STAR results from RHIC
Beam Energy Scan
Hiroshi Masui / Univ. of Tsukuba,
Sep/12/2013, J-PARC HI meeting

H. Masui, J-PARC HI meeting, Sep/12/2013

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RHIC Beam Energy Scan (BES)• Study the structure of QCD
phase diagram‣ large baryon chemical potential~ low beam energy➡ Beam Energy Scan !

• History & timeline‣ 2008: Test run at √sNN = 9.2

GeV (PRC81, 024901, 2010)‣ 2009: Proposal for BES Phase-I(arXiv:1007.2613)‣ 2010: First year of RHIC BES(7.7, 11.5, 39 and 62 GeV)
‣ 2011: Two further energies(19.6 and 27 GeV)
‣ 2012: Test run at 5 GeV
H. Masui, J-PARC HI meeting, Sep/12/2013

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Goals• At small baryon density, produced matter is characterized by‣ initial energy densities > critical values from lattice QCD‣ ~ ideal fluid flow (~ small shear viscosity to entropy density ratio)‣ opacity of jets

• 3 main goals for Beam Energy Scan at STAR

‣ Search for ‘turn-off’ signals of Quark-Gluon Plasma (QGP)- or onset of the QGP‣ Search for signals of phase boundary‣ Search for QCD critical point

H. Masui, J-PARC HI meeting, Sep/12/2013

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Observables• Search for ‘turn-off’ signals of
Quark-Gluon Plasma‣ Number of Constituent Quark(NCQ) scaling of v2‣ High pT suppression‣ Chiral magnetic effect• Search for signals of phaseboundary‣ Directed flow
‣ Azimuthal sensitive HBT• Search for QCD critical point‣ Fluctuations

H. Masui, J-PARC HI meeting, Sep/12/2013

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Solenoidal Tracker At RHIC
TOF

BEMC

TPC
VPD

EEMC
BBC

• Large & uniform acceptance at midrapidity‣ Full azimuth, |η| < 1• Excellent particle identification‣ TPC + TOF

H. Masui, J-PARC HI meeting, Sep/12/2013

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Challenges
Background from Beampipe Interactions
Hz
Au+Au @ 7.7~4GeV

~1 Hz

~7 Hz

eampipe Interactions• Event rate ~ O(1) Hz, fill length ~The RHIC experts tuned the beams and optimized the fiseveral weeks → huge increase in performance to the● For 7.7 and 11.5 GeV we reached our goals (~5M and ~● Much larger beam
● 39
width
compared
to 200GeV
GeV wasstable andSTARcollected 169M events (25●Au+Au @ 7.7 GeV

O(10) minutes

Expected situation from year 2009 test run‣ 9.2
Significant
the@GeV 25.03.2011 improvement towards
Alexander Schmah - LBNL
● Continuously online (High Level Trigger)end of runs (thanks to RHIC)
and offline (fast offline DST production)
monitoring of the background● Background well under control for analysis●• Huge background from beam-

25.03.2011

beampipe collisions

Alexander Schmah - LBNL

12

‣ e.g. total 100M events collected in ~ aMuch larger beam width compared to 200 GeV

month, ~95% is background at 7.7 GeV
H. Masui, J-PARC HI meeting, Sep/12/2013

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Search forTurn-off signals
of QGP
H. Masui, J-PARC HI meeting, Sep/12/2013

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%

unted for by mean field potentials where the K and
eel an attractive force while the Kþ and p feel alsive force.

compared to the particles. At sNN ¼ 11:5 GeV
difference becomes negative for the antiparticle(mT % m0 ) <1 GeV=c2 but the overall trend is still si
pffiffiffiffiffiffiffiffi
to the one of the particles andSTAR
to PRL110,sNN 143201¼ 62:4
GeV.
(2013)Breakdown of NCQ scaling

see also PRC88, 014902 (2013)

v2

0.2

11.5 GeV

62.4 GeV

(a)

(b)

0.1

p
Λ
Ξ
Ω

v2(B)-fit

0

Au+Au, 0%-80%

11.5 GeV

p
Λ
+
Ξ
+
Ω

(c)

-

π
K
0
Ks
φ

η-sub EP

62.4 GeV

(d)

π+
+
K
0
Ks
φ

0.05
0
-0.05
0

1

2

3

0

1

2

3

0

1

2

3

0

1

2

3

mT-m0 (GeV/c2)

3 (color online). The upper panels depict the elliptic flow v2 as a function of reduced transverse mass (mT % m0 ) for par
pffiffiffiffiffiffiffiffind (b), and antiparticles, (c) and (d), in 0%–80% central Au þ Au collisions at sNN ¼ 11:5 and 62.4 GeV. Simultaneous
Meson-baryon
splittingatThe
62difference
GeV -of NCQ
scaling
v2fits are shown in themesons except
the pions are shown as thedashed lines.
the baryon
v2 and the of
meson
ls.

•

‣ No difference between particles and anti-particles

for anti-particles at 11.5 GeV
142301-5• Meson-baryon splitting is gone
• NCQ scaling is broken between particles and anti-particles
H. Masui, J-PARC HI meeting, Sep/12/2013

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Large2 difference
PHYSICAL REVIEW
PRLv110,142301 (2013) for baryons

Mo
duce
the m
0.04
theorythe ptheory0.02
possibthat t
0
Coulo
(only
0
20
40
60
baryo
sNN (GeV)
out
FIG. 2 (color
Theparticles
differenceandin vanti-particles
2 between particles
Differenceof online).
v2 between
increaseobser! (see legend) as a
(X)
and
their
corresponding
antiparticles(
X)
In
in lower energies
pffiffiffiffiffiffiffiffi
function of sNN for 0%–80% central Au þ Au collisions. The"v2 f
Baryonsshow
largerdifferencethanmesonsdashed lines in the plot are fits with a power-law function. The
if diff
error bars depict the combined statistical and systematic errors.
H. Masui, J-PARC HI meeting, Sep/12/2013 9a/24
fun2

v2(X)-v (X)

0.06

•
•

STAR PRL110, 143201 (2013)see also PRC88, 014902 (2013)

Au+Au, 0%-80%
η-sub EP

-

+

Ξ -Ξ
p-p
Λ-Λ
K+-K
π+-π-

Disappearanceof RcpRCP
suppression
Open strange hadrons

Feng Zhao, Thu. Resonances

Statistical error only
𝐊 𝟎𝐒 , Λ, Ξ, RCP :(0~5%)/(40~60%)
Ω RCP in 19.6 and 27 GeV :(0~10%)/(40~60%)

Hijing Simulation• Significant suppression at 200 GeV
K R larger than unity for p R> 1.5increases
GeV/cwith decreasing beam•
R particle type (baryon/meson) difference at intermediate p (2~3 GeV/c)𝒔𝑵𝑵   ≤  11.5 GeV,



CP

CP

becomes less obvious
STAR PreliminaryT

cp

energiesT• Baryon-meson splitting reduces at17
low energies

H. Masui, J-PARC HI meeting, Sep/12/2013

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Disappearance of charge separation
 cos(q + q - 2 s) 

0.002

ALICE, arXiv:1207.0900
0.001

2.76 TeV Pb+Pb

62.4 GeV Au+Au

200 GeV Au+Au

39 GeV Au+Au

27 GeV Au+Au

11.5 GeV Au+Au

7.7 GeV Au+Au

γos

0.0005

Opposite sign0-0.0005

Same sign

γss

-0
0.002
001
0.00151

2

 cos(q + q - 2 s) 

γ = 〈cos(φ1+φ2-2Ψ)〉1

20.00150.001

19.6 GeV Au+Au

0.00050-0.0005-0.001
80

STAR preliminary
70

60

50

40

30

20

10

00

70

% Most Central

60

50

40

30

20

10

00

% Most Central

70

60

50

40

30

20

10

0

% Most Central• Charge separation (γ -γ ) at 200 GeVosss‣ chiral magnetic effect (deconfinement+chiral symmetry restoration) ?

• Separation decreases with decreasing beam energies.
Disappears at √sNN = 11.5 GeV or less

H. Masui, J-PARC HI meeting, Sep/12/2013

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Search forsignals ofphase boundary
H. Masui, J-PARC HI meeting, Sep/12/2013

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Directed flow - early predictions

ysics Letters B 458 (1999) 454–459

455
L. P. Csernai, D. Rohrich, PLB458, 454 (1999)
130

H.
Stocker, NPA750, 121 (2005)
H. Stöcker / Nuclear Physics A 750 (2005) 121–147

Ideal hydrodynamical modelwith 1st order phase transition

1.9

4.5

√sNN (GeV)
13.8

Fig. 7. Measured SIS and AGS proton (dpx /dy)-slope data compared to a three-fluid hydro calculation. A l
Fig. 1. Upper part: Definition of the measure softening, S, deextrapolation of the AGS data indicates a collapse of flow at ELab ≈ 30 A GeV, i.e., for the lowest SPS- an
scribing the deviation of Px Ž y . or Õ1Ž y . from the straight line
upper FAIR-energies at GSI [59].
<
Ž
.<
<
<
behavior, ay, around CM. S is defined as ay y Px y r ay . The
lower figure shows a typical example for fluid dynamical calculations with Hadronic and QGP EoS w3x. QGP leads to strong
softening, ;100%.

• Linear rapidity dependence without QGP at low energy‣ “Bounce-off” of spectators•stv
1 slope becomes flat with 1 order phase transition
Fig. 8 of w4x, Fig. 6b of w5x and Fig. 6 of w7x at or

Early predictions show minimum around √sNN ~5 GeV

slightly below 0.5 yrycm if QGP formation was‣allowed during the calculation. In sharp contrast, thesolutions with hadronic EoS did not show this effect,
and the maximum and minimum of the Px curve

H. Masui, J-PARC HI meeting, Sep/12/2013

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rgy loss in the TPC and from the time measurement by109
STAR’s time-of-flight barrel [25] in combination with the110
momentum.
111

same direction near mid-rapidity, which is argued to b
consistentwitha tiltedsource [18].
flow but deviate
at aemission
qualitative fromlevel fromthe observed
anismThes
increaproton flow.
ful in interp
1 be explained by the baryon
results
certainly cannot
stop
as possib
ping picture [15], since we observe large pionfar
flow
tha
ported baryo
v1 Slope at 7.7 GeV.
is not opposite to proton10-40%:
flow except
peof Fig.In3 andprotons)
E895:slopepripheral0.1collisions, a negativefor directed
flowbasofor proNA49: pprotons and pions at all energies
may have aslopedi↵eren
where r is t
STAR: porigin0.05
that is unrelated to STAR:a phasetransition [27].
among the apNon-monotonic behavior of v slope
123
124

141
142
143

112

0.02

v1

39 GeV :π0-10%
10-40%
40-80%

39 GeV : p
0-10%
10-40%
0.02
40-80%

0

113
114
115

0

STAR preliminary
-0.5

0

0.5

1

-1

-0.5

0

27 GeV : p

1

27 GeV : π
0.02

0

0

-0.02

-0.02

147

149

this equation
151of this obser
0152
present anal153
and ⇡ and
0.04
39 GeV(10-40%): p 0.04
39 GeV(10-40%): π 154
tinguishable
STAR Data
STAR Data-0.050.02
0.02
155
only slightly
156
ically, the su
00STAR preliminary
157
a proxy or b
-0.02
AMPT(D)-0.02
158
protons, and
AMPT(S)159
the net-proto
UrQMD
-0.04
-0.04
0.02
the
contribut
-1
-0.5
0
0.5
1
-1
-0.5
0
0.51601
0.04
0.04
27 GeVNet Proton
161: π recent study
27 GeV : p
0.02 p- p
0.02
162
tentials that
STAR: π-

150

v1

v1

0.5

-

0.02

146

dv1/dy’

-0.02

-1

145

148

116

-0.02

144

-1

-0.5

0

0.5

1

-1

-0.5

0

19.6 GeV : p

0 0

-0.02
-1

-0.5

0

0.5

1

-1

-0.5

0

11.5 GeV : p

11.5 GeV :π

0

-0.02

-0.02
0.5

7.7 GeV : p

0.02

1

-1

0

-0.02

-0.02
-1 -0.5

0

y

0.5

-0.5

0.02

0

1

0

163

• v slopes (dv /dy’) are negative
0

0.5

1

7.7 GeV : π-‣ except for protons at low energies• Net-proton v slope shows aminimum around 11.5-19.5 GeV
v1

0

AMPT
-0.02 AMPT(S)(String Melting)

v1

0

v1

0.02

-0.5

1

-

0.02

-1

0.5

URQMD

The lower
164
slope for tra
-0.02
165
comes negat
-0.04
-0.04
166
and 19.6 Ge
2-1-0.5 10 00.5
1-110 -0.5
0
0.5
1
167
at
7.7 GeV
0.04
0.04
√
s
NN
19.6 GeV : π 19.6 GeV : p
168
In contrast,
0.02
0.02
169
at all energie
FIG. 3: Directed flow slope (dv1 /dy 0 ) near mid-rapidity as170 of this remar00
1 a function of beam
1energy for mid-central Au+Au collisions,
171
model that
where the -0.02
primed quantity y 0 refers -0.02
to normalized rapidityy/ybeam . The upper panel reports slopes for protons, an-172 trends at lea
-0.04
-0.04
tiprotons andpions, including measurements
by prior ex-173 panel) also r
0.5
1
-1
-0.5
0
0.51741
atesstrongly
periments. 0.04
The -1lower-0.5panel0 shows
STAR’smeasurement
of
0.04
11.5
GeV
:
π
GeV
:p
this same slope for11.5
net
protons,
which is a representation175 ergy region
1
0.02
0.02
of the signal
for initial-state baryon number
transported to176 roughly coin
mid-rapidity, 0along with corresponding predictions
from the177 above the re0
UrQMD and AMPT models. The systematic uncertainty on178 from the par-0.02 measurements are shown-0.02 as a shaded bandthe net-proton
179
influences fl0centered on dv1 /dy = 0.
180transport mo
-0.04
-0.04
H.-1Masui,
Sep/12/2013
hadronic/24 phy14-0.5 J-PARC
0
0.5 HI meeting,
1
-1
-0.5
0
0.5181
1
v1

0

v1

0

dv1/dy’19.6 GeV : π
0.02

v1

1

-

0.02

-0.02

0.5

-1 -0.5

0

y

0.5

1

Evolution of initial spatial anisotropy
P. F. Kolb et al, PRC62, 054909 (2000)• Spatial anisotropy
⌦ 2
↵
2
y
x
"x = 2hy + x2 i(eccentricity) is sensitive toEquation Of State (EOS)• Non-monotonic behavior

could indicate the softest
point of EOS

with 1st order phase transition

H. Masui, J-PARC HI meeting, Sep/12/2013

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Monotonic decrease of freeze-out eccentricity
Excitation function for freeze-out eccentricity, εF
Kt = 0.15-0.6 GeV/c*Model centralities
correspond to data

εF

0.4

0.3

E895 - PLB 496, 2004 (7.4-29.7%)
CERES - PRC 78, 2008 (10-25%)
STAR (-0.5<y<0.5, 10-30%)
STAR (-1<y<-0.5, 10-30%)
STAR (0.5<y<1, 10-30%)
URQMD
Hybrid[BM]+UrQMD
Hybrid[HG]+UrQMD
2D hydro EoS-Q
2D hydro EoS-H
2D hydro EoS-I

0.2

0.1

0
STAR preliminary
1

•
60

61
62

10

102

3

sNN

10

p
Pion Figure
freeze-out
eccentricity
smoothlydecrease
as online).
a function
3: Freeze out eccentricity," f , as a functionof sNN for data
and models [9] (colorof beam energy

Summary dependence also studied to try to compare with CERES‣4. Rapidity
To summarize, ⇤-⇤ correlation function is presented. Fits to data with di↵erent potential
models suggest that ⇤-⇤ interaction is attractive. A negative
scattering length gives indication
H. Masui, J-PARC HI meeting, Sep/12/2013

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Search forsignals of
QCD critical point
H. Masui, J-PARC HI meeting, Sep/12/2013

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Higher (n>2) moments (or cumulants)• At critical point (with infinite system)‣ susceptibilities and correlation length diverge- both quantities cannot be directly measured• Experimental observables‣ Moment (or cumulant) of conserved quantities: net-baryons, net-

charge, net-strangeness, ...
‣ Moment product (cumulant ratio) ↔ ratio of susceptibility⌦

2 = ( N )
3
S =
⇠
2

2

↵

2

⌦

3

↵

⇠ ⇠ , 3 = ( N ) ⇠ ⇠
4
3
4
2
, K =
⇠
2
2
2

4.5

⌦

, 4 = ( N )

4

↵

2

3 h( N )i ⇠ ⇠ 7- directly related to the susceptibility ratios (Lattice QCD)
M. A. Stephanov, PRL102, 032301 (2009)- higher moments (cumulants) have higher sensitivity to correlation length• Signal = Non-monotonic behavior of moment products(cumulant ratios) vs beam energy

H. Masui, J-PARC HI meeting, Sep/12/2013

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Non-gaussian fluctuations
From Wikipedia• 3rd moment = Skewness S‣ Asymmetry• 4th moment = Kurtosis K‣ Peakedness• Both moments = 0 for gaussian distribution• Critical point induces non-gaussian fluctuations
H. Masui, J-PARC HI meeting, Sep/12/2013

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Net-protons

Moment Products: Energy Dependence !

•! Data
Deviations below Poisson !

‣ efficiency
uncorrected*
expectations
are observed
beyondstatistical and systematic errors in
Data
0-5%
most compared
central collisionsto
forvarious
κσ2andexpectations
Sσ above 7.7 GeV.

•
!

‣ For
Poisson!
peripheral collisions, the !
deviations above Poission expectations‣ (Negative-)Binomial*
are observed below 19.6 GeV.!
‣! Random sampling between p and! UrQMD model show monotonic !
pbar*
behavior for the moment products, in!
which
non-CP physics,
UrQMD
showssucha as!
monotonic
baryon conservation, hadronicenergydependence
scattering
effects,
are implemented.!

•• Need precision measurements
at low energies* under investigation (not shown here)
H. Masui, J-PARC HI meeting, Sep/12/2013

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Net-charge
Higher Moments of Net-charge• Data‣ efficiency uncorrected*• Data compared to various• Net-charge fluctuation – related tofreeze-out
parameters
expectations

Bazavov et al, PRL 109 (2012) 192302‣ Poisson• Data - efficiency uncorrected *‣ (Negative-)Binomial*• Data compared to various expectations- Poisson- (Negative-)Binomial *• Need precision measurements• Needprecision
measurements.
at lowenergies* Currently under investigation* under investigation (not shown here)

Aug. 19th, 2013

Future Trends Workshop, Beijing

X. Dong

18

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Summary on BES-I• Turn-off QGP signals‣ Breakdown of NCQ scaling (particles vs anti-particles)‣ Disappearance of high pT suppression‣ Disappearance of charge separation• Signals of phase transition‣ Non-monotonic behavior of net-proton v1 slope‣ Monotonic decrease of freeze-out eccentricity• Signals of QCD critical point‣ Ongoing study on several aspects (efficiency, baseline, ...)‣ Need precision measurements at low beam energies• BES phase-II will focus on beam energies below ~ 20 GeV
H. Masui, J-PARC HI meeting, Sep/12/2013

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Potential for luminosity improvement for low-energy RHIC

BES phase-II proposal

February 10, 2012averageluminosity
per store luminosity
Average(cm-2 s-1)
per store

will also come with heavy beam losses which was already an issue in RHIC operation at lower
energies even for nominal bunch intensities. On the contrary, the use of electron cooling should help
to minimize beam losses and make operation easier with longer stores in addition to a significant
luminosity improvement.
Simulationsof luminosity evolution with time for the three cases
A. Fedotov andM. Blaskiewicz
for luminosity
for low-energy
presented in Fig. 9Potential
are shownin Fig.improvement
10, for completeness.
1 10

26

1 10

25

1 10

24

• BES phase-II (2017-) will

RHIC operation with long bunches

cover the energy below ~ 20
GeV with improved statistics‣ Fill the gap between 11.5 and 19.6

GeV (ΔµB~100 MeV)

1 10

23

√sNN = 7.7 GeV

1 10

22

w/o e-coolingw/o e-cooling, long bunches
e-cooling + long bunches

0

0.5

1

1.5

• Electron cooling + longer

bunches will give 3-10 times
higher luminosity

2Gold Target
z-vertex cut (m)vertex cut [m]2.4 inch(6.10 cm) OD• Fixed targetSTARproposal - down
to √sSide~ 3 GeVNN
View:
Target Mount

Diagram is 1:1 Scale‣ Annular gold target, 2m away fromBeam Pipe:
Aluminum
OD:  3”  (7.62  cm)Thickness: 60 mil (1.5 mm)
sc
ID:  3.88”  (7.47cm)

Gold forAnnuluscenterofand
thespaceSTAR
Fig. 9. Average (per store) luminosity
111 bunches of Au ions inthe
RHIC
at =4.1
thick cooling and long bunches ( s=4.5 m, *=2 m,charge tune spread Q =0.05: 30
1) milelectron‣ Datatakingwith collider
mode at them, Ni=5e8) - blue, dash curve; 2) without cooling ( s=1.5m, *=6m, n,95%m,n,95%Ni=5e8)–Mountred,Sleeve:solid curve; 3) without
coolingwith higher bunch
intensityof eachfill, no1.8 inch (4.57
cm) IDbut longer bunches beginning
TargetAluminum( s=4.5m, *=6 m, n,95%m, Ni=1.5e9) – magenta, middle curvedisturbance(maxi mum luminosity
wasto normal
RHIC running
OD:  3.8”  (9.65  cm)dividedby a60factorof 4 to account for a very short luminosity lifetime – in simulations, theThickness:mil (1.5 mm)
ID:  3.68”  (9.35  cm)luminosity
was decreased by a factor of 4 after first 3 min as shown in Fig. 10).
Length:  1.2  “  (3.04  cm)Target Sheet:Gold

3 inch (7.62 cm) Aluminum Beam Pipe

H. Masui, J-PARC HI meeting, Sep/12/2013

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Conclusions• Several QGP signals turned off (v , R , ...)
2cp

➡ hadronic interactions become more important at low energies• Non-monotonic behavior on v slope• Need precision measurements for higher moment analysis
1• BES phase-II‣ Focus on √sNN < 20 GeV‣ Precision measurements on bulk observables- especially event-by-event fluctuations to search for QCD critical point

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