The calibration and alignment of the LHCb RICH system Antonis Papanestis STFC - RAL for the LHCb Collaboration

RICH / 21 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 / 21 The LHCb Detector A forward single arm spectrometer RICH1 RICH2 VELO Magnet Tracker Calorimeter Muon

RICH / 21 RICH detector facts  Full acceptance (300 mrad horizontal, 250 mrad vertical) coverage.  Two radiators:  Aerogel, 540 nm.  C 4 F 400 nm.  4 composite spherical mirrors.  16 glass flat mirrors.  196 HPDs.  Acceptance 120 mrad horizontal and 100 mrad vertical.  CF 4 radiator:  400 nm.  56 glass spherical mirrors.  40 glass flat mirrors.  288 HPDs. Two mirror (spherical and flat) system. HPDs for photon detectors. Magnetic shielding. Gas radiators at atmospheric pressure. RICH1 & RICH2 RICH1RICH2 HPD array

RICH / 21 RICH system HPD plane: 7 columns, 14 tubes each Magnetic shield box Vertical X-section Spherical Mirror Flat Mirror Photon funnel+Shielding Central Tube Mirror Support Panel Z Y X RICH1 RICH2

RICH / 21 Calibration with projected patterns  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. HPD schematic and picture (G. Aglieri Rinella et al., NIMA 553 (2005) 120)

RICH / 21 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 629 HPD 684

RICH / 21 Center of Gravity Analysis Immagine con fit lineari Rotated figure to fit vertical lines Horizontal 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 / 21 HPD QE monitor (with LED projector in RICH2) PMT signal vs lightHPD photon count vs light Use LED projector and PMTs to monitor HPD quantum efficiency Light emitted (a.u.) Blue LED light Green LED light Amount of light varied using neutral density filters 3 different HPDs Green LED light

RICH / 21 Alignment with tracks Mirror misalignment histograms RICH misalignment Cherenkov ring

RICH / 21 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. 2 mm Mirror movement during transport and installation Initial mirror alignment (50 rad)

RICH / 21 Test-beam Alignment (1)  C 4 F 10 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 283 HPD 265 HPD 282 HPD 222 Run 27 at Mirror Position 29 Run 28 at Mirror Position 30

RICH / 21 Test-beam alignment (2) After 1 st mirror alignment After mirror alignment and alignment of Si sensors according to data from the test centres HPD 283 HPD 265HPD 222 HPD 283 HPD 222 HPD 265 HPD 282

RICH / 21 T/B 2 nd Mirror Alignment Sigma similar to N 2 test-beam runs on single HPD (work in progress) Cherenkov theta (rad)

RICH / 21 Cherenkov angle (RICH2, “Forward” tracks > 80 GeV) Cherenkov angle resolution using MC info for photon-track association and particle type No Monte-Carlo information. Peak mean value gives refractive index Sigma gives Ch angle resolution Correct photon-track association Wrong photon-track association Sigma 0.64 mradSigma 0.80 mrad

RICH / 21 RICH1 Cherenkov angle ( “Forward” tracks > 80 GeV )  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%). Ch angle resolution (rad) Reconstructed Ch angle (rad) Sigma 1.4 mradSigma 2.3 mrad

RICH / 21 RICH particle ID calibration (with particle samples selected independently of the RICH) “Golden” kinematics easy to suppress the background in order to obtain a clean sample: Mass difference (M D* - M D0 )=145.4 MeV A dedicated D* trigger will provide about 10 7 D*+  D 0  +, D 0  K events per year. A kaon and two pions with different momenta from each event can be used for RICH calibration. A method to calibrate and study the performance of the RICH detector completely independent of the MC truth information using the D *+  D 0  +, D 0  K  decay chain (D*-D 0 ) mass (GeV)

RICH / 21 D* event selection cuts D0D0 p , K (GeV) p t , K (GeV) IP sig. , K   D 0 vertex D 0 mass (GeV) p t D 0 (GeV) >2 >0.3 >3 <16 (1.84,1.89) D*D* IP sig. slow  Dist PV - D0 DVsig.   D * vertex p t D * (GeV) D * mass (GeV) mass ( D*- D 0 ) (GeV) >1. >6 < (1.990, 2.030) (0.1445, ) SELECTION WITH KINEMATIC CUTS ONLY – NO RICH. Particles are attributed in turn K mass and  mass B d →D*X sample ; The cuts provide a very clean k and  sample (about 90% purity). Purity can be improved with reduced efficiency. Over 90% D 0 selected are true D 0

RICH / 21 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 p t.  →K, p K→K, p  e   e  Comparison with MC truth

RICH / 21 Kaons (p t >1 Gev) and pions (p t >1 GeV) identified using MC truth kaons and pions from the MC independent calibration sample (p t >1GeV), with slow pion p t >1 GeV DC06 sample; recent developments show significant improvement in overall RICH pID. K→K, p  →K, p  e   e  With P t cuts

RICH / 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.