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Beam Energy Scan Program at RHIC Michal Šumbera

Nuclear Physics Institute AS CR, Řež/Prague

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World’s (second) largest operational heavy-ion colliderWorld’s largest polarized proton collider

RHIC BRAHMSPHOBOSPHENIX

STAR

AGS

TANDEMS

Relativistic Heavy Ion ColliderBrookhaven National Laboratory (BNL), Upton, NY

Animation M. Lisa

Year System sNN [GeV]

2000 Au+Au 130

2001 Au+Au 200

2002 p+p 200

2003 d+Au 200

2004 Au+Aup+p

200, 62.4200

2005 Cu+Cu 200, 62.4, 22

2006 p+p 62.4, 200, 500

2007 Au+Au 200

2008d+Aup+p

Au+Au

2002009.2

2009 p+p 200, 500

2010 Au+Au 200, 62.4, 39, 11.5, 7.7

2011 Au+Aup+p

200,19.6,27500

2012 U+UCu+Au

p+p

193200

200,510

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Recorded Datasets

Fast DAQ + Electron Based Ion Source + 3D Stochastic cooling

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– Perfect liquid BRAHMS, PHENIX, PHOBOS, STAR, Nuclear Physics A757 (2005)1-283

– Number of constituent quark scaling PHENIX, PRL 91(2003)072301; STAR, PR C70(2005) 014904

– Jet quenching PHENIX, PRL 88(2002)022301; STAR, PRL 90(2003) 082302

– Heavy-quark suppression PHENIX, PRL 98(2007)172301, STAR, PRL 98(2007)192301

– Production of exotic systems• Discovery on anti-strange nucleus STAR, Science 328 (2010) 58

• Observation of anti-4He nucleus STAR, Nature 473 (2011) 353

– Indications of gluon saturation at small x STAR, PRL 90(2003) 082302; BRAHMS, PRL 91(2003) 072305; PHENIX ibid 072303

Remarkable discoveries at RHIC

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Introducing sQGP …

Festschrift in honor of B.L. Ioffe,”At the Frontier of Particle Physics / Handbook of QCD”, M. Shifman, ed., (World Scientific).

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1st/2nd order

QCD Phase Diagram

Crossover

Particle Physics

~21012K

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… and how was it discovered

<Nbinary>/sinelp+p

Nucleus-nucleus yield

AA hadronsleadingparticle suppressed

q

q

?

NULL Result

If R = 1 here, nothing “new” is going on

Scaling AA to pp (or central to peripheral)

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FERMILAB-Pub-82/59-

THY

Phys.Lett.B243(1990)432

Au + Au Experiment d + Au Control Experiment

• Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control experiment.

New state of matter is produced in central Au+Au collisions at √sNN=200GeV

Suppresion of leading hadrons at RHIC

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…and at LHC

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arXiv:1210.4520v1

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Single hadron RAA: RHIC vs LHC

RAA

RAA for both systems looks similar

…and at LHC

For pT < 8 GeV/c: RAA for p and K are compatible and they are smaller than RAA for proton.For pT > 10 GeV/c: the RAA for p, K and proton are compatible within systematic error.

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LHC: Suppression of inclusive jets

15Like for charged particles, high-pT jet RAA flat at ≈ 0.5

Fully unfolded inclusive jet RAA pp 2.76 TeV reference

CMS-PAS HIN-12-004

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Dihadron azimuthal correlations at RHIC

Azimuthal distribution of hadrons with pT > 2 GeV/c relative to trigger hadron with pT

trig > 4 GeV/c (background subtracted). Data are from p+p, central d+Au and central Au+Au collisions.

STAR, PRL 90(2003) 082302

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Photon tag:• Identifies jet as u,d quark jet• Provides initial quark direction• Provides initial quark pT

Jet (98 GeV)

Photon(191GeV)

… and g+jet at LHC

1818

Elliptic flow: off-plane or in-plane

xx px

pyy

x px

py

px

pyy

x

v2 < 0: for 100 AMeV ≤ Ebeam ≤ 5 AGev slowly moving spectator matter prevents the in-plane emission of participating nucleons or produced pions which appear to be sqeezed-out of the reaction zone

J.-Y. OllitraultPRD 46 (1992) 229, PRD 48 (1993) 1132

v2 > 0: at higher energies shadowing disappears and interactions among produced particles generate in-plane emission

W. Greiner & Co.PRC 25 (1982) 1873

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Elliptic Flow and CollectivityPressure gradient

Spatial Anisotropy

Momentum Anisotropy

INPUT

OUTPUT

Interaction amongproduced particlesdN

/df

f0 2p

dN/d

f

f0 2p

2v2

x

y

f

Free Streamingv2 = 0

s

Initial spatial anisotropy

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Energy Dependence of Elliptic Flow

ALICE: PRL 105 (2010) 252302

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V2(pT): LHC vs.RHIC

The same flow properties from √sNN=200 GeV to 2.76 TeV

ALICE: PRL 105 (2010) 252302

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1) QuenchingAll hard hadronic process are strongly quenched 2) FlowPanta rhei: All soft particles emerge from the common flow field

The ‘Standard Model’ of high energy heavy ion collisions

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0) Turn-off of sQGP signatures

1) Search for the signals of phase boundary 2) Search for the QCD critical point

Why to go to lower energies?

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The RHIC Beam Energy Scan Project

A landmark of the QCD phase diagram

• Since the original design of RHIC (1985), running at lower energies has been envisioned

• RHIC has studied the possibilities of running lower energies with a series of test runs: 19.6 GeV Au+Au in 2001, 22.4 GeV Cu+Cu in 2005, and 9.2 GeV Au+Au in 2008

• In 2009 the RHIC PAC approved a proposal to run a series of six energies to search for the critical point and the onset of deconfinement.

• These energies were run during the 2010 and 2011 running periods.

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Selected Results

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RAA of neutral pions

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RAA(pT) of neutral pions

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RAA(pT) of neutral pions

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(0-5

%/6

0-80

%)

STAR Preliminary

Suppression of Charged Hadrons …

PRL 91, 172302 (2003)

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(0-5

%/6

0-80

%)

STAR Preliminary

… and its Disappearance

RCP ≥ 1 at √sNN ≤ 27 GeV - Cronin effect?

PRL 91, 172302 (2003)

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STAR Preliminary

RCP : Identified Particles

RCP (K0s) < 1 @ √sNN > 19.6 GeV

RCP > 1 @ √sNN ≤ 11.5 GeV For pT > 2 GeV/c:

• Baryon-meson splitting reduces and disappears with decreasing energy

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W/f ratio falls off at 11.5 GeV

STAR Preliminary

Baryon/Meson Ratio

v1(y) is sensitive to baryon transport, space - momentum correlations and QGP formation

Azimuthal Anisothropy

1

1 2 cos ( )n nn

dN v nd

Generated already during the nuclear passage time

(2R/g≈.1 fm/c@200GeV)

⇒ It probes the onset of bulk collective dynamics during thermalization

Directed flow is quantified by the first harmonic:

rapidity

<px> or directed flow

Directed flow is due to the sideward motion of the particles within the reaction plane.

(preequilibrium)34

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STAR Preliminary

v 1Directed Flow of p and π

Mid-central collisions:Pion v1 slope: Always negative (7.7-39 GeV)(Net)-proton v1 slope: changes sign between 7.7 and 11.5 GeV - may be due to the contribution from the transported protons coming to midrapidity at the lower beam energies

p π

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Energy Dependence of v2

• The rate of increase with collision energy is slower from 7.7 to 39 GeV compared to that between 3 to 7.7 GeV

ALICE: PRL 105, 252302 (2010)PHENIX: PRL 98, 162301 (2007) PHOBOS: PRL 98, 242302 (2007) CERES: Nucl. Phys. A 698, 253c (2002).E877: Nucl. Phys. A 638, 3c(1998). E895: PRL 83, 1295 (1999). STAR 130 Gev:

Phys.Rev. C66,034904 (2002).STAR 200 GeV:

Phys.Rev. C72,014904 (2005).

STAR Preliminary

STAR, ALICE: v2{4} resultsCentrality: 20-30%

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v2(pT): First Result

STAR: Nucl.Phys. A862-863(2011)125

v2 (7.7 GeV) < v2 (11.5 GeV) < v2 (39 GeV) v2 (39 GeV) ≈ v2 (62.4 GeV) ≈ v2 (200 GeV) ≈ v2 (2.76 TeV)

⇒ sQGP from 39 GeV to 2.76 TeV

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v2(pT): Final ResultSTAR Coll.: e-Print arXiv:1206.5528

For pT < 2 GeV/c: v2 values rise with increasing √sNN For pT ≥ 2 GeV/c: v2 values are (within stat. errors) comparableThe increase of v2 with √sNN,could be due to change of chemical composition and/or larger collectivity at higher collision energy.

ALICE data: PRL 105, 252302 (2010)

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Corresponding anti-particlesParticles

v2 vs. mT-m0

Baryon–meson splitting is observed when collisions energy ≥ 19.6 GeV for both particles and the corresponding anti-particles For anti-particles the splitting is almost gone within errors at 11.5 GeV

STAR Preliminary

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Particles vs. Anti-particles Beam energy ≥ 39 GeV• Δv2 for baryon and anti-baryon within 10%• Almost no difference for mesons Beam energy < 39 GeV• The difference of baryon and

anti-baryon v2

→ Increasing with decrease of beam energy

At √sNN = 7.7 - 19.6 GeV • v2(K+)>v2(K-) • v2(π-) >v2(π+) Possible explanation(s)• Baryon transport to midrapidity?

ref: J. Dunlop et al., PRC 84, 044914 (2011)• Hadronic potential? ref: J. Xu et al., PRC 85, 041901 (2012)

The difference between particles and anti-particles is observed

STAR Preliminary

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Universal trend for most of particles – ncq scaling not broken at low energies ϕ meson v2 deviates from other particles in Au+Au@(11.5 & 7.7) GeV: ~ 2σ at the highest pT data point

Reduction of v2 for ϕ meson and absence of ncq scaling during the evolution the system remains in the hadronic phase [B. Mohanty and N. Xu: J. Phys. G 36, 064022(2009)]

NCQ Scaling Test

Particles STAR Preliminary

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Accessing Phase Diagram

T-mB:From spectra and ratios

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p, K, p Spectra

STAR Preliminary

Slopes: p > K > p. Proton spectra: without feed-down correctionp,K,p yields within measured pT ranges: 70-80% of total yields

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STAR PreliminarySTAR Preliminary

Strange Hadron SpectraX

Au+Au 39 GeVAu+Au 39 GeV

K0s L

Au+Au 39 GeV

f, K0s: Levy function fit

L, X : Boltzmann fit L: feed-down corrected

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STAR Preliminary

Chemical Freeze-out Parameters

Centrality dependence of freeze-out temperature with baryon chemical potential observed for first time at lower energies

THERMUS* Model:Tch and mB

Particles used: p, K, p, L, K0

s, X

S. Wheaton & J.Cleymans, Comp. Phys. Com. 180: 84, 2009.

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STAR Preliminary

Au+Au

Kinetic Freeze-out Parameters

Higher kinetic temperature corresponds to lower value of average flow velocity and vice-versa

Blast Wave: Tkin and <b>

Particles used: p,K,p

STAR Preliminary

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Beam Energy Scan Phase- II

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1% Au target

A. Fedotov, W. Fischer, private discussions, 2012.

BES Phase-II proposal Electron cooling will provide increased luminosity ~ 10 times Proposal BES-II (Years 2015-2017):

√sNN [GeV] μB [MeV] Requested Events(106)

Au+Au 19.6 206 150

Au+Au 15 256 150

Au+Au 11.5 316 50

Au+Au 7.7 420 70

U+U: ~20 ~200 100

- Annular 1% gold target inside the STAR beam pipe - 2m away from the center of STAR- Data taking concurrently with collider mode at beginning of each fill

No disturbance to normal RHIC running

Fixed Target Proposal:

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Fixed Target Set-up

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BES Program Summary

206 5851120 420

2.557.719.639

775

√sNN (GeV)

mB (MeV)

QGP

pro

perti

es

BES

phas

e-I

Test

Run

Fixe

d Ta

rget

BES

phas

e-II

Large range of mB in the phase diagram !!!

Explore QCD Diagram

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SummaryResults from BES program covering large mB range

provide important constraint on QCD phase diagram.

Different features show up:– Proton v1 slope changes sign between 7.7 GeV and 11.5 GeV– Particles-antiparticles v2 difference increases with decreasing √sNN

– f-meson v2 deviates from others for √sNN ≤ 11.5 GeV

Search for the critical point continues:- Proposed BES-II program - Fixed target proposal to extend mB coverage up to 800 MeV

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Back up

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Chemical Freeze-out : Inelastic collision ceases Particle ratios get fixed

★THERMUS : Statistical thermal model Ensemble used – Grand Canonical and Strangeness Canonical

To consider incomplete strangeness equilibration:

Extracted thermodynamic quantities: Tch, mB, ms and gS •Thermus, S. Wheaton & Cleymans, Comput. Phys. Commun. 180: 84-106, 2009.

For Grand Canonical: Quantum numbers (B, S, Q) conserved on average

For Strangeness Canonical: Strangeness quantum number (S) conserved exactly

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Kinetic Freeze-out : Elastic collision ceases Transverse momentum spectra get fixed Blast Wave : Hydrodynamic inspired model

Extracted thermodynamic quantities: Tkin and <β>

E. Schnedermann et al., Phys. Rev. C 48, 2462 (1993)

Particle spectra are fitted simultaneously

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Lattice Gauge Theory (LGT) prediction on the transition temperature TC is robust.

LGT calculation, universality, and models hinted the existence of the critical point on the QCD phase diagram* at finite baryon chemical potential.

Experimental evidence for either the critical point or 1st order transition is important for our knowledge of the QCD phase diagram*.

* Thermalization has been assumed M. Stephanov, K. Rajagopal, and E. Shuryak,

PRL 81, 4816(98); K. Rajagopal, PR D61, 105017 (00) http

://www.er.doe.gov/np/nsac/docs/Nuclear-Science.Low-Res.pdf

The RHIC Beam Energy Scan Motivation

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STAR Preliminary

K/p

Particle Ratio Fluctuations

Monotonic behavior of particle ratio fluctuations vs. √sNN

STAR Preliminary

STAR Preliminary

p/p

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Higher Moments: Net-protons

0-5% central collisions: Deviations below Poisson observed for √sNN > 7.7 GeV Peripheral collisions: Deviations above Poisson observed for √sNN < 19.6 GeV Higher statistics needed at 7.7 GeV and 11.5 GeV and possibly a new data point around ~15 GeV

s /

S s ~ /

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Higher Moments: Net-charge

s /

S s ~ /

Data lies in between Poisson and HRG model expectations

Higher statistics needed at 7.7 GeV and 11.5 GeV and possibly a new data point around ~15 GeV

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(C) Searching QCD Critical Point

√sNN

observab

le Enhanced Fluctuationsnear Critical Point

T. Andrews. Phil. Trans. Royal Soc., 159:575, 1869

CO2 nearliquid-gas transition

Particle ratio fluctuations (2nd moments) - K/p, p/p, K/p Conserved number fluctuations - Higher moments of net-protons, net-charge,..

Peak magnetic field ~ 1015 Tesla ! (Kharzeev et al. NPA 803 (2008) 227)

CSE + CME → Chiral Magnetic Wave: • collective excitation• signature of Chiral Symmetry Restoration

RPaddN

ff

sin21

A direct measurement of the P-odd quantity “a” should yield zero.

S. Voloshin, PRC 70 (2004) 057901

Directed flow: expected to be the same for SS and OS

Non-flow/non-parity effects:largely cancel out P-even quantity:

still sensitive to charge separation

Chiral Magnetic effect + Local Parity Violation

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TPC:Detects Particles in the |h|<1 rangep, K, p through dE/dx and TOFK0

s, L, X, W, f through invariant mass

Coverage: 0 < f < 2p |h| < 1.0Uniform acceptance: All energies and particles

M. Šumbera NPI ASCR 63

Detector performance generally improves at lower energies.

Geometric acceptance remains the same, track density gets lower.Triggering required effort, but was a solvable problem.

Year √sNN [GeV] events(106)

2010 39 130

2011 27 70

2011 19.6 36

2010 11.5 12

2010 7.7 5

2012* 5 Test Run

BES-I Data:

Central Au+Au at 7.7 GeV in STAR TPC

Uncorrected Nch

dNev

t / (N

evt d

Nch

)BES Data Taking

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STAR TPC - Uniform Acceptance over all RHIC EnergiesAu+Au at 7.7 GeV Au+Au at 39 GeV Au+Au at 200 GeV

Crucial for all analyses

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Particle IdentificationPID (TPC+TOF):π/K: pT~1.6 GeV/cp: pT~3.0 GeV/cStrange hadrons: decay topology & invariant mass

TPC TPC+TOF

Au+Au 39 GeV

dE/d

x (M

eV/c

m)

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Charged Hadrons v1: Beam Energy Dependence

Scaling behavior in v1 vs. η/ybeam and v1 vs. η’=η-ybeam

Data at 62.4&200GeV from STAR, PRL 101 252301 (2008)

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Cou

nts

Cou

nts

Cou

nts

Cou

nts

Cou

nts

Cou

nts

2.94 2.96 2.98 2.94 2.96 2.98

Minv(He3+p )(GeV)2.94 2.96 2.98

Minv(He3+p )(GeV) Minv(He3+p )(GeV)3 3.02 3.04 3.06 3.08 3.1-

02.94 2.96 2.98

60

40

20

100

80

140

120

160Signal

2 / ndf 64.2 / 34Yield 46.43 16.34Mean 2.991 0.001

Run11 27 GeV minbias

3 3.02 3.04 3.06 3.08 3.1Minv(He3+p-)(GeV)

0

60

40

20

120

100

80

160

140

Signal2 / ndf 25.8 / 32Yield 45.57 17.35Mean 2.991 0.001

Run10 7.7 GeV minbias

3 3.02 3.04 3.06 3.08 3.1-

0

10080

60

40

20

160

140

120

220

200

180

240Signal

2 / ndf 41.1 / 32Yield 88.12 20.98Mean 2.992 0.002

Run10 39 GeV minbias

3 3.02 3.04 3.06 3.08 3.1Minv(He3+p-)(GeV)

0

60

40

20

120

100

80

160

140

Signal2 / ndf 28.5 / 32Yield 41.18 17.29Mean 2.991 0.001

Run10 11.5 GeV minbias

3 3.02 3.04 3.06 3.08 3.1-02.94 2.96 2.98

100

50

150

200

250Signal

2 / ndf 75.3 / 34Yield 82.91 20.32Mean 2.991 0.000

Run10 200 GeV minbias

3 3.02 3.04 3.06 3.08 3.1Minv(He3+p-)(GeV)

02.94 2.96 2.98

40

20

80

60

100

120Signal

2 / ndf 60.7 / 34Yield 42.11 14.00Mean 2.991 0.001

Run11 19 GeV minbias

STAR PreliminarySignal

rotated backgroundsignal+background fit

STAR Preliminarysignal

rotated backgroundsignal+background fit

STAR PreliminarySignal

rotated backgroundsignal+background fit

STAR PreliminarySignal

rotated backgroundsignal+background fit

STAR Preliminarysignal

rotated backgroundsignal+background fit

STAR Preliminarysignal

rotated backgroundsignal+background fit

Hypertriton Production

H + H produced at √sNN = 7.7, 11.5, 19.6, 27, 39, 200 GeV (minbias)3L

3L

_

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Phase Boundary Search With Nuclei

Needs higher statistics to make conclusive statement

Strangeness Population Factor:

Beam energy dependence of S3 behaves differently in QGPand pure hadron gas

- S. Zhang et al., PLB 684 (2010) 224

- J. Steinheimer et al.,PLB 714 (2012) 85

S3 indicates (with 1.7σ )

an increasing trend

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With 1st

order P.T.Without 1st

Order P.T.

Time evolution of the collision geometry

Kolb and Heinz, 2003, nucl-th/0305084

Initial out-of-plane eccentricity Stronger in-plane pressure gradients drive preferential in-plane expansion Longer lifetimes or stronger pressure gradients cause more expansion and more spherical freeze-out shape

We want to measure the eccentricity at freeze out, εF, as a function of energy using azimuthal femtoscopic radii Rx and Ry:

Evolution of the initial shape depends on the pressure anisotropy ● - Freeze-out eccentricity sensitive to the 1st order phase transition.

Non-monotonic behavior could indicate a soft point in the equation of state.

Spatial eccentricity

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Azimuthal HBT: First result

Is there a non-monotonic behavior?

sNN (GeV)

J. Phys. G: Nucl. Part. Phys. 38 (2011) 124148

x

71Is the discrepancy due to centrality or rapidity range? - NO

-1.0<y<-0.5-0.5<y<0.50.5<y<1.0

Azimuthal HBT: More Data

…and at LHC

ALICE, Phys.Lett. B696 (2011)30

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