Calibration and Performance of the Upgraded ALICE Inner Tracking System

TL;DR

The upgraded ALICE Inner Tracking System (ITS2) with advanced pixel sensors significantly improves spatial resolution and readout rate, enabling high-quality data collection during LHC Run 3 with stable calibration procedures.

Contribution

This paper presents the design, calibration, and operational performance of the largest MAPS-based pixel tracker in high-energy physics, marking a major upgrade for ALICE at LHC.

Findings

Impact parameter resolution improved by a factor of 3

Readout rate increased to 67 kHz in Pb-Pb collisions

Successful commissioning and stable operation during LHC Run 3

Abstract

The ALICE Experiment at the Large Hadron Collider (LHC) underwent a major upgrade during the Long Shutdown 2. Several subsystems have been improved, including the ALICE Inner Tracking System (ITS), which has been entirely replaced. The new pixel-only tracker (ITS2) consists of 7 layers of Monolithic Active Pixel Sensors (MAPS) featuring a pixel size of 27x29 $\mu$m$^2$, with an intrinsic spatial resolution of 5 $\mu$m. With 24120 sensors and 12.5 billion pixels, this detector covers an active area of about 10 m$^2$ and represents the largest application of the MAPS technology in a high-energy physics experiment to date. The most significant improvements introduced by the ITS2 to the ALICE experiment include a reduction in the impact parameter resolution to approximately 30 $\mu$m in both the r$\varphi$ and z coordinates at a transverse momentum of 1 GeV/c. This is a factor of 3 improvement over the previous detector. Additionally, the standard ITS readout rate has been increased from 1 kHz to 67 kHz in Pb-Pb collisions and to 202 kHz in proton-proton collisions. To ensure stable operations and maintain high data quality a regular calibration is performed, which consists in establishing the charge threshold and the noisy channels of the detector. The ITS2 has been successfully commissioned for LHC Run3, and already operated during proton-proton and Pb-Pb collisions at LHC with excellent performance. This contribution gives an overview of the operational procedures required to maintain an optimal data quality, along with results obtained from calibration and the performance achieved during the LHC Run 3.