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The HERMES Spectrometer

K. Ackerstaff

TL;DR

The paper presents the design, construction, and performance of the HERMES forward‑angle spectrometer built to study polarised deep inelastic scattering on polarised gas targets. It details a robust tracking system (MSGC vertex chambers, drift chambers, and MWPCs in the magnet gap) and a comprehensive PID suite (calorimeter, pre‑shower, TRD, and threshold Cherenkov) together with a fast, Tree‑search based track‑reconstruction and a fast momentum lookup. The apparatus achieves high tracking accuracy (momentum resolution 0.7–1.25%, angle <0.6 mrad) and substantial hadron rejection (HRF ≥10⁴) enabling clean DIS positron samples and valuable semi‑inclusive measurements for flavour tagging and spin decompositions. The paper also covers luminosity monitoring, trigger architecture, data acquisition, and beam‑tuning procedures, illustrating a reliable, high‑performance experimental setup capable of delivering unique insights into quark spin contributions to nucleon spin and semi‑inclusive hadron production.

Abstract

The HERMES experiment is collecting data on inclusive and semi-inclusive deep inelastic scattering of polarised positrons from polarised targets of H, D, and He. These data give information on the spin structure of the nucleon. This paper describes the forward angle spectrometer built for this purpose. The spectrometer includes numerous tracking chambers (micro-strip gas chambers, drift and proportional chambers) in front of and behind a 1.3 T.m magnetic field, as well as an extensive set of detectors for particle identification (a lead-glass calorimeter, a pre-shower detector, a transition radiation detector, and a threshold Cherenkov detector). Two of the main features of the spectrometer are its good acceptance and identification of both positrons and hadrons, in particular pions. These characteristics, together with the purity of the targets, are allowing HERMES to make unique contributions to the understanding of how the spins of the quarks contribute to the spin of the nucleon.

The HERMES Spectrometer

TL;DR

The paper presents the design, construction, and performance of the HERMES forward‑angle spectrometer built to study polarised deep inelastic scattering on polarised gas targets. It details a robust tracking system (MSGC vertex chambers, drift chambers, and MWPCs in the magnet gap) and a comprehensive PID suite (calorimeter, pre‑shower, TRD, and threshold Cherenkov) together with a fast, Tree‑search based track‑reconstruction and a fast momentum lookup. The apparatus achieves high tracking accuracy (momentum resolution 0.7–1.25%, angle <0.6 mrad) and substantial hadron rejection (HRF ≥10⁴) enabling clean DIS positron samples and valuable semi‑inclusive measurements for flavour tagging and spin decompositions. The paper also covers luminosity monitoring, trigger architecture, data acquisition, and beam‑tuning procedures, illustrating a reliable, high‑performance experimental setup capable of delivering unique insights into quark spin contributions to nucleon spin and semi‑inclusive hadron production.

Abstract

The HERMES experiment is collecting data on inclusive and semi-inclusive deep inelastic scattering of polarised positrons from polarised targets of H, D, and He. These data give information on the spin structure of the nucleon. This paper describes the forward angle spectrometer built for this purpose. The spectrometer includes numerous tracking chambers (micro-strip gas chambers, drift and proportional chambers) in front of and behind a 1.3 T.m magnetic field, as well as an extensive set of detectors for particle identification (a lead-glass calorimeter, a pre-shower detector, a transition radiation detector, and a threshold Cherenkov detector). Two of the main features of the spectrometer are its good acceptance and identification of both positrons and hadrons, in particular pions. These characteristics, together with the purity of the targets, are allowing HERMES to make unique contributions to the understanding of how the spins of the quarks contribute to the spin of the nucleon.

Paper Structure

This paper contains 36 sections, 30 figures, 4 tables.

Table of Contents

  1. Introduction.
  2. General Description.
  3. The Target Region.
  4. The Magnet.
  5. The Tracking System
  6. Vertex Chambers
  7. The Drift Chambers
  8. The Front Drift Chambers.
  9. The Back Chambers.
  10. The Drift Vertex Chambers
  11. The Magnet Chambers
  12. Alignment
  13. Laser Alignment System
  14. Track Reconstruction
  15. The Fast Pattern Recognition Algorithm
  16. ...and 21 more sections

Figures (30)

  • Figure 1: Schematic side view of the HERMES spectrometer. See the text for the meaning of the labels.
  • Figure 2: Schematic of the target region. See the text for more explanation.
  • Figure 3: Schematic view of a drift cell in a MicroStrip Gas Counter, showing the field lines. This figure has been taken from ref.VC-vdMarel.
  • Figure 4: Vertex reconstruction in the transverse coordinates using the VC alone. This plot was made using two-track events only to get a well defined vertex. The outline of the target cell is shown as the ellipse.
  • Figure 5: FC resolution as function of drift distance.
  • ...and 25 more figures