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. Two RICH detectors provide
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 , 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
Silica Aerogel
n=1.03
1-10 GeV/c
C4F10 gas
n=1.0014
Up to ~70 GeV/c
CF4 gas
n=1.0005
Beyond ~100 GeV/c
Expected photon yields – for isolated saturated particles
Aerogel
C4F10
CF4
6.7
30.3
21.9
RICH1:
25250 mrad vertical
25 300 mrad horizontal
RICH2:
15100 mrad vertical,
15 120 mrad horizontal

8. The LHCb RICH Detectors4m 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 Carbon Fibre Mirrors: 1.5% radiation length
Spherical mirrors
Glass planar mirrors
Carbon Fibre Mirrors: 1.5% radiation length

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

18. The RICH with beam

19. Alignment Detector RICH1 RICH2 Si sensors 196 288 Photo-detectors
Mirrors 20 96

20. Magnetic field corrections
HPD Image distortion due to
magnetic field
Projection of test pattern without
and with magnetic field to extract
correction parameters
RICH2
Before
After
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. PID calibration samplesKs L
D from D*
Calibration data give unique p/K/p samples → allow PID performance in efficiency and purity to be evaluated with data

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 )
with kinematic cuts only
Without PID dominated by:

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

30. Conclusions There are exciting times ahead !
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 ~ 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. Magnetic Distortion calibration
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  B┴ B║
Axial
Transverse