The LHCb RICH detectors

1. The LHCb RICH detectors Neville Harnew University of Oxford On behalf of the LHCb RICH Collaboration IoP Tyndel-fest RAL, 1st July 2011

2. Outline of the talk • The LHCb Experiment & a brief UK history • The LHCb RICH detectors • RICH1 • RICH2 • The Hybrid Photon Detectors (HPDs) • LHCb data taking • RICH calibration and performance • Summary

3. The LHCb Detector Forward spectrometer (running in pp collider mode). A dedicated B-physics experiment at the LHC Acceptance Vertical 250mrad Horizontal 300mrad to 10 mrad Two RICH detectors provide p/K/p identification

4. LHCb detector : data-taking in progress RICH2 RICH1

5. Why Particle ID at LHCb ? • Crucial to identify the flavour of B decay products • Many decay modes have identical topology, e.g. B0 → +- and B0 → K+- • Flavour tagging Monte Carlo simulation of the invariant mass of B→hh decays

6. A brief history of UK LHCb RICH involvement • 1994 – Beauty 2004 conference (Mont St. Michel) : Cambridge, Imperial, Oxford first discussed joining the LHCb initiative. Cemented at Beauty 1995. • Between 1995-2000, RAL, Bristol, Glasgow & Edinburgh also joined. • These institutes formed the RICH project with CERN, Milano and Genoa (Liverpool also joined LHCb, but worked on the VELO project … which was later augmented by Glasgow, Warwick, Manchester and Birmingham) • LHCb was approved by the (then) PPARC in 2000 • The UK RICH team have been involved in all aspects of RICH 1 & 2 : the design, prototyping mechanical construction, electronics, DAQ, reconstruction, calibration etc.

7. Two RICHes, three radiators B meson decays Expected photon yields – for isolated saturated particles C4F10 gas n=1.0014 Up to ~70 GeV/c CF4 gas n=1.0005 Beyond ~100 GeV/c Silica Aerogel n=1.03 1-10 GeV/c RICH1: 25250 mrad vertical 25 300 mrad horizontal RICH2: 15100 mrad vertical, 15 120 mrad horizontal

8. The LHCb RICH Detectors 4m 1m

9. RICH1 schematic : “vertical” geometry Photon detector plane 14 by 7 Hybrid Photon Detectors (HPDs) Upper Magnetic Shielding Protects HPDs from B field, supports upper HPDs 4m Quartz Window Spherical Mirrors Lightweight carbon fibre mirrors 1.5% radiation length Glass Planar Mirrors VELO Exit Window 2mm aluminium.. Sealed to gas enclosure. No RICH entrance window. Beryllium beampipe (defines RICH1 inner acceptance) RICH1 Exit Window Carbon fibre & PMMI foam Sealed direct to the beampipe. Gas Enclosure supports mirrors and aerogel, contains C4F10 Lower Photon detector plane Mounted on lower shield Lower Magnetic Shielding mounted on cavern floor, supports lower HPDs and Gas Enclosure

10. RICH1 components Beryllium beampipe, VELO exit window and seal and planar mirrors Gas enclosure and mirrors installed in LHCb pit Gas Enclosure before installation

11. The RICH1 Mirrors Spherical mirrors Glass planar mirrors Carbon Fibre Mirrors:1.5% radiation length

12. 8m RICH2 schematic : “horizontal” geometry Flat Mirrors each made from 20 square glass segments Spherical Mirrors each made from 21 glass hexagonal segments Magnetic Shields protect the HPD planes HPD planes of 9 by 16 HPDs RICH2 entrance / exit windows carbon fibre and foam sandwich Gas Enclosure Contains CF4 gas radiator and the optical system

13. RICH2 to the pit-Nov 2005 • Mirrors aligned to 150 mrad before move • After move, mirror movement ~100 mrad • cf. RICH-2 Cherenkov angle resolution ~ 700 mrad

14. Pixel Hybrid Photon Detectors • Pixel HPDs developed in collaboration with industry (Photonis-DEP lead partner) • Combines vacuum technology with silicon pixel readout (quartz window with S20 photocathode). • 484 HPDs occupy a total area of 3.3m2 with 2.5 x 2.5 mm2 granularity • Factor 5 demagnification @ 18 kV. • Encapsulated 32x32 pixel silicon sensor Bump-bonded binary readout chip running @40 MHz • 200-600 nm wavelength coverage

15. HPDs continued • Excellent QE • Nevertheless ion feedback has been problematic

16. The HPD readout chain • All HPDs arranged in columns with ancillary front-end electronics • LV & HV boards power the HPDs • “Level-0” board passes triggered data to the “Level-1” off-detector board via an ~100m optical link • Level-1 board receives and zero-suppresses the data and passes to the DAQ HPD column assembly

17. pb-1 pb-1 2010 2011 2010/2011 LHCb Integrated Luminosity @ 7 TeV • Recorded 37.7 pb-1 in 2010 → 389 pb-1 so far in 2011 • Data taking efficiency > 90%

18. The RICH with beam

19. Alignment

20. Magnetic field corrections • HPD Image distortion due to magnetic field • Projection of test pattern without and with magnetic field to extract correction parameters Before After RICH2 Δx=0.18 pixel RICH1 Pixels Pixels

21. Cherenkov angle resolution (i) Single photon resolution, satuated tracks -Typical run σ= 1.62 mrad σ= 0.68 mrad Monte Carlo

22. Cherenkov angle resolution (ii) Resolution distribution of all 2010 runs Very good stability in time

23. Particle ID reconstruction • Global event likelihood algorithm (particle ID algorithm fits the event as a whole in both RICHes). • Likelihood function includes expected contributions from signal plus background for every pixel.

24. Cherenkov angle vs momentum Using isolated rings

25. Ks L PID calibration samples • Calibration data give unique p/K/p samples → allow PID performance in efficiency and purity to be evaluated with data D from D*

26. Multiple interactions Particle ID performance of the RICH detectors for different number of primary vertices Robust performance in a challenging environment

27. Effect of PID on f → K+ K- 900 GeV pp collisions with & without RICH 7 TeV with RICH

28. Physics analysis with the RICH Measurement of direct CP violation in charmless charged two-body B decays at LHCb (LHCb-CONF-2011-011) with kinematic cuts only Without PID dominated by:

29. Physics Analysis (ii) Measurement of the relative yields of the decay modes B0→D−π+ ,B0 → D−K+ , B0s → D−s π + (LHCb-CONF-2011-013) Very little combinatorial background

30. Conclusions The LHCb RICH detectors are operating well and the LHCb detector is extracting exciting physics results. The particle identification performance has been evaluated with data and is consistent with expectations. The RICH detectors have very robust performance in a high background and harsh environment. The ability to distinguish pions from kaons from protons over a wide momentum range using the RICH system is crucial for the B physics programme of LHCb There are exciting times ahead !

31. Spare slides

32. The RICH1 Aerogel Radiator • 16 aerogel tiles for RICH1. Produced by Boreskov Institute of Catalysis Novisibirsk • 200x200x50 mm tiles – the largest ever • n=1.03 ; gives p/K separation up to ~10 GeV/c • Exceptional clarity C ~ 0.005m4cm-1 [I/I0 = A exp –(Ct/l4) for thickness t] • Excellent homogeneity s(n-1)/(n-1) <1% • Tiles have undergone extensive ageing studies Test installation into RICH1

33. B┴  B║ • Magnetic fields distort the electron trajectories of the HPDs. • We see significant distortions in the LHCb RICHes (max 24 Gauss in RICH1) . • Tubes are individually shielded in mu-metal cylinders to mitigate these effects. • Below: RICH2 projection system Magnetic Distortion calibration Axial Transverse