# Mechanical stability of the CMS strip tracker measured with a laser alignment system

## TL;DR

This paper reports on the mechanical stability of the CMS silicon strip tracker during operation, using a laser alignment system and particle tracks to measure displacements and temperature effects with micron-level precision.

## Contribution

It introduces a dedicated laser alignment system for real-time monitoring of the tracker’s mechanical stability during LHC operations.

## Key findings

- Tracker components moved less than 30 microns during stable operation.
- Temperature changes caused displacements of about 2 microns per degree Celsius.
- Displacements largely reverted when temperature was restored.

## Abstract

The CMS tracker consists of 206 square meters of silicon strip sensors assembled on carbon fibre composite structures and is designed for operation in the temperature range from -25 to +25 degrees C. The mechanical stability of tracker components during physics operation was monitored with a few micron resolution using a dedicated laser alignment system as well as particle tracks from cosmic rays and hadron-hadron collisions. During the LHC operational period of 2011-2013 at stable temperatures, the components of the tracker were observed to experience relative movements of less than 30 microns. In addition, temperature variations were found to cause displacements of tracker structures of about 2 microns/degree C, which largely revert to their initial positions when the temperature is restored to its original value.