Anisotropic Flow of Charged particles at High Transverse Momentum in 2.76TeV Pb-Pb Collisions in ALICE Experiment Anitha Nyatha (IITBOMBAY) for the ALICE COLLABORATION outline…. • Anisotropic Flow • Motivation • Experimental Methods • ALICE Experiment • Analysis procedure • Charged particle vn at high pT . • Identified particle vn at high pT . • Summary 7/6/12 1 Anisotropic Flow ∞ ⎞ d 3N 1 d 2 N ⎛ E 3 = ⎜1+ ∑ 2v n cos[n(φ − Φn )]⎟ d P 2π pt dpt dy ⎝ n =1 ⎠ v n ≡ cos n(φ − Φn ) € € spatial" anisotropy" momentum" anisotropy" 7/6/12 εn" vn" 2 Motivation • The Anisotropic flow develops at early times of heavy-ion collision and is a powerful tool to understand collision dynamics. • The large elliptic flow at RHIC and LHC provides evidence for strongly interacting matter, which appears to be almost perfect liquid. • The magnitude and transverse momentum dependence of vn is sensitive to a value of η/s (shear viscosity over entropy density ratio). Data suggest small η/s, a few times 1/4π. • The measurement of v2 , v3, v4 provides strong constraints on the profile of the initial energy density, fluctuations and η/s ratio. 7/6/12 3 • Time Projection Chamber (TPC): Tracking and particle identification • Time OF Flight : Particle identification 7/6/12 • VZERO detector: Centrality Selection 4 Event Selection Data: 2.76TeV Pb-Pb collisions Event Selection: 11M Minimum Bias events (0-80%) Abs(Zvertex) <10cm Track Selection: Pseudorapidity Range: |η| < 0.8 PT Range: 0.2 < PT < 20Gev/c Centrality selection : VZERO detector multiplicity fitted with Glauber model 7/6/12 5 Experimental Methods Event Plane Method: • Calculate Flow Vector, Q = (Qx, Qy): Qn,x = i i Qn,y = ∑ w i sin(nφ i ) i 1 −1⎛ Qn,y ⎞ ψ n = tan ⎜ ⎟ n Q ⎝ n,x ⎠ • Event plane angle: • Observed nth harmonic flow: € φ − ψ n )] v nobs = cos[n( • Event Plane Resolution: Rn = cos n(ψ n − Φn ) € € • Flow Coefficients: ∑ w cos(nφ ) € € Cumulants: i v nobs vn = Rn • 2-particle and 4-particle azimuthal correlations for an event: 2 ≡ cos n(φ i −€ φ j) 4 ≡ cos n(φ i + φ j − φ k − φ l ) φi ≠ φ j ≠ φk ≠ φl • Averaging over all events: € Cn {2} ≡ 2 = v n2 + δ n € 7/6/12 € Cn {4} = 4 − 2 2 € 2 = −v n4 6 € Analysis Procedure Flow components(v2, v3, v4) are calculated with event plane from VZERO detector and 4-­‐ parIcle cumulants. Event Plane Method: Charged parIcles from TPC |η|<0.8 are correlated with the event plane from VZERO-­‐A (2.8<η<5.1 )and VZERO-­‐C (-­‐3.7<η<-­‐1.7), the observed flow is corrected with event plane resoluIon. Event Plane Resolu2on: ResoluIon is calculated by three sub-­‐event plane method VZERO-­‐ A, VZERO-­‐C and TPC event planes. cos n(ψ na − Φn ) = cos n(ψ na − ψ nb ) cos n(ψ na − ψ nc ) cos n(ψ nb − ψ nc ) Where a= VZERO-­‐A (VZERO-­‐B), b=VZERO-­‐B (VZERO-­‐A) and c=TPC € The combined results from VZERO-­‐A and VZERO-­‐C are vn{EP, V0} ∑v vn = i ∑ (1/σ i 7/6/12 2 / σ n,i v n,i 2 v n,i ) σv2n = 1 ∑ (1/σ i 2 v n,i ) 7 Event Plane resolution 7/6/12 8 Charged particle Vn at High PT Ref:arXiv:1205.5761v1 • v2 at high pT (pT>8 GeV/c) is finite and positive with values increasing from central to mid-peripheral events • v2{EP} for pT>10 GeV/c reproduced by WHDG model calculations (W. A. Horowitz, M. Gyulassy, J. Phys. G 38, 124114 (2011)) • v3 is smaller than v2 in magnitude (except 0-5%) and exhibits much weaker centrality dependence. • v4(ψ4) does not depend strongly on centrality compared to v4(ψ2) • v4 at high pT (pT>8GeV/c) is consistent with zero within relatively large uncertainties 7/6/12 9 ParIcle IdenIficaIon (Proton and pion) at High PT • PID at low pT: • Asymmetric β-cut in TOF detector to select high purity π, Κ and P • Additional 2σ cut in th eTPC dE/dx to clean the data -Contamination <5% for π, Κ and P • PID at hight pT: • PID based on the ionization energy loss in the TPC: Δπ=dE/dx-<dE/dx>π • Select ranges where the contamination is small: -Pions: contamination <1% 7/6/12 -Protons: contamination <15% 10 Proton and pion v2 at High PT • (Anti-)Proton v2 and v3 higher than that of pion out to at least pT=8GeV/c • Charged pion v2 reproduced by WHDG π0 predictions for pT>7GeV/c • Charged pion v2 consistent with PHENIX π0 v2 (PRL 105, 142301 (2010)) 7/6/12 Ref:arXiv:1205.5761v1 11 Summary ALICE has measured charged particle flow up to 20GeV/c • v2 and v3 are finite positive and approximately constant for pT>8GeV/c • v4 for pT>8GeV/c is consistent with zero within the large uncertainties • v2 , v3, v4 behave as expected from fluctuations in the initial geometry, which provides new strong experimental constraints on η/s (at low pT) and initial conditions ALICE has measured identified particle flow up to pT=16GeV/c • 7/6/12 (Anti-)proton v2 and v3 higher than that of the pion at least out pT=8GeV/c 12