The calibration and alignmentof the LHCb RICH system

Antonis PapanestisSTFC - RAL

for the LHCb Collaboration

RICH 2007 2 / 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 2007 3 / 21

The LHCb DetectorA forward single arm spectrometer

RICH1

RICH2

VELO

Magnet

Tracker

CalorimeterMuon

RICH 2007 4 / 21

RICH detector facts

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.

• Two mirror (spherical and flat) system.• HPDs for photon detectors.• Magnetic shielding.• Gas radiators at atmospheric pressure.

RICH1 & RICH2

RICH1 RICH2

HPD array

RICH 2007 5 / 21

RICH system

HPD plane:7 columns,

14 tubes each

Magnetic shield box

VerticalX-section

Spherical Mirror

Flat Mirror

Photon funnel+Shielding

Central Tube

Mirror Support Panel

Z

Y

X

RICH1 RICH2

RICH 2007 6 / 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 2007 7 / 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 2007 8 / 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 2007 9 / 21

0

40

80

120

160

200

240

0 10 20 30 40 50 60 70 80 90 100 110

Light emitted [a.u.]

An

alys

is p

eak

[-m

V]

Blue

Green

HPD QE monitor(with LED projector in RICH2)

PMT signal vs light

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80 90 100 110

Light emitted [a.u.]

Po

isso

n m

ean

(va

riab

le e

rro

r)

HPD 628

HPD 629

HPD 684

HPD photon count vs light

Use LED projector and PMTs to monitor HPD quantum efficiency

Light emitted (a.u.) Light emitted (a.u.)

Blue LED light

Green LED light

Amount of light varied using neutral density filters

3 different HPDsGreen LED light

RICH 2007 10 / 21

Alignment with tracks

)sin()cos( chch BA

Mirror misalignment histogramsRICH misalignment

Cherenkovring

RICH 2007 11 / 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 2007 12 / 21

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 283

HPD 265

HPD 282

HPD 222

Run 27 at Mirror Position 29

Run 28 at Mirror Position 30

RICH 2007 13 / 21

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 222HPD 265HPD 282

RICH 2007 14 / 21

T/B 2nd Mirror Alignment

Sigma similar to N2 test-beam runs on single HPD (work in progress)

Cherenkov theta (rad)

RICH 2007 15 / 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 indexSigma gives Ch angle resolution

Correct photon-track association

Wrong photon-track association

Sigma 0.64 mrad Sigma 0.80 mrad

RICH 2007 16 / 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 mrad Sigma 2.3 mrad

RICH 2007 17 / 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(MD* - MD0)=145.4 MeV A dedicated D* trigger will provide about 107 D*+ D0 +, D0 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*+ D0 +, D0 K decay chain

(D*-D0) mass (GeV)

RICH 2007 18 / 21

D* event selection cuts

D0 p , K (GeV)pt , K (GeV)

IP sig. , K D0 vertex

D0 mass (GeV)pt D0 (GeV)

>2 >0.3 >3<16(1.84,1.89) 1.250

D* IP sig. slow Dist PV - D0 DVsig.D* vertexpt D* (GeV)

D* mass (GeV)mass ( D*- D0) (GeV)

>1.>6<161.250 (1.990, 2.030) (0.1445, 0.1465)

SELECTION WITH KINEMATIC CUTS ONLY – NO RICH. Particles are attributed in turn K mass and mass

Bd→D*X sample ; The cuts provide a very clean k and sample (about 90% purity) . Purity can be improved with reduced efficiency.

Over 90% D0 selected are true D0

RICH 2007 19 / 21

Kaons and pions identified using MC truthKaons and pions from the MC independent calibration sampleBiases introduced by the method are negligible if the efficiency and the misidentification rates are considered both as a function of p and pt.

→K, p

K→K, p e

e

Comparison with MC truth

RICH 2007 20 / 21

Kaons (pt>1 Gev) and pions (pt>1 GeV) identified using MC truthkaons and pions from the MC independent calibration sample (pt>1GeV), with slow pion pt>1 GeVDC06 sample; recent developments show significant improvement in overall RICH pID.

K→K, p

→K, p

e

e

With Pt cuts

RICH 2007 21 / 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.