TPC ExB distortion at LHC-ALICE experiment

Yasuto Horifor the ALICE-TPC collaboration

Center for Nuclear Study, University of Tokyo

1

Outline

2

• Introduction: ALICE-TPC and ExB distortion

• Calculation technique of ExB distortion

• Many kinds of ExB distortion • ExB Twist distortion

• Summary and Outlook

Overview of ALICE-TPC

100kVA sideROC

C side(muon-side)ROC

E field

• Main central tracking device • diameter x length = 5 m x 5 m,|h| < 0.9, 2p azimuthal coverage• Max drift length ~ 2.5 m, Central electrode ~100kV E ~ 400V/cm • gas Ne-CO2-N2(85.7%:9.5%:4.8%) low diffusion & high drift velocity(drift time 92msec), but strong temperature dependent drift velocity• temperature stability & homogenity < 0.1K cooling system and P,T monitor• UV laser system used for drift velocity, ExB, alignment

B field

beamIP

zgx

gyUV Laser tracks in TPC drift volume

3

ExB distortion in ALICE-TPC

E &B fieldDrift electrons

Track

Pad plane

Distorted drift electron

• Electrons are drifted at the E field direction if ExB is zero.• If ExB is non-zero Space point distortion on the Pad plane (or x-y, r- phi) occurs• This distortion may degrade DCA-to-Vertex distribution, DCA-btw-2tracks distribution, ITS-TPC matching efficiency,..

Gating Grid wire

Calculation technique via Langevin equation• Langevin eq. which describes this distortion can be decomposed into E field part and B field part if Ez Er,E≫ f & Bz Br,B≫ f• We have to know parameter T1, T2 of Ne:CO2:N2

Next slide

dzBzB

dzBzBr

cc

cc

dzEzE

dzEzEr

cc

cc

r

r

21

12

01

10

5

22

2)(1

10

Tc

22

1

1

)(11

TT

c

22

1

222

)(1

)(2

TT

c

E field distortion B field distortionSpace point distortion

]/[

sec]/[][10

cmVE

cmvkGausB d

BBEBEEEm

eu ˆ)ˆˆ()(]ˆˆ[ˆ

)(1

1 22

Decompose + wt tensor

Steady-state langevin eq

T1, T2 extraction via Laser track distortion with Gating Grid Voltage (GGV) Scan

Procedure1. Calibrate Voltage at GG plane by measuring C1&C0 using B=0 laser tracks ( C1&C0 must be 0 & 1 at B=0 ) 2. Measure change of track distortion with GGV scan at B=5kGaus3. Compare with calculation using T1 and T2

T1 = 1.0 +- 0.1, T2=1.0 +-0.3 is obtained!

Z [cm]Radi [cm]

6

Zigzag pattern of Real data is from Pad geometry

T1 = 1.0T2 = 1.0GGV= 10 V

drf [cm]

Radi [cm]

Dis

torti

on c

hang

e [c

m]

ExB distortion Sources and Models

7

• B field non-uniformity B field is measured, non-zero Br, Bf, corrected to ~0.3mm

• Twist between E and B field axis TPC is tilted from the B field axis

• E field distortion by misalignment of TPC components ( field cage, each rod, ROC, central electrode )• GGV Error from normal setting• E field distortion because of space charge

Next 3 slides

gX [cm]gY [cm]

drf [mm] dr [mm]

R [cm]

Z [cm]

B field

E field

TPC

Twist between E and B axis• space point distortion drift length and angle between E and B axis∝• track distortion is similar to translation, but A side track is translated to the opposite direction of C side track translation

gX[cm]

gX[cm]

gY[cm]

gY[cm]Z[cm]

Z[cm]

dr [cm]

dr f [cm]

Zdrift = 250 cm

Zdrift = 250 cm

8

ZdriftBy

x

cc

cc

dzBzBy

dzBzBx

cc

cc

y

x

21

12

21

12

R [cm]

R [cm]

Value to be measured

A/C vertex shift due to ExB Twist

• By ExB Twist Distortion, x-y position of vertex reconstructed by only A side tracks is shifted from that of only C side tracks

9

fast simulation full simulation (small statistics)

TPC pad plane

Distorted A trackOriginal trackDistorted C track

A/C Vertex shift

Inner wall

Outerwall

A/C vertex shift is almost proportional to Twist angle !!

Twist angles extraction and verification at p+p collision data

10

• A/C vertex shift can be found at pp collision data!• Position of A /C vertex are swapped with B field polarity• Primary vertex (by ITS&TPC all tracks) lays on the middle of the A/C vertex• Twist angles are extracted and verified !!

A vertex

A vertex

C vertex

C vertex

B positive data B negative data A Vertex C Vertex Primary Vertex

Non correctedVertex shift [cm]

Extractedqx[mrad]

Extractedqy[mrad]

B pos.

0.312 -2.3 -0.40

B neg.

0.33 -1.7 -0.94

CorrectedVertex shift [cm]

0.04

0.03

Summary and Outlook

11

• We construct framework for calculation of ExB distortion • T1&T2 values are extracted by Laser track distortion with GGV scan• List up many kinds of ExB distortion • ExB distortion due to B non-uniformity is already corrected• ExB Twist distortion is measured via A/C vertex shift in p+p collision real data

• measure A/C ROC rotation and translation, which causes A/C vertex shift• estimate how Rods shift (dominant part of E field distortion) causes A/C vertex shift• finalize Twist angles • ExB due to E field distortion like Rods shift, … • Space charge at PbPb collision data

- back up -

12

Misalignment between A side ROC & C side ROC

dY

dX

df

13

T1, T2 parameters measurement via Laser track distortion with Gating Grid Voltage (GGV) Scan 1 Change Gating Grid Voltage from normal setting ~ 70Volt Change “effective” voltage at GG plane “effective” means that voltage at GG plane depend on GGV setting itself as well as E field leakage from under GG plane E field distortion change ExB space point distortion change, which can be observed by laser tracks and calculated using T1 and T2 parameters

Z [cm]Radi [cm]

14

Zigzag pattern of Real data is from Pad geometry

T1 = 1.0T2 = 1.0GGV= 10 V

drf [cm]

Radi [cm]

Dis

torti

on c

hang

e [c

m]

T1, T2 parameters measurement via Laser track distortion with Gating Grid Voltage (GGV) Scan 2

• “effective” voltage at GG plane can be calculated by garfield simulation GGVeff [V] = 0.9175*GGVsetting [V]+ 3.91

• This can be confirmed by measuring C0 & C1

value at B=0 data, which must be 1 & 0 Reasonable agreement (left chi2 plot)

15

• With above relation, T1&T2 values are extracted by B=5kGaus laser data T1= 1.0 +- 0.1 T2= 1.0 +- 0.3 Good agreement with garfield simulation

1616

Sigma of DCA-to-Vertex distribution with 2 sets of correction parameters.

Black line is almost consistent to MC

With Old corr.With New corr.