Calibration of the Upgraded ALICE Inner Tracking System

TL;DR

This paper details the calibration process of the upgraded ALICE Inner Tracking System, a large pixel detector, focusing on threshold tuning and noise measurement to ensure high data quality during LHC Run 3.

Contribution

It introduces a novel calibration framework for the largest pixel detector in high energy physics, enabling efficient in situ calibration of over 24,000 sensors.

Findings

Successful in situ calibration of pixel thresholds

Effective noise channel identification

Stable detector operation achieved

Abstract

The ALICE Experiment has replaced its Inner Tracking System with a 7-layer pixel-only tracker made out of more than 24000 monolithic active pixel sensor chips, in order to fulfill the requirements of the physics program of the LHC Run 3. The upgraded Inner Tracking System (ITS2) has been installed in the ALICE experiment during the LHC long shutdown 2 and has started to take data with the beginning of Run 3 in July 2022, with proton-proton collisions at $\sqrt{s}$ = 13.6 TeV. With its 12.5 billion pixels it is the largest pixel detector installed in a high energy physics experiment to date. To guarantee stable operation and a consistently high data quality, a regular calibration of the detector has to be performed. The main part of the calibration program consists of a tuning and subsequent measurement of the pixel thresholds and a determination of the noisy channels. In particular the complexity of the threshold scan depends linearly on the number of pixels, which is why the threshold scan of the ITS2 is an unprecedented challenge. This work describes the architecture of the calibration framework, which has been developed using the detector control system of the ITS2 and the ALICE data processing layer. Results of first threshold and noise calibrations done in situ are shown as well.