Alignment of the CMS silicon tracker during commissioning with cosmic rays

Abstract

The CMS silicon tracker, consisting of 1440 silicon pixel and 15 148 silicon strip detector modules, has been aligned using more than three million cosmic ray charged particles, with additional information from optical surveys. The positions of the modules were determined with respect to cosmic ray trajectories to an average precision of 3-4 microns RMS in the barrel and 3-14 microns RMS in the endcap in the most sensitive coordinate. The results have been validated by several studies, including laser beam cross-checks, track fit self-consistency, track residuals in overlapping module regions, and track parameter resolution, and are compared with predictions obtained from simulation. Correlated systematic effects have been investigated. The track parameter resolutions obtained with this alignment are close to the design performance.

Prompt and non-prompt J/ψ production in pp collisions at $\sqrt{s} = 7$ TeV

“…The effect of uncertainties in the measured misalignment of the tracker modules is estimated by reconstructing the data several times using different sets of alignment constants. These sets reflect the uncertainty in the constants and, in particular, explore possible deformations of the tracker which are poorly constrained by the data [21]. The largest difference between the results with the nominal set of constants and with these sets is taken as a systematic uncertainty.…”

“…The detector simulation includes the trigger, as well as the effects of the finite precision of alignment and calibration, as determined using LHC collision data and cosmic-ray muon events [21].…”

“…In addition to calibrations already applied to the data [21,22], residual effects can be determined by studying the dependence of the reconstructed dimuon peak shapes on the muon kinematics. The transverse momentum corrected for the residual scale distortion is parametrized as…”

First measurement of the underlying event activity at the LHC with $\sqrt{s} = 0.9$ TeV

“…The p T resolution for 1 GeV/c charged particles is between 0.7% at η = 0 and 2% at |η| = 2.5 [17]. The modules composing the tracker system were aligned with cosmic ray data taken prior to LHC commissioning, with a precision of 3-4 µm in the barrel region [18].…”

CMS Alignment Tracking and Physics Performance Results

“…The core of the track-based alignment is the minimization of the χ 2 of the hit-track residuals with respect to the parameters describing the sensor positions. Two algorithms have been developed and are used in CMS [6]. The Millipede II algorithm performs a global minimization of the χ 2 including also the track parameters which are optmized together with the sensor position parameters.…”