TPC ExB distortion at LHC-ALICE experiment Yasuto Hori for 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 100kV A side ROC C side (muon-side) ROC E field Main central tracking device diameter x length = 5 m x 5 m,|  | < 0.9, 2  azimuthal coverage Max drift length ~ 2.5 m, Central electrode ~100kV  E ~ 400V/cm gas Ne-CO 2 -N 2 (85.7%:9.5%:4.8%)  low diffusion & high drift velocity(drift time 92  sec), 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 beam IP z gx gy UV Laser tracks in TPC drift volume 3

ExB distortion in ALICE-TPC E &B field Drift 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  & Bz ≫ Br,B  We have to know parameter T 1, T 2 of Ne:CO 2 :N 2  Next slide 5 E field distortionB field distortion Space point distortion Decompose +  tensor Steady-state langevin eq

T 1, T 2 extraction via Laser track distortion with Gating Grid Voltage (GGV) Scan Procedure 1.Calibrate Voltage at GG plane by measuring C 1 &C 0 using B=0 laser tracks ( C 1 &C 0 must be 0 & 1 at B=0 ) 2. Measure change of track distortion with GGV scan at B=5kGaus 3. Compare with calculation using T 1 and T 2  T 1 = , T 2 = is obtained! Z [cm] Radi [cm] 6 Zigzag pattern of Real data is from Pad geometry T 1 = 1.0 T 2 = 1.0 GGV= 10 V dr  [cm] Radi [cm] Distortion change [cm]

ExB distortion Sources and Models 7 B field non-uniformity  B field is measured, non-zero Br, B , 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] dr  [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] gY[cm] Z[cm] dr [cm] dr  [cm] Zdrift = 250 cm 8 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 track Original track Distorted C track A/C Vertex shift Inner wall Outer wall 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 C vertex B positive data B negative data A Vertex C Vertex Primary Vertex Non corrected Vertex shift [cm] Extracted  x[mrad] Extracted  y[mrad] B pos B neg Corrected Vertex shift [cm]

Summary and Outlook 11 We construct framework for calculation of ExB distortion T 1 &T 2 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

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Misalignment between A side ROC & C side ROC dY dX dd 13

T 1, T 2 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 T 1 and T 2 parameters Z [cm] Radi [cm] 14 Zigzag pattern of Real data is from Pad geometry T 1 = 1.0 T 2 = 1.0 GGV= 10 V dr  [cm] Radi [cm] Distortion change [cm]

T 1, T 2 parameters measurement via Laser track distortion with Gating Grid Voltage (GGV) Scan 2 “effective” voltage at GG plane can be calculated by garfield simulation  GGV eff [V] = *GGV setting [V] This can be confirmed by measuring C 0 & C 1 value at B=0 data, which must be 1 & 0  Reasonable agreement (left chi2 plot) 15 With above relation, T 1 &T 2 values are extracted by B=5kGaus laser data  T 1 = T 2 = Good agreement with garfield simulation

16 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.