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"
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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