CMS Bulletin

MUON DETECTORS: RPC

The RPC system is operating with a very high uptime, an average chamber efficiency of about 95% and an average cluster size around 1.8. The average number of active channels is 97.7%. Eight chambers are disconnected and forty are working in single-gap mode due to high-voltage problems. The total luminosity lost due to RPCs in 2012 is 88.46 pb^–1.

One of the main goals of 2012 was to improve the stability of the endcap trigger that is strongly correlated to the performances of the detector, due to the 3-out-3 trigger logic.

At beginning of 2011 the instability of the detector efficiency was about 10%. Detailed studies found that this was mainly due to the strong correlation between the performance of the detector and the atmospheric pressure (P). Figure XXY shows the linear correlation between the average cluster size of the endcap chamber versus P. This effect is expected for gaseous detectors and can be reduced by correcting the applied high-voltage working point (HV_app) according to the following equation: HV_app = HV_eff (1–α+α*P/P0), where α is a parameter to estimate from the data (α < 1.0) and HV_eff is the effective HV working point (WP).

Figure 1: Cluster size versus atmospheric pressure before (red) and after (blue) the automatic HV working-point correction (endcaps: above; barrel: below).

Many improvements have been introduced since 2011 to stabilise the detector performance and they are: a “slow” (once per fill) WP automatic correction, with α = 1, (July 2011); a “fast” WP automatic correction (anytime it changes of 3 V) (July 2012); and an α value equal to 0.8, estimated with the first data of the 2012 (November 2012). Thanks to the first two steps the chamber efficiency instability went down from 10% to 4–5% and, finally, with α equal to 0.8, the amplitude of the oscillations due to atmospheric pressure variations has been reduced by a factor of 10 for the cluster size and by a factor of 4 for the efficiency (1–2%).

A detailed study has shown that the contribution of RPC hits to the standard Muon Reco is in a range of 1% to 7%. The contribution reaches a value of 5–7 % in some specific η regions: overlap between wheel 0 and ±1 and overlap barrel endcap.

A new muon object, based on the matching between a Tracker track and RPC hits, has been released in CMSSW 6.1.X.

The RE4 upgrade project is progressing well. The first batch of gaps has been delivered at CERN, Ghent and BARC (India) and is being tested right now. Few imperfections on the graphite surface of the gaps have been detected and some improvements in the gap production have been already introduced.

A first chamber has been produced at the laboratory in B904 at CERN and will be tested with cosmic rays in December. Ghent will begin to assemble chambers in December, and BARC in January. The power system has been ordered and will be delivered at CERN in May 2012.

The organisation chart for 2013 and 2014 is under preparation and will be presented at the next CMS Week. The Run Coordination team will be re-organised and put side-by-side with the Commissioning Coordination team for this special period, keeping in mind the regular maintenance tasks as well as installation and commissioning of RE4.