Off-line time calibration of the ATLAS RPC system

Resistive Plate Chambers (RPC's), operated in saturated avalanche regime, are used in the Muon Spectrometer of the ATLAS experiment to produce the first level of the muon trigger in the central region, $|eta| <$ 1.05. The trigger logic, based on a coincidence of hits in three layers of detector doublets, takes advantage of the very good time resolution of these detectors which allows to easily identify the LHC bunch crossing. The RPC readout electronics, based on a 320 MHz clock, allow to store a very granular time information, making the RPC system, potentially, the detector providing the most accurate time measurement in ATLAS. To fully exploit the intrinsic time resolution of detector and readout electronics, a careful calibration of the system is needed, involving about 330,000 channels. The ATLAS data recorded during 2011 in LHC p-p collisions at $sqrt{s}$ = 7 TeV have been used to show that, after applying an off-line calibration procedure, a time resolution uniform over the entire detector and stable in time, can be reached. A simulation of the various contributions to the observed time of flight per single channel, implemented in the software for the ATLAS simulation, allows to understand the resolution measured in data. The time resolution is understood in terms of the intrinsic detector resolution, the digitization error and the various components relevant at different levels of refinement of the time calibration procedure. Achieving the ultimate timing resolution of the RPC system is a very powerful way of extending the physics potential of ATLAS experiment, for example, in searches for particles moving with low velocity from the interaction point. In addition, good time resolution may be a key ingredient for background rejection, which may become of overwhelming importance in future scenarios of increased LHC luminosity.