CMS Bulletin

MUON DETECTORS: ALIGNMENT

A new muon alignment has been produced for 2012 A+B data reconstruction. It uses the latest Tracker alignment and single-muon data samples to align both DTs and CSCs. Physics validation has been performed and shows a modest improvement in stand-alone muon momentum resolution in the barrel, where the alignment is essentially unchanged from the previous version. The reference-target track-based algorithm using only collision muons is employed for the first time to align the CSCs, and a substantial improvement in resolution is observed in the endcap and overlap regions for stand-alone muons. This new alignment is undergoing the approval process and is expected to be deployed as part of a new global tag in the beginning of December.

The p_T dependence of the φ-bias in curvature observed in Monte Carlo was traced to a relative vertical misalignment between the Tracker and barrel muon systems. Moving the barrel as a whole to match the Tracker cures this p_T dependence, leaving only the φ dependence induced by the Tracker. Two temporary misalignment scenarios will be used to reprocess the Z′ samples, which were the most affected. One simply takes the existing scenario, which has large uncertainties, and corrects for the vertical displacement. A second scenario smears chamber positions according to the statistical precision of the 2011 track-based alignment. The former scenario is too conservative, while the latter is too optimistic as it does not take into account systematic uncertainties. The optimal solution is to estimate the systematic uncertainties in the current alignment procedure, which includes a systematic study of internal chamber alignment and angular residuals, and to use this information to provide a new set of APEs (alignment position errors) and a more realistic MC scenario. This work is in progress.