in these data is found to be $20(26) \pm 4(5)$~ps for side A(C). This represents a superb time resolution for a detector operating a few millimetres from the LHC beams. The difference of the primary vertex z-position measured by ATLAS and the value obtained by the ToFs is studied. The distribution of the difference constitutes of a background component from combinatorics since the level of pile-up is not negligible and a significantly narrower signal component from events where protons from interactions taking place in the primary vertex are detected in ToF. The fits are performed to the distributions yielding resolution of about $6.2 \pm 1.0$~mm to $8.5 \pm 3.4$~mm depending on applied data selection which is within uncertainties in agreement with the expectation based on single-channel resolutions.