RPCs: the choreography of precise timing

We present a comprehensive overview of all steps to be taken in order to achieve precise timing of the HARP RPCs. After briefly recalling the salient features of the RPC mechanics and electronics, we discuss the results from the dedicated calibration scan in a -12 GeV/c beam which gave valuable first information on efficiency and uniformity of response, charge attenuation, the global time slewing correction, effective strip transit time, relative delays between strips of a pad, and intrinsic time resolution. The latter was determined to be ~140 ps which sets the scale for the control of systematic effects, so as not to deteriorate significantly the overall time-of-flight resolution of the RPCs. From physics data, primarily from `neutral' and `charged' hits in RPC overlap regions, further refinements are derived, in particular padring-specific modifications of the global time slewing correction, and manifestations of the `knock-on' and noise' effects. With a view to a bias-free determination of pad-specific t0 constants which transform RPC time stamps together with the time of arrival of the beam particle at the target into a time-of-flight measurement, time spectra of `neutral' hits are analyzed in conjunction with an analytic simulation of the RPC electronics response. After all corrections and including the beam particle timing, the overall RPC time-of-flight resolution is ~180Â ps.