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The calibration and alignment of the LHCb RICH system

This study focuses on the calibration and alignment process of the LHCb RICH system for precise particle identification. The alignment techniques include tracking with test patterns, magnetic field corrections, HPD quantum efficiency monitoring, and refraction index adjustments. The system comprises two radiators, composite mirrors, and HPD arrays for photon detection. The approach involves projected test patterns to assess distortions and calibration parameters. The study also analyzes Cherenkov angle resolution, mirror misalignment histograms, and test-beam alignment results. The research aims to optimize the system for accurate particle identification in high-energy physics experiments.

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The calibration and alignment of the LHCb RICH system

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  1. The calibration and alignmentof the LHCb RICH system Antonis Papanestis STFC - RAL for the LHCb Collaboration

  2. Outline • Quick overview of the LHCb RICH system. • Calibration & Alignment with projected test patterns: • Magnetic field corrections. • HPD Quantum efficiency monitoring. • Alignment without tracks. • Calibration & Alignment with tracks (data): • Alignment with tracks. • Refractive index. • Cherenkov angle resolution. • Particle ID calibration. RICH 2007

  3. The LHCb DetectorA forward single arm spectrometer Calorimeter RICH2 Muon Tracker RICH1 VELO Magnet RICH 2007

  4. Full acceptance (300 mrad horizontal, 250 mrad vertical) coverage. Two radiators: Aerogel, n=1.03 @ 540 nm. C4F10,n=1.0014 @ 400 nm. 4 composite spherical mirrors. 16 glass flat mirrors. 196 HPDs. Acceptance 120 mrad horizontal and 100 mrad vertical. CF4 radiator: n=1.0005 @ 400 nm. 56 glass spherical mirrors. 40 glass flat mirrors. 288 HPDs. RICH detector facts RICH1 & RICH2 HPD array • Two mirror (spherical and flat) system. • HPDs for photon detectors. • Magnetic shielding. • Gas radiators at atmospheric pressure. RICH1 RICH2 RICH 2007

  5. RICH system RICH2 RICH1 VerticalX-section Mirror Support Panel Spherical Mirror Y X Z HPD plane: 7 columns, 14 tubes each Flat Mirror Central Tube RICH 2007 Magnetic shield box Photon funnel+Shielding

  6. Calibration with projected patterns HPD schematic and picture • HPDs operate in the fringe field of the LHCb magnet. • Magnetic shielding is used to minimise the magnetic field, however, image distortions are still possible. • Test pattern will be projected on detector planes with magnet off and on to test for distortions and obtain calibration parameters. • RICH2 will use an off the self projector. RICH1 will scan an array of LEDs in front of the HPDs. • There are 3 PMTs in each HPD panel in RICH2 to locate the pattern. (G. Aglieri Rinella et al., NIMA 553 (2005) 120) RICH 2007

  7. Test pattern projection Finding clusters in the presence of non-uniform background. Highly attenuated light (2-3 hits/HPD/25 ns) Results from test setup with 1 HPD column. HPD 684 HPD 629 RICH 2007

  8. Center of Gravity Analysis Horizontal lines Immagine con fit lineari Rotated figure to fit vertical lines Use the same technique for HPD to HPD alignment. Can extract mirror orientation parameters in RICH2 by projecting the pattern via the mirrors. RICH 2007

  9. HPD QE monitor(with LED projector in RICH2) Use LED projector and PMTs to monitor HPD quantum efficiency PMT signal vs light HPD photon count vs light Blue LED light 3 different HPDs Green LED light Green LED light Light emitted (a.u.) Light emitted (a.u.) Amount of light varied using neutral density filters RICH 2007

  10. Alignment with tracks Mirror misalignment histograms RICH misalignment Cherenkov ring RICH 2007

  11. 2 mm Mirror alignment • Emission point of photons is not known. • Cannot correct mirror misalignment for photons that cannot identify the mirrors they were reflected on. • Mirror segments must form a uniform mirror. Mirror movement during transport and installation Initial mirror alignment (50 mrad) RICH 2007

  12. Test-beam Alignment (1) • C4F10 runs with rings that cover multiple HPDs. • Preliminary investigations into two runs: • Run 27 (ring on 3 HPDs). • Run 28 (ring on 4 HPDs). • Two effects to distinguish: • Global misalignment caused by mirror position. • Effects of misalignment of individual HPDs. HPD 282 Run 28 at Mirror Position 30 HPD 265 HPD 283 Run 27 at Mirror Position 29 HPD 222 RICH 2007

  13. Test-beam alignment (2) After 1st mirror alignment After mirror alignment and alignment of Si sensors according to data from the test centres HPD 283 HPD 265 HPD 222 HPD 283 HPD 282 HPD 265 HPD 222 RICH 2007

  14. T/B 2nd Mirror Alignment Sigma similar to N2 test-beam runs on single HPD (work in progress) RICH 2007 Cherenkov theta (rad)

  15. Cherenkov angle (RICH2, “Forward” tracks > 80 GeV) No Monte-Carlo information. Peak mean value gives refractive index Sigma gives Ch angle resolution Cherenkov angle resolution using MC info for photon-track association and particle type Correct photon-track association Sigma 0.64 mrad Sigma 0.80 mrad Wrong photon-track association RICH 2007

  16. RICH1 Cherenkov angle(“Forward” tracks > 80 GeV) Sigma 1.4 mrad Sigma 2.3 mrad Reconstructed Ch angle (rad) Ch angle resolution (rad) • Hardware monitoring for gas radiators: • Gas temperature (5 sensors in RICH1, 20 in RICH2, 0.2°C). • Pressure relative to atmospheric (0.1 mbar). • Speed of sound technique for gas purity (1%). RICH 2007

  17. “Golden” kinematics easy to suppress the background in order to obtain a clean sample: Mass difference (MD* - MD0)=145.4 MeV A dedicated D* trigger will provide about 107D*+  D0p+, D0 Kp events per year. RICH particle ID calibration (with particle samples selected independently of the RICH) A method to calibrate and study the performance of the RICH detector completely independent of the MC truth information using the D*+ D0p+, D0 K p decay chain (D*-D0) mass (GeV) A kaon and two pions with different momenta from each event can be used for RICH calibration. RICH 2007

  18. D* event selection cuts Over 90% D0 selected are true D0 • Bd→D*X sample ; • The cuts provide a very clean k and p sample (about 90% purity) . • Purity can be improved with reduced efficiency. SELECTION WITH KINEMATIC CUTS ONLY – NO RICH. Particles are attributed in turn K mass and p mass RICH 2007

  19. Comparison with MC truth K→K, p p->emp p→K, p K->emp Kaons and pions identified using MC truth Kaons and pions from the MC independent calibration sample Biases introduced by the method are negligible if the efficiency and the misidentification rates are considered both as a function of p and pt. RICH 2007

  20. With Pt cuts p->emp K→K, p p→K, p K->emp Kaons (pt>1 Gev) and pions (pt>1 GeV) identified using MC truth kaons and pions from the MC independent calibration sample (pt>1GeV), with slow pion pt>1 GeV DC06 sample; recent developments show significant improvement in overall RICH pID. RICH 2007

  21. Conclusions • The LHCb RICH system requires a number of calibration parameters to reach its full potential. • Alignment, refractive index, Cherenkov angle resolution. • These parameters can be extracted from data and the algorithms required have been implemented and tested. • Hardware monitoring will assist and confirm the calibration. • It is possible to evaluate the particle ID performance of the RICH system using particles of known type selected independently of the RICH. • An alignment challenge is expected in early 2008, where simulation data will be produced with the whole LHCb detector misaligned. RICH 2007

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