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Alignment of the ALICE Inner Tracking System with cosmic-ray tracks

ALICE Collaboration

TL;DR

This study demonstrates a comprehensive alignment program for the ALICE ITS, combining survey data with track-based methods to control thousands of alignment parameters across six silicon layers. It validates survey accuracy with cosmic-ray tracks, applies the Millepede global algorithm and an independent iterative local method, and shows that the SPD can reach effective spatial resolutions near the intrinsic sensor performance, while SSD survey precision and SDD calibration are crucial steps toward full ITS–TPC integration. The work establishes a robust framework for ITS alignment prior to LHC collisions and lays out a clear path for incorporating SDD calibrations and collision data to achieve uniform, high-precision track parameter measurements critical for heavy-flavor physics. The approach has immediate practical impact on the quality of track and vertex reconstruction in ALICE and informs alignment strategies for complex multi-technology tracking systems.

Abstract

ALICE (A Large Ion Collider Experiment) is the LHC (Large Hadron Collider) experiment devoted to investigating the strongly interacting matter created in nucleus-nucleus collisions at the LHC energies. The ALICE ITS, Inner Tracking System, consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two layers of pixel detectors, two layers each of drift, and strip detectors. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 micron in some cases (pixels). The sources of alignment information include survey measurements, and the reconstructed tracks from cosmic rays and from proton-proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10^5 charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnet switched off.

Alignment of the ALICE Inner Tracking System with cosmic-ray tracks

TL;DR

This study demonstrates a comprehensive alignment program for the ALICE ITS, combining survey data with track-based methods to control thousands of alignment parameters across six silicon layers. It validates survey accuracy with cosmic-ray tracks, applies the Millepede global algorithm and an independent iterative local method, and shows that the SPD can reach effective spatial resolutions near the intrinsic sensor performance, while SSD survey precision and SDD calibration are crucial steps toward full ITS–TPC integration. The work establishes a robust framework for ITS alignment prior to LHC collisions and lays out a clear path for incorporating SDD calibrations and collision data to achieve uniform, high-precision track parameter measurements critical for heavy-flavor physics. The approach has immediate practical impact on the quality of track and vertex reconstruction in ALICE and informs alignment strategies for complex multi-technology tracking systems.

Abstract

ALICE (A Large Ion Collider Experiment) is the LHC (Large Hadron Collider) experiment devoted to investigating the strongly interacting matter created in nucleus-nucleus collisions at the LHC energies. The ALICE ITS, Inner Tracking System, consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two layers of pixel detectors, two layers each of drift, and strip detectors. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 micron in some cases (pixels). The sources of alignment information include survey measurements, and the reconstructed tracks from cosmic rays and from proton-proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10^5 charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnet switched off.

Paper Structure

This paper contains 17 sections, 4 equations, 15 figures, 1 table.

Table of Contents

  1. Introduction
  2. ITS detector layout
  3. Silicon Pixel Detector (SPD)
  4. Silicon Drift Detector (SDD)
  5. Silicon Strip Detector (SSD)
  6. Alignment target and strategy
  7. Cosmic-ray run 2008: data taking and reconstruction
  8. Validation of the survey measurements with cosmic-ray tracks
  9. Double points in SSD module overlaps
  10. Track-to-point residuals in SSD
  11. ITS alignment with Millepede
  12. General principles of the Millepede algorithm
  13. Millepede for the ALICE ITS
  14. Results on alignment quality
  15. Prospects for inclusion of SDD in the Millepede procedure
  16. ...and 2 more sections

Figures (15)

  • Figure 1: General layout of the ALICE experiment aliceJINST.
  • Figure 2: Layout of the ITS (left) and orientation of the ALICE global (middle) and ITS-module local (right) reference systems. The global reference system has indeed its origin in the middle of the ITS, so that the $z$ direction coincides with the beam line.
  • Figure 3: SPD drawings. Left: the SPD barrel and the beam pipe (radius in mm). Right: a Carbon Fibre Support Sector.
  • Figure 4: Left: scheme of the SDD layers. Right: scheme of a SDD module, where the drift direction is parallel to the $x_{\rm loc}$ coordinate. Units are millimeters.
  • Figure 5: View of one SSD ladder (from layer 5) as described in the AliRoot geometry.
  • ...and 10 more figures