Investigating the CME using the Event Shape Engineering technique in Pb-Pb collisions at snn = 2.76 TeV

Transcription

1 Investigating the CME using the Event Shape Engineering technique in Pb-Pb collisions at snn = 2.76 TeV Jaap Onderwaater for the ALICE collaboration QCD Workshop on Chirality, Vorticity and Magnetic Field in Heavy Ion Collisions March 28, 2017

2 Overview Introduction Analysis method Event Shape Engineering (ESE) technique Results for CME observable with ESE Model descriptions of CME and v2 Fit results for CME signal Summary & outlook 2

3 Chiral magnetic effect Strong magnetic field B B~1015 T for heavy ions at LHC QCD domains with P and CP symmetries locally broken momentum Charge separation perpendicular to the reaction plane spin Spin alignment along the magnetic field reversed momentum of left-handed quarks 3

4 CME observable Charge-dependent two particle correlation wrt reaction plane: Sensitive to <α2> + reaction plane dependent background sources e.g. local charge conservation with flow Idea: Vary magnitude of flow at fixed centrality to help disentangle signal and background contributions How: Employ event shape engineering technique Phys. Rev. Lett. 110 (2013)

5 Event shape engineering Event-by-event fluctuations result in a range of the magnitude v 2 for events with similar impact parameter (b) The q-vector is a proxy for the v2 magnitudes: 5

6 The ALICE detector Tracking and vertex determination with Inner Tracking System (ITS) and Time Projection Chamber (TPC) Forward multiplicity with scintillator arrays (V0) Triggering with V0 and ITS Data set: 12.5M MB Pb-Pb events at snn = 2.76 TeV Track selection: η < < pt < 5.0 GeV/c q2 measurement with V0C -3.7 < η < -1.7 Ψ2 measurement with V0A 2.8 < η < 5.1 6

7 q2 measurement q2 can be measured in different kinematic ranges TPC V0C fluctuations in q2 comparable to mean value small (large) q2 in one range shows correlation with small (large) q2 in another part of the event is a global event property Phys. Rev. C 93, (2016) 7

8 v2 with ESE By selecting on the estimate of q2 measured with V0C, the measured v2 at mid-rapidity can be made to vary ~25% from the unbiased selection Rapidity gap suppresses non-flow Event plane method; same plane Ψ2V0C as used in the γab measurement 8

9 v2 with ESE By selecting on the estimate of q2 measured with V0C, the measured v2 at mid-rapidity can be made to vary ~25% from the unbiased selection Rapidity gap suppresses non-flow Event plane method; same plane Ψ2V0C as used in the γab measurement Events are classified in 10% q2 bins 9

10 Charge-dependent correlations & ESE 3-particle correlator: γab = cos(φa +φb 2 Ψ2 ) a1,a a1,b +Bin plane Bout plane Correlators contain potential CME signal as well as background effects Background contributions in γ are ab suppressed at the level of ~v2 γab depends weakly on the event shape selection in a given centrality bin 10

11 Charge-dependent correlations & ESE 3-particle correlator: γab = cos(φa +φb 2 Ψ2 ) a1,a a1,b +Bin plane Bout plane 2-particle correlator: δab = cos(φa φb ) a1,a a1,b +Bin plane +Bout plane Correlators contain potential CME signal as well as background effects Background contributions in γ are ab suppressed at the level of ~v2 γab depends weakly on the event shape selection in a given centrality bin δab shows similar values for ESE and unbiased in a given centrality bin large non-flow contribution 11

12 Charge difference & ESE γab = cos (φa + φb 2Ψ2 ) γab (opp-same) quantifies the charge separation Difference is positive for all centrality classes and decreases with centrality and v2 (in a given centrality bin) 12

13 Charge difference & ESE γab = cos (φa + φb 2Ψ2 ) γab (opp-same) quantifies the charge separation Difference is positive for all centrality classes and decreases with centrality and v2 (in a given centrality bin) Scale by dn/dη to remove the dilution from combinatorics Difference approximately scales with v2 and multiplicity mostly background contribution Need to better understand observed charge dependence on v2 and q2 for signal and background dn/dη: ALICE, PRL 106, (2011) 13

14 MC study of v2 and B Use models to calculate for different centralities (impact parameter): v2 (ε2) distributions magnetic field component along 2nd order symmetry plane Glauber: M. Miller et al, ARNPS 57, 205 (2007) MC-Glauber, MC KLN CGC and EKRT are studied with parameters tuned to ALICE results Magnetic field is calculated at origin using spectators with the proper time τ=0.1 fm D. Kharzeev et al, NPA 803, 227 (2008) < B 2cos(2(ΨB-Ψ2 ))> represents the expected contribution of the CME to γab and shows a strong dependence on v2 14

15 Relating data and models γab (opp-same) vs v2 < B 2 cos(2(ψb -Ψ2 ))> vs v2 Glauber: M. Miller et al, ARNPS 57, 205 (2007) Fit γab (opp-same) and < B 2cos(2(ΨB-Ψ2))> with a linear function to disentangle the potential CME signal from background P1 (v2)= p0 (1+ p1 (v2 v2 )/ v2 ) 15

16 Slopes of data and model fits Glauber: M. Miller et al, ARNPS 57, 205 (2007) EKRT: H. Niemi et al, PRC 93, (2016) TRENTO: J. Moreland et al, PRC 92, (2015) KLN: H. Drescher et al, PRC 76, (2007) mckt code v.32, IC.htm Extract the CME fraction, fcme relating the slopes of data and model fits according to: fcme p1,mc +(1 fcme ) 1= p1,data Assumption: background contribution scales linearly with v2 and the corresponding slope is unity 16

17 CME fraction Glauber: M. Miller et al, ARNPS 57, 205 (2007) EKRT: H. Niemi et al, PRC 93, (2016) TRENTO: J. Moreland et al, PRC 92, (2015) KLN: H. Drescher et al, PRC 76, (2007) mckt code v.32, IC.htm CME fraction in 0-10% and 50-60% is currently statistically limited Combining the points from 10-50% despite possible centrality dependence gives: fcme (Glauber) = ± fcme (KLN) = ± fcme (EKRT) = ±

18 Summary Employed ESE technique for the first time to study charge-dependent correlations relative to the reaction plane (γab) γab (opp-same) approximately scales with v2 and multiplicity large background contribution Scaling of the CME contribution to γab is estimated using Monte Carlo simulations with magnetic field Scaling depends weakly on the initial state models CME fraction in γab for 10-50% is found to be fcme (Glauber) = ± fcme (KLN) = ± fcme (EKRT) = ±