Recent highlights from STAR

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Recent highlights from STAR

WangmeiZha1,_{for the STAR Collaboration}

1_{Department of modern physics, University of Science and Technology of China, Hefei, 230026, China}

Abstract.The Solenoidal Tracker at RHIC (STAR) experiment takes advantage of its excellent tracking and particle identification capabilities at mid-rapidity to explore the properties of strongly interacting QCD matter created in heavy-ion collisions at RHIC. The STAR collaboration presented 7 parallel and 2 plenary talks at Strangeness in Quark Matter 2017 and covered various topics including heavy flavor measurements, bulk ob-servables, electro-magnetic probes and the upgrade program. This paper highlights some of the selected results.

1 Introduction

Relativistic heavy-ion collisions provide a unique tool to study the most intriguing aspects of the strong interaction in the laboratory. In high energy heavy-ion collisions performed at RHIC and the LHC, the achieved energy densities are extremely high so that a new form of matter, referred to as the quark-gluon plasma (QGP) [1], can be created. This matter has been observed to quickly thermalize and evolve as a perfect fluid with small viscosity over entropy ratio. The STAR heavy-ion program at top RHIC energies is aimed to quantitatively study the properties of the strongly-interacting QGP matter created at vanishingly small baryon density. While at finite baryon density from the Beam Energy Scan (BES) phase-I program at lower energies, the nature of the phase transition is being studied.

At Strangeness in Quark Matter 2017, the STAR collaboration has presented results from two recently installed detectors for heavy-flavor studies, the Heavy Flavor Tracker (HFT) and Muon Tele-scope Detector (MTD). The HFT, which facilitates topological reconstruction of open heavy flavor hadrons, has enabled the first measurement ofD0_{azimuthal anisotropy at RHIC, which helps constrain}

the charm quark spatial diffusion coefficient in the QGP. The MTD, which enables identification and

triggering of muons, opens the door to measure quarkonia via the di-muon decay channel at STAR, providing insights to the understanding of the dissociation and regeneration phenomena of quarkonia in the QGP. With the full BES phase-I datasets, the collectivity of medium has been systematically studied via directed, elliptic and higher order harmonics together with femtoscopy measurements. In non-central relativistic heavy-ion collisions, the created medium possesses strong vorticity, which causes the spin of particles to trend to align with the system’s global angular momentum. The first differential measurement of theΛpolarization as a function ofΛazimuthal angle relative to the

re-action plan has been reported at this conference. For the electro-magnetic probes, the first dielectron measurement at very lowpT has been shown at this conference.

_{e-mail: wangmei@rcf.rhic.bnl.gov}

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(GeV/c)

T

p

0 1 2 3 4 5 6 7

2

Anisotropy Parameter, v 0 0.1 0.2

0 STAR D SUBATECH TAMU c-quark diﬀ. TAMU no c-quark diﬀ.

=7 s TD

π

Duke - 2 LBT PHSD 3D viscous hydro

= 200 GeV

NN

s STAR Au+Au

0-80%

)

2

(GeV/c

q

) / n

0

- m

T

(m

0 0.5 1 1.5 2 2.5

q

/ n2

0

D

-Ξ

Λ S

K

= 200 GeV

NN

s STAR Au+Au

10-40%

) 2 )/NCQ (GeV/c 0 -m T (m

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

3/2

/NCQ3

v

0.01

−

0 0.01 0.02 0.03 0.04 0.05 0.06

STAR Preliminary

Au+Au 200 GeV 0-80%

0

D p+

K

+ π φ Λ

Figure 1. Left:v2as a function ofpTfor D0mesons in 0-80% centrality [3]. Middle (right):v2/nqin 10-40%

centrality (v3/(nq)3/2in 0-80% centrality) as a function of (mT−m0)/nqfor D0and light flavor hadrons [3].

2 Heavy flavor measurements

In heavy-ion collisions at RHIC, heavy flavor quarks are predominantly produced by initial hard parton scatterings, and subsequently experience the whole evolution of the system. Therefore they can serve as calibrated probes to study the properties of the strongly coupled Quark Gluon Plasma (sQGP). In particular, the anisotropic flow of open heavy flavor hadrons is sensitive to the interactions of heavy flavor quarks with the medium and could also help constrain the magnitude of their spatial diffusion

coefficientDsin sQGP [2]. Shown in the left panel of Fig. 1 are thev2of D0mesons as a function of pTin the 0-80% centrality interval [3]. The measuredv2are compared to theoretical calculations [4–

9], which use different approaches to describe the transportation or interaction of charm quarks in the

medium. The hydrodynamic model [4] with parameters tuned to reproduce the light flavor hadron

v2 can reasonably describe theD0 v2, indicating that charm quarks may have achieved full thermal

equilibrium in these collisions. The transport models [6–9] with a temperature dependent diffusion

coefficient in the range 2<2πT Ds <12 forTc <T <2Tcare also able to reproduce the measured

values. The left (middle) panel of Fig. 1 shows thev2(v3) values scaled by the number of constituent

quarks (NCQ,nq) forD0mesons and a number of light flavor hadrons, as a fucntion of (mT−m0)/nq.

The scaledv2andv3for all the species includingD0falls on the same trend, which also suggests that

charm quarks flow with the medium. The yield ratios ofD±

s/D0andΛ±c/D0in √sNN=200 GeV Au+Au collisions at RHIC are shown

in Fig. 2. These measurements can help us to understand the hadronization mechanism of charm quarks in the QGP medium [10–12]. Both ratios show significant enhancements compared to those in p+p collisions predicted by PYTHIA, suggesting that charm quarks could hadronize via the

coales-cence mechanism in the QGP. ForΛ±_c/D0, model calculations with coalescence of thermalized charm

quarks in the QGP medium are compatible with data within uncertainties [10], while forD±

s/D0 the

TAMU model calculation for the 10-40% centrality interval with coalescence of charm quarks seems to under-predict the measured values [9].

Quarkonium dissociation by the color-screening effect in the hot and dense medium was initially

proposed as signature of the QGP formation [13]. However, the interpretation of J/ψsuppression observed in experiments has remained a challenge due to the regeneration from the coalescence of deconfined heavy quark pairs in the medium, the cold nuclear matter (CNM) effects and feed-down

contributions. In order to quantify those effects, we present (i) measurements of nuclear modification

factors for inclusiveJ/ψproduction in p+Au collisions at√s_NN=200 GeV; (ii) the first measurement

of the double ratio of inclusiveψ(2S) andJ/ψproduction rates at mid-rapidity between p+p and p+Au

collisions at √sNN =200 GeV; and (iii)measurements ofΥnuclear modification factors in Au+Au

collisions at √sNN=200 GeV.

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(GeV/c)

T

p

0 1 2 3 4 5 6 7

2

0 STAR D SUBATECH TAMU c-quark diﬀ. TAMU no c-quark diﬀ.

=7 s TD

π

Duke - 2 LBT PHSD 3D viscous hydro

= 200 GeV

NN s STAR Au+Au 0-80% ) 2 (GeV/c q

) / n

0

- m

T

(m

0 0.5 1 1.5 2 2.5

q

/ n2

Anisotropy Parameter, v 0 0.05 0.1 0 D -Ξ Λ S K

= 200 GeV

NN s STAR Au+Au 10-40% ) 2 )/NCQ (GeV/c 0 -m T (m

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

3/2 /NCQ3 v 0.01 − 0 0.01 0.02 0.03 0.04 0.05 0.06 STAR Preliminary

Au+Au 200 GeV 0-80%

0 D p+ K + π φ Λ

Figure 1. Left: v2as a function ofpT for D0mesons in 0-80% centrality [3]. Middle (right): v2/nqin 10-40%

centrality (v3/(nq)3/2in 0-80% centrality) as a function of (mT−m0)/nqfor D0and light flavor hadrons [3].

2 Heavy flavor measurements

In heavy-ion collisions at RHIC, heavy flavor quarks are predominantly produced by initial hard parton scatterings, and subsequently experience the whole evolution of the system. Therefore they can serve as calibrated probes to study the properties of the strongly coupled Quark Gluon Plasma (sQGP). In particular, the anisotropic flow of open heavy flavor hadrons is sensitive to the interactions of heavy flavor quarks with the medium and could also help constrain the magnitude of their spatial diffusion

coefficientDsin sQGP [2]. Shown in the left panel of Fig. 1 are thev2of D0mesons as a function of pTin the 0-80% centrality interval [3]. The measuredv2are compared to theoretical calculations [4–

9], which use different approaches to describe the transportation or interaction of charm quarks in the

medium. The hydrodynamic model [4] with parameters tuned to reproduce the light flavor hadron

v2 can reasonably describe theD0 v2, indicating that charm quarks may have achieved full thermal

equilibrium in these collisions. The transport models [6–9] with a temperature dependent diffusion

coefficient in the range 2 <2πT Ds <12 forTc <T <2Tcare also able to reproduce the measured

values. The left (middle) panel of Fig. 1 shows thev2(v3) values scaled by the number of constituent

quarks (NCQ,nq) forD0mesons and a number of light flavor hadrons, as a fucntion of (mT−m0)/nq.

The scaledv2andv3for all the species includingD0falls on the same trend, which also suggests that

charm quarks flow with the medium. The yield ratios ofD±

s/D0andΛ±c/D0in √sNN=200 GeV Au+Au collisions at RHIC are shown

in Fig. 2. These measurements can help us to understand the hadronization mechanism of charm quarks in the QGP medium [10–12]. Both ratios show significant enhancements compared to those in p+p collisions predicted by PYTHIA, suggesting that charm quarks could hadronize via the

coales-cence mechanism in the QGP. ForΛ±_c/D0, model calculations with coalescence of thermalized charm

quarks in the QGP medium are compatible with data within uncertainties [10], while forD±

s/D0 the

TAMU model calculation for the 10-40% centrality interval with coalescence of charm quarks seems to under-predict the measured values [9].

Quarkonium dissociation by the color-screening effect in the hot and dense medium was initially

proposed as signature of the QGP formation [13]. However, the interpretation of J/ψsuppression observed in experiments has remained a challenge due to the regeneration from the coalescence of deconfined heavy quark pairs in the medium, the cold nuclear matter (CNM) effects and feed-down

contributions. In order to quantify those effects, we present (i) measurements of nuclear modification

factors for inclusiveJ/ψproduction in p+Au collisions at√s_NN=200 GeV; (ii) the first measurement

of the double ratio of inclusiveψ(2S) andJ/ψproduction rates at mid-rapidity between p+p and p+Au

collisions at √sNN =200 GeV; and (iii)measurements ofΥnuclear modification factors in Au+Au

collisions at √sNN=200 GeV.

(GeV/c) T p

0 1 2 3 4 5 6 7 8

0 /Ds D 0 0.2 0.4 0.6 0.8 1

Au+Au @ 200 GeV STAR Preliminary 0-10% 10-40% ee/pp/ep average PYTHIA(ver. 6.4) TAMU(b=7.24fm)

0 1 2 3 4 5 6

Baryon-to-Meson Ratios 1 − 10 1 ) 0 D + 0 (D ) c Λ + c Λ ( 10-60% PYTHIA /D) c Λ Greco ( Ko: three-quark 0-5% Ko: di-quark SHM STAR Preliminary

Au+Au @ 200 GeV

0

/ Dc

Λ

(GeV/c) T Transverse Momentum p

Figure 2. Yield ratios ofDs/D0 in 0-10% and 10-40% Au+Au collisions (left) andΛc/D0in 10-60% central

Au+Au collisions (right) as a function ofpT.

[GeV/c] T p

0 2 4 6 8 10

pAu R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 |y|<0.5 0-100% pAu STAR R STAR Global uncertainty

|y|<0.35 0-100% dAu PHENIX R PHENIX Global uncertainty

=4.2mb Ferreiro et al. abs

σ nDSg + dAu

RpAu EPS09NLO Ma & Vogt RdAu EPS09NLO Lansberg & Shao R

nCTEQ15 Lansberg & Shao dAu R STAR Preliminary [GeV/c] T p

0 1 2 3 4 5 6 7 8 9 10

(1s) ψ σ (1s) ψ ll /B (2s) ψ σ (2s) ψ ll B 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

STAR p+p 200 GeV |y|<0.5 0-10 GeV/c PHENIX p+p 200 GeV |y|<0.35 PHENIX p+p 200 GeV 1.2<|y|<2.2 HERA-B p+A(C,Ti,W) 41.6 GeV (e channel)

channel)

µ

HERA-B p+A(C,Ti,W) 41.6 GeV ( , 1.8 TeV p CDF p+

ICEM p+p 200 GeV |y|<1 Ma & Vogt

STAR Preliminary

rapidity y 2

− −1.5 −1 −0.5 0 0.5 1 1.5 2

pp (1s) ψ σ (2s) ψ σ / pA (1s) ψ σ (2s) ψ σ 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

STAR p+Au |y|<0.5 0-100% PHENIX p+Au -2.2<y<-1.2 & 1.2<y<2.2 PHENIX d+Au |y|<0.35 p+Au co-mover Ferreiro

Au-going direction

← p-going direction →

Figure 3. Left: nuclear modification factorRpAuas a function ofpT for inclusive J/ψin the di-muon decay.

Middle: ratio ofψ(2S) toJ/ψproduction rates as a function ofpT. Right: double ratio ofψ(2S) toJ/ψproduction

rates as a function of rapidity in p+Au and d+Au collisions with respect to that in p+p collisions..

Shown in the left panel of Fig. 3 is theRpAu of inclusiveJ/ψin p+Au collisions. The measured

RpAuis generally consistent with theRdAuresult reported by PHENIX [14] within uncertainties. The

theoretical predictions with only the nuclear PDF effect using the nCTEQ15 [15] or EPS09NLO [16]

nuclear PDFs sets can touch the upper limit of the data. However, the calculation including an ad-ditional nuclear absorption effect [17] is favored by the data. The middle panel of Fig. 3 shows the

ratio of inclusive J/ψandψ(2S) production cross sections as a function of pT in p+p collisions at

√_s

=200 GeV. The measurement follows the trend of the world-wide data and could be described by

the ICEM calculation [18]. Shown in the right panel of Fig. 3 is the double ratio ofψ(2S) andJ/ψ

production rates between p+p and p+Au collisions as a function of rapidity. Taking into account the

large uncertainties, the measurement is consistent with the published PHENIX results [19] and the co-mover model calculation [20].

The RAA ofΥ(1S) as a function of the number of participating nucleons (Npart) is shown in the

left panel of Fig. 4. The measurement is compared to that in Pb +Pb collisions at √sNN =2.76

TeV from CMS [21] and they show similar suppression. Please note that the feed-down contributions from excited states are not excluded from theΥ(1S)RAA. The STAR results are also compared to

different theoretical model calculations [22–24], as shown in the middle panel of Fig. 4. Two limiting

cases for the in-medium potential between the quark-antiquark pairs that are often used are the internal energy corresponding to a strongly bound scenario (SBS), and the free energy corresponding to a more weakly bound secnario (WBS). Our data seem to favor the SBS scenario toward central collisions. ThepT dependence ofΥ(1S)RAAfrom STAR is shown in the right panel of Fig. 4 and also compared

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part N

0 100 200 300 400

pA, AA

R

0 0.2 0.4 0.6 0.8 1 1.2

1.4 30-60% 0-60%10-30% 0-10% (1S): STAR Au+Au@200 GeV |y|<0.5 ϒ

(1S): CMS Pb+Pb@2.76 TeV |y|<2.4 ϒ

(1S+2S+3S): STAR p+Au@200 GeV |y|<0.5 ϒ

uncertainty coll STAR N

part N

0 100 200 300 400

AA

(1S) R

ϒ

0 0.2 0.4 0.6 0.8 1

1.2 30-60% 0-60%_{STAR Au+Au@200 GeV |y|<0.5}10-30% 0-10%

uncertainty coll STAR N Strickland WBS Rapp SBS Strickland SBS Liu-Chen

(GeV/c) T p

0 2 4 6 8 10

AA

R

0 0.2 0.4 0.6 0.8 1 1.2

STAR Au+Au@200 GeV (0-60%)

-µ

+

µ → ϒ (1S) |y|<0.5 ϒ

CMS Pb+Pb@2.76 TeV (0-100%) (1S) |y|<2.4 ϒ STAR Preliminary

Figure 4. Left: nuclear modification factor,RAA, ofΥ(1S) as a function ofNpartin Au+Au collisions. Middle:

the same data as in the left panel compared to different model calculations. Right: RAAofΥ(1S) as a function

ofpTin 0-60% central Au+Au collisions at RHIC and compared to that in 0-100% central Pb+Pb collisions at

LHC.

part N

0 100 200 300 400

AA

R

0 0.2 0.4 0.6 0.8 1 1.2

1.4 30-60% 0-60%10-30% 0-10%

95% C. L.

(2S+3S): STAR Au+Au@200 GeV |y|<0.5 ϒ

(2S): CMS Pb+Pb@2.76 TeV |y|<2.4 ϒ

(3S): CMS Pb+Pb@2.76 TeV |y|<2.4 ϒ

(GeV/c) T p

0 2 4 6 8 10

AA

R

0 0.2 0.4 0.6 0.8 1 1.2

STAR Au+Au@200 GeV (0-60%)

-µ

+

µ → ϒ (2S+3S) |y|<0.5 ϒ

CMS Pb+Pb@2.76 TeV (0-100%) (2S) |y|<2.4 ϒ

90% C. L.

Figure 5. Left: RAAofΥ(2S+3S) as a function ofNpartin Au+Au collisions. Right: RAAofΥ(2S+3S) as a

function ofpTin 0-60% central Au+Au collisions.

to that from CMS. They show similar suppression and possess no significant pT dependence within

uncertainties. The firstΥ(2S+3S)RAA measurements from STAR as a function ofNpartand pT are

shown in the left and right panels of Fig. 5, respectively. In comparison with CMS measurements [21], there seems to be a hint of less suppression at RHIC. AlsoΥ(2S+3S) shows a significantly stronger

suppression in the most central collisions at RHIC than that forΥ(1S), which is consistent with the

sequential melting phenomenon.

3 Bulk observables

Experimentally, the dynamics and the collective behaviour of the sQGP have been studied by mea-suring the azimuthal anisotropy of the produced particles. Strange hadrons, especially multistrange hadrons, due to their small hadronic interaction cross section and freeze-out temperature closer to the phase transition temperature, are believed to be more sensitive to the partonic stage in heavy-ion col-lisions. Recent developments indicate that higher order flow harmonics can reveal information about initial state fluctuations, which could help us to better extract the QGP properties. Shown in Fig. 6 are

vn(pT) ofKs0,φandΛ +Λ¯ at mid-rapidity for 0-10%, 10-40% and 40-80% central U+U collisions at

√_s

NN =193 GeV and Au+Au collisions at √sNN =200 GeV [25]. A clear centrality dependence of

v2(pT) is observed forK0s,φandΛ +Λ¯, which is similar to that in Au+Au collisions. Higher order

flow harmonicsv3andv4show much weaker centrality dependence, if any.

New measurements of rapidity-even dipolar flow (veven

1 ) from STAR are presented in this

con-ference. These measurements could serve as constraints to distinguish between different initial-state

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part N

0 100 200 300 400

pA, AA

R

0 0.2 0.4 0.6 0.8 1 1.2

1.4 30-60% 0-60%10-30% 0-10% (1S): STAR Au+Au@200 GeV |y|<0.5 ϒ

(1S): CMS Pb+Pb@2.76 TeV |y|<2.4 ϒ

(1S+2S+3S): STAR p+Au@200 GeV |y|<0.5 ϒ

part N

0 100 200 300 400

AA

(1S) R

ϒ

0 0.2 0.4 0.6 0.8 1

1.2 30-60% 0-60%_{STAR Au+Au@200 GeV |y|<0.5}10-30% 0-10%

uncertainty coll STAR N Strickland WBS Rapp SBS Strickland SBS Liu-Chen

(GeV/c) T p

0 2 4 6 8 10

AA

R

0 0.2 0.4 0.6 0.8 1 1.2

STAR Au+Au@200 GeV (0-60%)

-µ

+

µ → ϒ (1S) |y|<0.5 ϒ

CMS Pb+Pb@2.76 TeV (0-100%) (1S) |y|<2.4 ϒ STAR Preliminary

Figure 4. Left: nuclear modification factor,RAA, ofΥ(1S) as a function ofNpartin Au+Au collisions. Middle:

the same data as in the left panel compared to different model calculations. Right: RAAofΥ(1S) as a function

ofpT in 0-60% central Au+Au collisions at RHIC and compared to that in 0-100% central Pb+Pb collisions at

LHC.

part N

0 100 200 300 400

AA

R

0 0.2 0.4 0.6 0.8 1 1.2

1.4 30-60% 0-60%10-30% 0-10%

95% C. L.

(2S+3S): STAR Au+Au@200 GeV |y|<0.5 ϒ

(2S): CMS Pb+Pb@2.76 TeV |y|<2.4 ϒ

(3S): CMS Pb+Pb@2.76 TeV |y|<2.4 ϒ

(GeV/c) T p

0 2 4 6 8 10

AA

R

0 0.2 0.4 0.6 0.8 1 1.2

STAR Au+Au@200 GeV (0-60%)

-µ

+

µ → ϒ (2S+3S) |y|<0.5 ϒ

CMS Pb+Pb@2.76 TeV (0-100%) (2S) |y|<2.4 ϒ

90% C. L.

Figure 5. Left: RAAofΥ(2S+3S) as a function ofNpartin Au+Au collisions. Right: RAAofΥ(2S+3S) as a

function ofpTin 0-60% central Au+Au collisions.

to that from CMS. They show similar suppression and possess no significant pT dependence within

uncertainties. The firstΥ(2S+3S)RAA measurements from STAR as a function ofNpart andpT are

shown in the left and right panels of Fig. 5, respectively. In comparison with CMS measurements [21], there seems to be a hint of less suppression at RHIC. AlsoΥ(2S+3S) shows a significantly stronger

suppression in the most central collisions at RHIC than that forΥ(1S), which is consistent with the

sequential melting phenomenon.

3 Bulk observables

Experimentally, the dynamics and the collective behaviour of the sQGP have been studied by mea-suring the azimuthal anisotropy of the produced particles. Strange hadrons, especially multistrange hadrons, due to their small hadronic interaction cross section and freeze-out temperature closer to the phase transition temperature, are believed to be more sensitive to the partonic stage in heavy-ion col-lisions. Recent developments indicate that higher order flow harmonics can reveal information about initial state fluctuations, which could help us to better extract the QGP properties. Shown in Fig. 6 are

vn(pT) ofKs0,φandΛ +Λ¯ at mid-rapidity for 0-10%, 10-40% and 40-80% central U+U collisions at

√_s

NN =193 GeV and Au+Au collisions at √sNN=200 GeV [25]. A clear centrality dependence of

v2(pT) is observed for K0s,φandΛ +Λ¯, which is similar to that in Au+Au collisions. Higher order

flow harmonicsv3andv4show much weaker centrality dependence, if any.

New measurements of rapidity-even dipolar flow (veven

1 ) from STAR are presented in this

con-ference. These measurements could serve as constraints to distinguish between different initial-state

(GeV/c) T p 0 1 2 3 4 5

2

v

0.0 0.1 0.2

0.3 0

s K

0 1 2 3 4 5

3

v

0.0 0.1 0.2

0 1 2 3 4 5

4

v

0.0 0.1

0.2 U+U, 193 GeV0-10% 10-40% 40-80%

0 1 2 3 4 5

|y| < 1.0

f

0 1 2 3 4 5

L + L

0 1 2 3 4 5

STAR Preliminary

0 1 2 3 4 5

Au+Au, 200 GeV 0-10% 10-40% 40-80%

Figure 6.v2(top panels),v3(middle panels) andv4(bottom panels) vspTofK0s,φandΛ +Λ¯ for 0-10%, 10-40%

and 40-80% centrality in U+U collisions at√sNN=193 GeV and Au+Au collisions at √sNN=200 GeV [25].

0 0.05 0.1 0.15

0 1 2

v

even 1

〈Mult 〉= 270 (a)

0

0.05 0.1 0.15

0 1 2

veven 1

〈Mult 〉= 140 (b)

STAR Preliminary

p_T(GeV/c)

0

0.05 0.1 0.15

0 1 2

veven 1

〈Mult 〉∼25 (c)

U+U Au+Au Cu+Au Cu+Cu d+Au p+Au

Figure 7. veven

1 as a function ofpT for differentNChselections for different collision systems at √sNN ∼200

GeV.

models and aid a more reliable extraction of the viscosity over entropy density ratioη/s. Theveven₁

for different collision systems are shown in Fig. 7 for different magnitude ofNch. For each selected

Nchclass,veven₁ (pT) does not show a significant dependence on the collision system, which suggests

that the fluctuation-driven initial-state eccentricityε1is similar for the six collision systems.

Two-particle interferometry measurement, known as femtoscopy or HBT, is a standard method of studying space-time characteristics of the created system from heavy-ion collisions. Recently col-lected high-statistic datasets from STAR allow measurements with heavier, but less abundant parti-cles. Fig. 8 shows a comparison of kaon and pion [26] source radii as a function ofmT for 0-5% and

30-40% Au+Au collisions. TheR_out values for kaons are systematically larger than those of pions,

while theRsidevalues for kaons and pions are comparable, and themT-dependence ofRlongis diff

er-ent between kaons and pions. These observations suggest different space-time configurations at the

kinetic freeze-out for the pions and kaons. However, further understanding of the observed behaviors

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] 2 c [GeV/ T m

0.0 0.5 1.0

[fm]

out

R

2 3 4 5

6 STAR preliminary

] 2 c [GeV/ T m

0.0 0.5 1.0

[fm]

side

R

2 3 4 5

STAR preliminary

] 2 c [GeV/ T m

0.0 0.5 1.0

[fm]

long

R

2 3 4 5 6

7 _{STAR preliminary}

ππ

ππ with ToF KK

Centrality 0 -5% Centrality 30 -40%

Figure 8.The comparison of the kaon and pion [26] source radii in Au+Au collisions at √sNN=200 GeV from

STAR.

obs Y

-f

0 0.5 1 1.5

H

P

0 0.005 0.01

Au+Au 200GeV 20-50% STAR preliminary

signal L

oﬀ mass-peak L

obs Y

-f

0 0.5 1 1.5

H

P

0 0.005 0.01

Au+Au 200GeV 20-50% STAR preliminary

combined L and L

signal oﬀ mass-peak

Figure 9.Left:PHforΛand ¯Λas a function ofφ−Ψobsin 20-50% central Au+Au collisions at √sNN=200 GeV.

Right: mergedPH betweenΛand ¯Λas a function ofφ−Ψobsin 20-50% central Au+Au collisions at √sNN=

200 GeV.

require a comparison with the full three-dimensional hydrodynamic calculations with rescattering in the hadronic phase.

A non-zero globalΛpolarization has been observed in Au+Au collisions by STAR [27] for the

first time, revealing that the matter produced in heavy-ion collisions is by far the most vortical system ever observed. Such vorticity is expected to be maximal at the equator, leads to a larger in-plane than out-of-plane polarization for hyperons. Shown in the left panel of Fig. 9 are theΛand ¯Λpolarization

(PH) as a function ofφ−Ψobsat mid-rapidity in 20-50% central Au+Au collisions at √sNN =200

GeV, whereΨobs is the observed angle of the first-order event plane as an estimate of the reaction

plane angle. The right panel of Fig. 9 shows the combinedΛand ¯ΛPHas a function ofφ−Ψobs. The

significance of∆PH, forΛand ¯Λcombined, betweenφ−Ψobsbin of [0,π/8] and [3π/8,π/2] is 4.7σ.

The larger in-plane than out-of-plane polarization is consistent with the picture of maximum vorticity in the equator and low viscosity of the system.

4 Electro-magnetic probes

Recently, the ALICE collaboration reported a significantJ/ψexcess yield at very low pT (pT <0.3

GeV/c) in peripheral Pb+Pb collisions at √s_NN=2.76 TeV [28]. This remarkable excess can not be

explained by the hadronic production ofJ/ψmodified by the cold and hot medium effects, however,

could be qualitatively explained by coherent photonuclear production mechanism [29]. Shown in the left panel of Fig. 10 are the extracted J/ψRAAas a function of pT in Au+Au collisions at √sNN =

200 GeV and U+U collisions at √s_NN =193 GeV from STAR. A huge enhancement ofJ/ψ pro-duction has been observed in peripheral (40-80%) Au+Au and U+U collisions for pT <0.1 GeV/c,

(7)

] 2 c [GeV/ T m

0.0 0.5 1.0

[fm] out R 2 3 4 5

6 STAR preliminary

] 2 c [GeV/ T m

0.0 0.5 1.0

[fm] side R 2 3 4 5 STAR preliminary ] 2 c [GeV/ T m

0.0 0.5 1.0

[fm] long R 2 3 4 5 6

7 _{STAR preliminary}

ππ

ππ with ToF KK

Centrality 0 -5% Centrality 30 -40%

Figure 8.The comparison of the kaon and pion [26] source radii in Au+Au collisions at √sNN=200 GeV from

STAR.

obs Y

-f

0 0.5 1 1.5

H P 0 0.005 0.01 Au+Au 200GeV 20-50% STAR preliminary signal L signal L oﬀ mass-peak L oﬀ mass-peak L obs Y -f

0 0.5 1 1.5

H P 0 0.005 0.01 Au+Au 200GeV 20-50% STAR preliminary combined L and L signal oﬀ mass-peak

Figure 9.Left:PHforΛand ¯Λas a function ofφ−Ψobsin 20-50% central Au+Au collisions at √sNN=200 GeV.

Right: mergedPH betweenΛand ¯Λas a function ofφ−Ψobsin 20-50% central Au+Au collisions at√sNN=