MUON DETECTORS: ALIGNMENT

The main developments in muon alignment since March 2010 have been the production, approval and deployment of alignment constants for the ICHEP data reprocessing.

In the barrel, a new geometry, combining information from both hardware and track-based alignment systems, has been developed for the first time. The hardware alignment provides an initial DT geometry, which is then anchored as a rigid solid, using the link alignment system, to a reference frame common to the tracker. The “GlobalPositionRecords” for both the Tracker and Muon systems are being used for the first time, and the initial tracker-muon relative positioning, based on the link alignment, yields good results within the photogrammetry uncertainties of the Tracker and alignment ring positions. For the first time, the optical and track-based alignments show good agreement between them; the optical alignment being refined by the track-based alignment. The resulting geometry is the most complete to date, aligning all 250 DTs, and is closer than ever to the final design alignment strategy.

In the endcaps, new optical alignment constants with (x,y) CSC positions were approved. Unfortunately, a technical mistake when combining these measurements with link alignment measurements of ME12 and ME11 chambers and CRAFT09 measurements for other degrees of freedom caused the final alignment database to be incorrect, and it was not fixed in time for ICHEP reprocessing. However, a successful beam-halo alignment based on overlapping tracks and photogrammetry information has been validated and deployed for CSCs.

The alignment efforts will now concentrate on the combination of hardware and track-based results in the endcaps, the automation of the entire data chain leading to alignment databases and tags, further refinements of alignment results and on extending the validation techniques.

What are the alignment constants? How can they be visualized?
The answer to the first question is simple: the CMS reconstruction needs to know where to place, in space, the hits recorded from the active tracking elements. For many reasons such as mounting tolerances, thermal effects, gravitational forces, magnetic forces, etc…, these tracking detectors are not exactly at their design position. In the case of the muon alignment, the alignment constants are a set of “corrections” with respect to this ideal alignment. They consist of six numbers (called degrees of freedom) for each layer of each muon chamber: three translations and three rotations, with respect to the nominal, ideal alignment. That’s a lot of numbers! Which brings us to the second question: what do they look like? Well, one could make a list, and then they would look something like this:

But a nicer way to look at them is show them organized by chamber and by degree of freedom in such a way that the visualization becomes a little more intuitive. The images below are nothing more than a graphical representation of the alignment constants in the current CMS reconstruction database for muon all chambers (ignoring internal layers):


by G.Gomez