elliptic and higher order flow measured in a large phase...
TRANSCRIPT
Elliptic and higher order flow measured in a
large phase space in large phase space in
Pb+Pb Collisions2.76NN
s TeV=
Soumya Mohapatra
for the ATLAS Collaboration
Stony Brook University
• 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
Introduction and Motivation
dN
d φ∝1+ 2v
ncosn φ − Ψ
n( )n
∑Singles:
dN
d ∆φ∝1+ 2v
n
a vn
b cos n∆φ( )∑Pairs:
EP method
2PC method
2
• The harmonics vn carry information about the medium: initial geometry, η/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
d ∆φ∝1+ 2v
nv
ncos n∆φ( )
n
∑Pairs: 2PC method
ATLAS Detector3.3<|η|<4.8
3
• Tracking coverage : |η|<2.5
• FCal coverage : 3.3<|η|<4.8 (used to determine Event Planes)
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
|η|<2.5
Event Plane Method4
Bands indicate
systematic errors
Centrality dependence of vn
•5% Centrality bins + 0-1%
centrality bin
•v2 has a stronger centrality
dependence.
•Other vn are flatter.
5
•In most central collisions, v3,v4
can be larger than v2 at high
enough pT
pT Dependence of vnv
n
6
• Similar trend across all harmonics (increase till 3-4GeV then decrease)
• In most central collisions(0-5%): v3, v4 can be larger than v2.
Soumya Mohapatra : Stony Brook University
vn
vn scaling(v
n1
/n /
v2
1/2
)
7
• 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)
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
v2 Comparison to RHIC8
• Similar magnitude and pT dependence in overlapping pT range
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
|η|<1
η Dependence of vn
•weak dependence on
η (~5% drop within
acceptance)
9
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
• For Correlations:
relation is true only if
the η dependence is
weak
,
, v v va b a b
n n n n= ×
Two Particle ∆η−∆φ correlations
Near-side jet peak
is always visible
Ridge seen in
central and mid-
central collisions,
weak η
dependence
Ridge strength first
increases then
decreases with
10
Away side has
double hump
structure in most
central events
Peripheral events have near side peak truncated
decreases with
centrality
Peripheral events
have jet related
peaks only
Obtaining harmonics from correlationsa) The 2D correlation function in
∆η,∆φ.
b) The corresponding 1D
correlation function in ∆φ for
2<|∆η|<5 ( the |∆η| cut
removes near side jet)
c) The vn,n obtained using a
Discrete Fourier
11
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
Discrete Fourier
Transformation(DFT)
d) Corresponding vn values
( ) ( ), v = v ,a a a
n T n n T Tp p p
Bands indicate systematic errors
Comparison between the two methods 12
Centrality Dependence
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
pT Dependence of v2
13
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
�The 2PC vn for |∆η|<0.5 deviates from the EP results (for all pT)
� Good agreement seen for |∆η|>2 at pT<4 GeV.
�See deviations for pT>4 GeV even for |∆η|>2 due to increased away-side
jet contribution (which swings along ∆η).
pT Dependence of other vn
14
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
We see similar trend as v2
Recovering the correlations from EP vn
From 2PC method From EP method
C ∆φ( ) = b2P (1+ 2v1,1
2P cos ∆φ + 2 vn
EPvn
EP cosn∆φn=2
6
∑ )
� Chose v1,1 and
normalization to be
same as original
correlation function, but
all other harmonics are
15
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
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.
• 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 η/s.
• Noted that v2 doesn’t change drastically from RHIC to LHC
• Observed that the vn follow a simple scaling relation: vn1/n ∝ v2
1/2.
Summary16
∝
• Concluded that the features in two particle correlations for |∆η|>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
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
• 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:
BACKUP SLIDES
BACKUP SLIDES17
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
18
Vn of Charged Hadrons: from RHIC to LHC
vn(Ψn)
From
Arkadiy Taranenko
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
Arkadiy Taranenko
EPIC@LHC Talk
∆η dependence of vn
•Repeat procedure
in narrow ∆η slices
to obtain vn vs ∆η.
•vn values peak at
low ∆η, due to jet
bias.
19
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
•Relatively flat
afterwards, so we
require a |∆η| >2
gap (to remove
near-side jet).
Bands indicate systematic errors
v1,1
20
• Left panel: : v1(pTa) vs ∆η 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
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
Universality of vn
vn,n
pT
a ,pT
b( )=vn
pT
a( )vn
pT
b( )
vn
pT
a( ) = vn,n
pT
a ,pT
a( )
21
� vn,n is expected to factorize
into single vn for flow
� Obtain vn using “fixed pT”
correlations
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
vn
pT
b( )=v
n,np
T
a ,pT
b( )v
np
T
a( )
� cross-check via “mixed-pT”
correlation
� Indeed, vn,n factorizes! (above
certain ∆η)
v2 out to 20GeV
central
22
• Charged hadrons, pT=0.5-20 GeV, mid-rapidity, |η|<1
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
peripheral
pT evolution of ∆φ correlations 23
• pT evolution of two-particle ∆φ correlations for 0-10% centrality selection, with a large rapidity gap (|∆η|>2) to suppress the near-side jets and select only the long range components.
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
vn,n and vn vs ∆η for other centralities24
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
pT Dependence of v3 (2PC)25
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
pT Dependence of v4 (2PC)26
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
pT Dependence of v5 (2PC)27
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
ATLAS Detector3.3<|η|<4.8
28
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
• Tracking coverage : |η|<2.5
• FCal coverage : 3.3<|η|<4.8 (used to determine Event Planes)
• “Flipped side FCal” used to study η dependence (reduces Jet bias)
• The EP is determined with the Q-vector method using ET
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
Q vector example
,1
, ,
,
1cos( ) ; sin( ) ; tan ( )
y n
x n i i y n i i n
i i x n
QQ E n Q E n
n Qφ φ −= = Ψ =∑ ∑
29
• In Central and mid-central collisions and for higher harmonics, the ring blurs out
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
Two Particle Correlations30
� 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.
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
ForegroundPairs( )( )
MixedPairs( )C
φφ
φ
∆∆ =
∆
Recovering 0-1% correlation31
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
Recovering 0-5% correlation32
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration
33
Soumya Mohapatra : Stony Brook University : ATLAS Collaboration