Commissioning and Performance of the CMS Pixel Tracker with Cosmic Ray Muons

TL;DR

The paper reports the commissioning and early performance of the CMS silicon pixel tracker using cosmic-ray muons in a 3.8 T magnetic field. It details calibration procedures for the readout electronics, ADC-to-charge response, and thresholds, along with alignment and timing strategies; data from the CRAFT run are used to validate hit efficiency, charge collection, Lorentz-angle effects, and track-resolution metrics. Key findings include hit efficiencies above 96%, intrinsic pixel resolutions around 19 μm in the transverse plane and 31 μm in z, and a transverse impact parameter resolution of about 18 μm for high-momentum tracks—all in line with design goals. The results demonstrate robust detector performance and proper calibration, confirming the pixel detector’s readiness for operation with proton-proton collisions.

Abstract

The pixel detector of the Compact Muon Solenoid experiment consists of three barrel layers and two disks for each endcap. The detector was installed in summer 2008, commissioned with charge injections, and operated in the 3.8 T magnetic field during cosmic ray data taking. This paper reports on the first running experience and presents results on the pixel tracker performance, which are found to be in line with the design specifications of this detector. The transverse impact parameter resolution measured in a sample of high momentum muons is 18 microns.

Figures (12)

• Figure 1: Sketch of the CMS pixel detector (a) and exploded view of a barrel module (b).
• Figure 2: (a) ADC values corresponding to the address encoding for a single ROC, where six peaks corresponding to the levels are visible and well separated. (b) RMS of each peak for all active ROCs in the detector. (c) Separation between adjacent peaks.
• Figure 3: (a) Example of the ADC response as a function of injected charge in VCAL units ($\approx$ 65.5 electrons) for one pixel. Distribution of gains (b) and pedestals (c) for all pixels.
• Figure 4: (a) Efficiency S-curve as a function of injected charge in VCAL units ($\approx$65.5 electrons). (b) Distribution of ROC-mean threshold in the endcap and barrel detectors.
• Figure 5: Number of hits associated to a track detected in each ROC for the first (a), second (b) and third (c) barrel layers. Bins in white correspond to readout chips excluded from data taking. On average each ROC had about 60 hits when integrated over the 85 000 tracks traversing the barrel pixel detector. The plot origin corresponds to $\phi=0$ and $z=-26.7$ cm.