Optimization of the operating parameters of the LHCb muon system

LHCb is a $B$ physics experiment at the Large Hadron Collider (LHC) at CERN. The LHCb muon detector has a total area of about 435 m2 and is divided into five stations with four regions of different granularity. The whole system is composed of 1380 Multi Wire Proportional Chambers (MWPCs). To obtain a good trigger performance each chamber should have an efficiency of approximately 99% which translates into a time resolution < 3.5 ns r.m.s.. To achieve the required efficiency and time resolution the muon chambers have to be operated at the lowest possible threshold. To minimize ageing effects at the same time the chambers have also to be operated at the lowest possible voltage. In order to optimize the operating parameters of the muon system it is of particular importance to have a profound understanding of the effects that limit the performance. This work describes a detailed study of all the resolution limiting parameters of the MWPCs in order to find out how they contribute to time resolution and therefore affect the threshold settings. The study was carried out using the drift chamber simulation program GARFIELD. The simulations have been compared with actual measurements to ensure that the simulation reproduces the measurement well. It was of particular interest to understand the dependence of time resolution and efficiency on electronics noise and threshold. The simulation study showed that large deposit fluctuations have a big influence on time resolution because of the time slewing effect. The effect of a time slewing correction is shown. Since ageing will not play a role during the first phase of operation with LHC beams, one should concentrate first of all on the threshold settings. Therefore an optimization of the electronics thresholds for the different chamber types was performed using the information obtained by the simulations. In this context, the noise characteristics of the front-end (FE) electronics have been studied. The threshold settings of the past were improved by setting individual thresholds for the 122k FE channels of the muon system. It is shown that the thresholds keep the noise in the system at an acceptable level while conserving time resolution and efficiency. Knowing this one can now start to optimize the high voltage settings. The last chapter finally concentrates on the presentation of results obtained by analysing cosmics data and first collision data aquired in LHCb to get a first overview of the performance of the muon system with the final threshold settings.