Aligning the CMS Muon Chambers with the Muon Alignment System during an Extended Cosmic Ray Run

TL;DR

This paper reports on the successful commissioning and performance of the CMS muon chamber alignment system during a cosmic ray run, achieving near-precision alignment under magnetic forces.

Contribution

It presents the first full commissioning of the CMS muon alignment system during an extended cosmic ray run, demonstrating its capability to track and align chambers with high precision.

Findings

Chamber movements of up to 18 mm tracked

Alignment precisions of 140-350 microns achieved

Systematic errors estimated at 340-590 microns

Abstract

The alignment system for the muon spectrometer of the CMS detector comprises three independent subsystems of optical and analog position sensors. It aligns muon chambers with respect to each other and to the central silicon tracker. System commissioning at full magnetic field began in 2008 during an extended cosmic ray run. The system succeeded in tracking muon detector movements of up to 18 mm and rotations of several milliradians under magnetic forces. Depending on coordinate and subsystem, the system achieved chamber alignment precisions of 140-350 microns and 30-200 microradians, close to the precision requirements of the experiment. Systematic errors on absolute positions are estimated to be 340-590 microns based on comparisons with independent photogrammetry measurements.