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Systematic investigation of partonic collectivitythrough centrality dependence of elliptic flow

of multi-strange hadron in Au+Au collision at sNN =200 GeV in STAR

Quark Matter Washington D.C August 12-18, 2012

Outline• Introduction & motivation• STAR experiment at RHIC• Results• Summary

VECC

Md. Nasim(for the STAR Collaboration) Variable Energy Cyclotron Centre Kolkata , India

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Elliptic Flow (v2)

Reaction plane

Px

Py

Pz

Momentum anisotropy( v2)Initial spatial anisotropy ( ε2 )

x

y

Pressure gradient

P. Klob, U. W. HeinzNucl. Phys. A715, (2003) 653c

– Interactions among constituents transform the initial spatial anisotropy into momentum anisotropy

– Process quenches itself → sensitive to early times in the evolution of the system

– Sensitive to the equation of state

v2 cos(2( rp ))

Interactions

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Motivation Low interaction cross-section with other hadrons BW Model results support early freeze out of multi-strange hadrons Tk ~ Tch ~ Tc( Lattice QCD) v2( ϕ, Ξ, Ω) is a good probe of partonic collectivity

STAR Collaboration : Nuclear Physics A 757 (2005)

Partonic collectivity : Centrality dependence of NCQ scaling Partonic collectivity vs. System size.

v2(p)/v2(ϕ) > 1 ~ mass ordering of v2(hydro-like) < 1 ~ hadronic re-scattering effectComparison of v2(ϕ) with v2(p) can be used to study the effect of hadronic re-scattering.

Hadronic re-scattering effect :

STAR Collaboration: Phys.Rev.Lett.99 (2007) 112301

T Hirano et.al Phys. Rev. C 77, 044909 (2008)

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STAR Detectors

Full 2π coverage Excellent particle identification capabilities (Using TOF and TPC)

Data Set:Au+Au 200 GeV, collected in 2010Total # of analyzed events = 240 M

dE/dx clean PID: pion/kaon: pT ~ 0.6 GeV/c and proton pT~ 1.0 GeV/cWith TOF : pion/kaon: pT ~ 1.6 GeV/c and proton pT~ 3.0 GeV/c

p

K

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Multi-strange hadron & v2 measurement

Ω and Ξ are reconstructed by decay topology method + invariant mass method-mesons are reconstructed by invariant mass method

Ω -> Λ + K Ξ -> Λ + π ϕ -> K+ +K-

v2 vs. invariant mass method:

v2total (minv ) v2

Sig .Sig

SigBg(minv )v2

Bg .Bg

SigBg(minv )

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Centrality Dependence

Centrality dependence for multi-strange hadrons v2 observed

STAR Preliminary

Au+Au 200 GeV, η sub EP

|y | < 1.0

, Δη = 0.1 unit

Note : Only statistical errors

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v2 vs. mT-m0

Centrality 0-30% : Baryon–meson splitting at intermediate mT-m0.Centrality 30-80% : Deviations of multi-strange hadrons from baryons and mesons groups at intermediate mT-m0. Multi-strange hadron v2 smaller than other strange hadrons v2 at mT-m0>1 GeV/c2 (v2(Ξ)<v2(Λ), v2(ϕ)<v2(K0

S))

Ref: v2 of π, K , p, K0s and Λ – Poster Presentation by Hiroshi Masui (poster #145)

Au+Au 200 GeV, η sub EP

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Number-of-Constituent-Quark Scaling: pT/nq Au+Au 200 GeV, η sub EP

Centrality 0-30% : -meson is below from fit line 4 ± 7 % for pT/nq > 1 GeV/c.Centrality 30-80%: -meson is below from fit line 14 ± 5 % for pT/nq > 1GeV/c

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(mT-m0)/nq scalingAu+Au 200 GeV, η sub EP

Centrality 0-30% : -meson below from fit line 4 ± 6 % for (mT-m0)/nq > 0.6 GeV/c2.Centrality 30-80%: -meson below from fit line 11 ± 4 % for (mT-m0)/nq > 0.6 GeV/c2.

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Deviation from K0s

–meson seems to show larger deviation in 30-80% than 0-30%

STAR Preliminary

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Mass ordering of elliptic flow

Mass ordering observed in 0-80%, except between proton and -meson for pT < 1.0 GeV/c Similar trend is observed for 0-30% and 30-80% centrality.

STAR Preliminary

Au+Au 200 GeV, η sub EP

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v2()/v2(p) vs. pT

Au+Au 200 GeV, η sub EP

The effect is stronger in central collisions

T Hirano, Phys. Rev. C 77, 044909 (2008)

Hydro+Cascade

At low pT the v2() tends to be higher than v2(p) for central collisions. This trend is consistent with the hadronic re-scattering scenario as proposed in Phys. Rev. C 77, 044909 (2008).

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Summary

First systematic measurement of centrality dependence of multi- strange hadrons ( ϕ(ssbar),Ξ(dss), and Ω (sss)) at mid-rapidity in Au+Au collisions at √sNN=200 GeV.

v2()/v2(p) tends to be greater than unity at low pT. Consistent with hadronic re-scattering effect as proposed in theoretical model.

Multi-strange hadron v2 at 30-80% centrality is smaller than other strange hadrons v2 at mT-m0>1 GeV/c2 (v2(Ξ)<v2(Λ), v2(ϕ)<v2(K0

S)) –meson seems to show larger deviation in 30-80% than 0-30% in NCQ scaling.

May indicate smaller contribution to thecollectivity from partonic phase in peripheral collisions.

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Back-up

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Number-of-Constituent-Quark Scaling: pT/nq Au+Au 200 GeV, η sub EP

Particle Deviation from fit line in 0-30% for pT/nq > 1GeV/c

Deviation from fit line in 30-80% for pT/nq > 1 GeV/c

4 ± 7% 14 ± 5%

Λ 8 ± 3% 6 ± 5%

Ξ 14 ± 3% 16 ± 6 %

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(mT-m0)/nq scalingAu+Au 200 GeV, η sub EP

Particle Deviation from fit line in 0-30% for (mT-m0)/nq > 0.6 GeV/c2

Deviation from fit line in 30-80% for (mT-m0)/nq > 0.6 GeV/c2

4 ± 6% 11 ± 4%

Λ 4 ± 3 % 3 ± 4 %

Ξ 11 ± 4% 16 ± 4%