1/11

Measurement of Centrality
dependence of identified
hadron Elliptic Flow in √sNN
= 200 GeV Au+Au collisions
at RHIC - PHENIX
Hiroshi Masui / Univ. of Tsukuba
for theCollaboration
JPS, 62nd Annual Meeting
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Why Elliptic Flow ?
Zionpl
ane

Y

ctea

• Initial density + EOS• System size (numberof collisions)Pz

Px

JPS, Sep/24/2007• Sensitive probe in
the early stage ofheavy ion
collisions– Initial geometry
overlap (eccentricity)

RPyX

2/11– Final momentum
anisotropy (v2)

Hiroshi Masui / Univ. of Tsukuba

Universal Scaling of v2 ?

3/11

QM2006
A. Taranenko

At mid-rapidity

JPS, Sep/24/2007• Universal scaling of v2 suggest– v2 for a variety of particle species
at RHIC are determined through(ε, KET/nq)– Its magnitude is established inpartonic level before
hadronization

Hiroshi Masui / Univ. of Tsukuba

Motivation

4/11• Universal scaling of v2 has been observed byassuming– ε ∝ 〈v2〉 of non-identified charged hadrons– v2 ∝ F(KET/nq)• Questions– Are these assumptions really correct ?
– The scaling of v2 is tested from central to midcentral. How about peripheral collisions ?

• Study the validity of the scaling of v2 in a wide
range of centrality for identified hadrons
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

PHENIX Experiment

5/11• Vertex, Centrality– BBC, ZDC• Event plane– BBC (full azimuth, |η| = 3.0 - 3.9)
⇒ Large rapidity interval• Tracking– DC, PC

• PID
– TOF (|φ|<π/4, |η|<0.35), σt ~ 120ps• π, p : pT < 4 GeV/c, K : pT < 3GeV/c

JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Centrality dependence of v2(pT)• v2 increase with increasing centrality• Similar mass ordering for all centrality
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

6/11

KET + NCQ scaling of v2• Scaling holds for all centrality ?
– Ratio of data to fit (next slide)
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

7/11

v2(Data)/v2(Fit)

8/11• Scaling works within systematic errors, except for low KET– Radial flow ?
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Eccentricity

9/11• Eccentricity (ε) isestimated by
Glauber MC
simulation

Systematic error on ε ~ 10 - 20 %JPS, Sep/24/2007– ε value’s are notchanged so much inb = 5 - 9 fm (10 - 50% centrality). Δε/ε ~10 %– Quite different fordifferent definitionsof ε at central andperipheral

Hiroshi Masui / Univ. of Tsukuba

Eccentricity scaling

10/11• Scaled v2 withparticipant eccentricity• Start to break even in
mid-central at high KET– Scaling works at low
KET within systematicerrors• Clear difference
between central andperipheral– The difference of v2 isalso observed fordifferent eccentricity* Only statistical errors are shown* Systematic error on 〈ε〉 ~ 10 - 20 %JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Summary11/11• Study the scaling of v2(pT) for identified hadrons
in a wide range of centrality• KET + quark number scaling– Scaling holds within systematic errors, except for low
KET– New RXNP detector could help us to• reduce systematic errors• add more statistics at peripheral events• Eccentricity scaling– Scaling breaks even in mid-central at high KET• works for low KET within systematic errors– Clear difference between central and peripheral• Suggest that 〈v2〉 ∝ ε ⇒ 〈v2〉 ∝ ε × f(Npart)
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Back up

JPS, Sep/24/2007

12/11

Hiroshi Masui / Univ. of Tsukuba

Elliptic Flow at RHIC

13/11

• Substantial elliptic flow
signals are observed fora variety of particle
species at RHIC
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Event plane resolution• Event plane resolution is determined by multiplicity
and v2– Maximum at mid-central• High multiplicity, small v2 at central• Low multiplicity, large v2 at peripheral
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

14/11

Terminology

15/11• Definition of eccentricity– εstd : Standard eccentricity– εvar (participant eccentricity εpart)– ε2 (event plane eccentricity) : εvar,
subtract auto-correlation event-byevent (an idea from ShinIchi)• Weighting– εpart (εcoll)• Calculated by weighting with Npart(Ncoll) distribution• Averaging– ε{2} ≡ √〈ε2〉• Averaging of ε2 over all events,
then take square root
• More natural definition likemeasured v2• Total : 3 × 2 × 2 = 12 definitions

JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

Eccentricity• Estimated by Glauber MC simulation– εcoll > εpart due to steeper Ncoll distributions
compared to Npart
JPS, Sep/24/2007

Hiroshi Masui / Univ. of Tsukuba

16/11