CMS Bulletin

MUON DETECTORS: ALIGNMENT

Alignment efforts in the first few months of 2011 have shifted away from providing alignment constants (now a well established procedure) and focussed on some critical remaining issues. The single most important task left was to understand the systematic differences observed between the track-based (TB) and hardware-based (HW) barrel alignments: a systematic difference in r-φ and in z, which grew as a function of z, and which amounted to ~4-5 mm differences going from one end of the barrel to the other. This difference is now understood to be caused by the tracker alignment. The systematic differences disappear when the track-based barrel alignment is performed using the new “twist-free” tracker alignment. This removes the largest remaining source of systematic uncertainty.

Since the barrel alignment is based on hardware, it does not suffer from the tracker twist. However, untwisting the tracker causes endcap disks (which are aligned using tracker tracks) to rotate around the beam line by about 1 mrad in opposite directions, resulting in a poor relative alignment between barrel and endcap. A new endcap alignment has been produced which makes CSCs consistent with the new tracker alignment and which improves the relative barrel-endcap alignment.

There has also been progress in the barrel alignment using stand-alone muon tracks. An initial procedure has been tested on Monte Carlo and is now being tested on data. It aligns the entire barrel in sequence: all DTs inside each sector of each wheel first, neighbouring sectors within each wheel next, neighbouring wheels last. A new effort has begun to provide realistic alignment position errors (APEs) for all chambers. These APEs should help improve the dynamic truncation fitter, a new reconstruction for highly energetic muons which can undergo significant radiative losses.