soumya mohapatra for the atlas collaboration stony brook university

Measurement of elliptic and higher order flow at Pb+Pb Collisions

with the ATLAS detector2.76NNs TeV

Soumya Mohapatrafor the ATLAS CollaborationStony Brook University

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• Initial spatial fluctuations of nucleons lead to higher moments of deformations in the fireball, each with its own orientation.

• The spatial anisotropy is transferred to momentum space by collective flow

• The harmonics vn carry information about the medium: initial geometry, h/s

• Understanding of higher order vn can shed light on the physics origin of “ridge” and “cone” seen in 2P correlations

Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

Introduction and Motivation

Singles:

Pairs:

EP method

2PC method

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• Tracking coverage : |h|<2.5• FCal coverage : 3.3<|h|<4.8 (used to determine Event Planes)

ATLAS Detector


|η|<2.5

3.3<|η|<4.8

4Event Plane Method

Bands indicate systematic errors

5Centrality dependence of vn

•5% Centrality bins + 0-1% centrality bin

•v2 has a stronger centrality dependence.

•Other vn are flatter.

•In most central collisions, v3,v4 can be larger than v2 at high enough pT

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• Similar trend across all harmonics (increase till 3-4GeV then decrease)• In most central collisions(0-5%): v3, v4 can be larger than v2.

pT Dependence of vn

Soumya Mohapatra : Stony Brook University

v nv n

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• Observe scaling: vn1/n =kv2

1/2, where “k” is only weakly dependent on pT.• R.Lacey et al. (http://arxiv.org/abs/1105.3782)

vn scaling


(vn 1

/n /v

21/2 )

8vn from RHIC to LHC


FromXiaoyang GongQuark Matter 2011 Talk

vn(Ψn) vn(Ψn)

9h Dependence of vn


•weak dependence on h (~5% drop within acceptance)

• For Correlations:

relation is true only if the h dependence is weak

,, v v va b a bn n n n

10Two Particle Dh-Df correlationsNear-side jet peak is always visible

Ridge seen in central and mid-central collisions, weak h dependence

Away side has double hump structure in most central events

Peripheral events have near side peak truncated

Ridge strength first increases then decreases with centrality

Peripheral events have jet related peaks only

11Obtaining harmonics from correlations


a) The 2D correlation function in Dh,Df.

b) The corresponding 1D correlation function in Df for 2<|Dh|<5 ( the |Dh| cut removes near side jet)

c) The vn,n obtained using a Discrete Fourier Transformation(DFT)

d)Corresponding vn values

( ( , v = v ,a a an T n n T Tp p p


12Dh dependence of vn


•Repeat procedure in narrow Dh slices to obtain vn vs Dh.

•vn values peak at low Dh, due to jet bias.

•Relatively flat afterwards, so we require a |Dh| >2 gap (to remove near-side jet).


13Universality of vn


vn,n is expected to factorize into single vn for flow

Obtain vn using “fixed pT” correlations

cross-check via “mixed-pT” correlation

Indeed, vn,n factorizes! (above certain Dh)

14Comparison between the two methods


Centrality Dependence

15pT Dependence of v2


The 2PC vn for |Dh|<0.5 deviates from the EP results (for all pT)

Good agreement seen for |Dh|>2 at pT<4 GeV.

See deviations for pT>4 GeV even for |Dh|>2 due to increased away-side jet contribution (which swings along Dh.

16pT Dependence of other vn


We see similar trend as v2

17Recovering the correlations from EP vn


From 2PC method From EP method

Chose v1,1 and normalization to be same as original correlation function, but all other harmonics are from EP analysis.

Correlation function is well reproduced, ridge and cone are recovered!

Common physics origin for the near and away-side long range structures.

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• Measured v2-v6 by both correlation and event-plane analysis. – Significant and consistent v2-v6 were observed by the two methods. – Measured in phase space much larger than at RHIC.– Each vn can act as independent cross-check for h/s.

• Noted that v2 doesn’t change drastically from RHIC to LHC

• Observed that the vn follow a simple scaling relation: vn1/n v∝ 2

1/2.

• Concluded that the features in two particle correlations for |Dh|>2 at low and intermediate pT (pT<4.0GeV) can be accounted for by the collective flow of the medium. – Double hump and ridge arise due to interplay of even and odd

harmonics

Summary


• ATLAS vn analysis note : http://cdsweb.cern.ch/record/1352458• ATLAS HI public results page : • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsPublicResults

For more results see:

http://cdsweb.cern.ch/record/1352458

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BACKUP SLIDES

BACKUP SLIDES


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• Left panel: : v1(pTa) vs Dh for four fixed-pT correlations.

– We see that v1 ,1(pTa ) can become negative showing eta dependence of v1

• Right panel: v1(pTb) vs for target pT in (1.4,1.6) GeV

– We see that v1(pTb) depends on pT

a showing the breakdown of the scaling relation

Breakdown of v1,1 scaling


21Universality of vn


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• Similar magnitude and pT dependence in overlapping pT range

v2 Comparison to RHIC


|η|<1

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• Charged hadrons, pT=0.5-20 GeV, mid-rapidity, |η|<1

v2 out to 20GeV


central

peripheral

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• pT evolution of two-particle Df correlations for 0-10% centrality selection, with a large rapidity gap (|Dh|>2) to suppress the near-side jets and select only the long range components.

pT evolution of Df correlations


25vn,n and vn vs Dh for other centralities


26pT Dependence of v3 (2PC)


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• The EP is determined with the Q-vector method using E T flow in FCal

• In mid-central collisions, the Q2 vector is distributed in a ring-like structure indicating the excellent ability of the FCal in determining the reaction plane

• In Central and mid-central collisions and for higher harmonics, the ring blurs out

Q vector example


,1, ,

,

1cos( ) ; sin( ); tan ( )y nx n i i y n i i n

i i x n

QQ E n Q E n

n Qf f -

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The correlations are constructed by dividing foreground pairs by mixed background pairs.

Mixed background pairs account for detector acceptance. Final correlation contains only physical effects.

The detector acceptance causes fluctuations ~ 0.001 in the foreground pairs, which mostly cancels out in the ratio.

Two Particle Correlations


Foreground Pairs( )( )Mixed Pairs( )

C fffD

D D

31Recovering 0-1% correlation


32Recovering 0-5% correlation


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soumya mohapatra for the atlas collaboration stony brook university

Documents

higher order flow

collective flow

higher moments of deformations

momentum space

harmonics vn

spatial anisotropy

large phase space

motivation singles