1 Strangeness in Quark Matter, 27 June - 1 July 2016
Strangeness in STAR at RHIC
Shusu Shi 施梳苏 for the STAR collaboration
Central China Normal University
华中师范大学
2 Strangeness in Quark Matter, 27 June - 1 July 2016
Outline Ø Introduction
Ø Collectivity Ø Multi-strange hadron and ϕ meson v2 Ø Energy dependence of v2
Ø Strangeness production Ø Nuclear modification factors RCP Ø Baryon/meson ratio
Ø Summary
3 Strangeness in Quark Matter, 27 June - 1 July 2016
Ø Strange, multi-strange hadrons and ϕ meson
Less sensitive to late hadronic interactions Ø Ω hyperons and ϕ meson minimal distortion from decay feed- down Good probe for QGP properties and QCD phase transition
φ Ω Ξ Λ
partonic hadronic
π K p
Strangeness
Ø Key observables Ø Event anisotropy v2 Collectivity Ø Nuclear modification factor
Partonic energy loss and recombination Ø Baryon/meson ratio
Parton recombination
STAR measurements at the beam energies of 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV
4 Strangeness in Quark Matter, 27 June - 1 July 2016
Large acceptance at mid-rapidity! Time Projection Chamber: Full azimuthal coverage, (|η|<1.0)
Excellent particle identification!
STAR Detectors
5 Strangeness in Quark Matter, 27 June - 1 July 2016
Particle Identification
Ø Reconstruction techniques used Ø ϕ mesons: invariant mass Ø Weak decay particles (KS
0, Λ, Ξ, Ω): secondary vertex + invariant mass
)2) (GeV/c-,K+M(K1 1.05 1.1
)-1 )2
dN/d
M (
1 (M
eV/c
0
2
4
6
610×
φ
same event mixed event
)2) (GeV/c-πM(p,1.1 1.15
)-1 )2
dN/d
M (
2 (M
eV/c
0
5
10
15
20610×
Λ
)2) (GeV/c-π,ΛM(1.3 1.35 1.4
)-1 )2
dN/d
M (
1 (M
eV/c
0
0.2
0.4
0.6
610×-Ξ
)2) (GeV/c-,KΛM(1.65 1.7 1.75
)-1 )2
dN/d
M (
1 (M
eV/c
0
2
4
6
8310×
-Ω
)2) (GeV/c-π,+πM(0.45 0.5 0.55
)-1 )2
dN/d
M (
1 (M
eV/c
0
5
10
15
610×
s0K
)2) (GeV/c+π,pM(1.1 1.15
)-1 )2
dN/d
M (
2 (M
eV/c
0
2
4
6
610×
Λ
)2) (GeV/c+π,ΛM(1.3 1.35 1.4
)-1 )2
dN/d
M (
1 (M
eV/c
0
0.1
0.2
0.3
610×+
Ξ
)2) (GeV/c+,KΛM(1.65 1.7 1.75
)-1 )2
dN/d
M (
1 (M
eV/c
0
2
4
310×+
Ω
Au + Au at 62.4 GeV
STAR: Phys. Rev. C 88, 014902 (2013)
6 Strangeness in Quark Matter, 27 June - 1 July 2016
Ø Proton and pion v2 compared with Ω baryon and ϕ meson v2 Ø High precision data prove that Ω follows the baryon/meson splitting
at intermediate pT range, 2 < pT < 5 GeV/c . First time! The major part of collectivity has been built-up at partonic stage!
STAR: Phys. Rev. Lett.116, 062301 (2016)
Ω Baryon and ϕ Meson v2 Ω and ϕ: good probes of early partonic stage of collision
7 Strangeness in Quark Matter, 27 June - 1 July 2016 7
v2(ϕ) versus v2(p)
Before hadronic rescattering
After hadronic rescattering
Ø Ideal hydro + hadron cascade (JAM) Ø Small hadron cross section + hadronic rescattering effect on v2 Mass ϕ > mass p è v2(ϕ) >v2(p) è Break mass ordering for ϕ mesons and protons
Model calculations: T. Hirano et al., ; PRC77, 044909 (2008)
8 Strangeness in Quark Matter, 27 June - 1 July 2016
Ø Model study indicates with increasing hadronic cascade time (more hadronic re-scattering), the v2(ϕ)/v2(p) ratio increases Ø The ratio v2(ϕ)/v2(p) Is at pT = 0.52 GeV/c in 0-30% -> Possibly due to the effect of late hadronic interactions on the proton v2
v2(ϕ) versus v2(p)
STAR: Phys. Rev. Lett.116, 062301 (2016) Model calculations: T. Hirano et al., ; PRC77, 044909 (2008)
9 Strangeness in Quark Matter, 27 June - 1 July 2016
RHIC: NCQ scaling holds within 10% Ø LHC: The deviation from the NCQ scaling at the
level of +/-20% Ø Coalescence is the dominant hadronization
mechanism at RHIC in the intermediate pT range
NCQ Scaling - RHIC
(c) 0-30%
= 200 GeVNNsAu+Au,
(d) 30-80%
q/n 2v
0
0.05
0.1
0.15π
KS0K
pφ
Λ
Ξ
Ω
(a) 0-30% (b) 30-80%
2 vS0Fit to K
0 1 2
Dat
a/Fi
t
0.60.8
11.2 (e)
0 1 26
8
1
2 (g)
0 1 2
(f)
0 1 2Fit Error
(h)
(GeV/c)q/nT
p )2 (GeV/cq)/n0-mT
(mSTAR: Phys. Rev. Lett.116, 062301 (2016) ALICE: JHEP06(2015)190
Number of Constituent Quark Scaling
10 Strangeness in Quark Matter, 27 June - 1 July 2016
(GeV)NNs0 20 40 60
))X( 2(X
)-v2
(v
0
0.02
0.04
0.06Au+Au, 10%-40%-sub EPη
+Ξ--Ξpp-Λ-Λ
--K+K-π-+π
(a)
(GeV)NNs0 20 40 60
))X( 2(X
)-v2
(v
0
0.02
0.04
pp-0%-10%10%-40%40%-80%
(b)
(GeV)NNs0 20 40 60
2norm
))/v
X( 2(X
)-v2
(v
0
0.2
0.4
0.6
0.8 pp-0%-10%10%-40%40%-80%
(c)
0 20 40 60
)X( 2(X
)-v2v
0
0.02
0.04
0.06 0-80%Λ-Λpp-
--K+K-π-+π
(GeV)NNs
Ø Significant difference of baryon and anti- baryon v2 observed Ø New data from 14.5 GeV fit the energy dependency curve
STAR: Phys. Rev. C 93, 014907(2016)
Particle vs. Anti-particle v2
11 Strangeness in Quark Matter, 27 June - 1 July 2016
Baryon/Meson Separation
Ø A splitting between baryons and mesons is observed at all energies except 7.7 GeV and all centralities.
Ø At 7.7 GeV we are limited by the number of events.
0 1 2 3 40
0.10.20.3 0%-10%7.7 GeV
0 1 2 3 40
0.10.20.3 11.5 GeV
0 1 2 3 40
0.10.20.3 14.5 GeV
0 1 2 3 40
0.10.20.3
2v 19.6 GeV
0 1 2 3 40
0.10.20.3 27 GeV
0 1 2 3 40
0.10.20.3 39 GeV
0 1 2 3 40
0.10.20.3 62.4 GeV
0 1 2 3 40
0.10.20.3 10%-40%p
Λ-Ξ -Ω
+π +Ks0K
φ
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3 40%-80%
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
)2 (GeV/c0-mTm
Particles
STAR: Phys. Rev. C 93, 014907(2016)
12 Strangeness in Quark Matter, 27 June - 1 July 2016
Baryon/Meson Separation
Ø The splitting between baryons and mesons is observed for all energies above 14.5 GeV and also at 14.5 GeV for 40–80%.
Ø For these energies below 11.5 GeV, we are limited by the number of events.
STAR: Phys. Rev. C 93, 014907(2016)
0 1 2 3 40
0.10.20.3 0%-10%7.7 GeV
0 1 2 3 40
0.10.20.3 11.5 GeV
0 1 2 3 40
0.10.20.3 14.5 GeV
0 1 2 3 40
0.10.20.3
2v 19.6 GeV
0 1 2 3 40
0.10.20.3 27 GeV
0 1 2 3 40
0.10.20.3 39 GeV
0 1 2 3 40
0.10.20.3 62.4 GeV
0 1 2 3 40
0.10.20.3 10%-40%p
Λ+Ξ +Ω
-π -Ks0K
φ
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3 40%-80%
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
0 1 2 3 40
0.10.20.3
)2 (GeV/c0-mTm
Antiparticles
13 Strangeness in Quark Matter, 27 June - 1 July 2016
Outline Ø Introduction
Ø Collectivity Ø Multi-strange hadron and ϕ meson v2 Ø Energy dependence of v2
Ø Strangeness production Ø Nuclear modification factors RCP Ø Baryon/meson ratio
Ø Summary
14 Strangeness in Quark Matter, 27 June - 1 July 2016
Au+Au 200 GeV
Ø Nuclear modification factors at Au+Au 200 GeV Ø Less than unity at high pT
Ø Baryon/meson follow different trends
-> Partonic energy loss and recombination
Ø Baryon/meson ratio at Au+Au 200 GeV Ø Baryon enhancement at
intermediate pT in central collisions
-> Parton recombination
STAR: arXiv:1007.2613
Strangeness Production
Strangeness is sensitive probe
15 Strangeness in Quark Matter, 27 June - 1 July 2016
STAR Preliminary
STAR Preliminary
STAR Preliminary
Strangeness Enhancement
Ø K-, anti-Λ, anti-Ξ and ϕ yields increase significantly with respect to the pion yield as the beam energy increases
STAR Preliminary
most central (0-5%), mid-rapidity
Statistical Hadron gas Model: A. Andronic et al., Nucl. Phys. A 772, 167 (2006)
16 Strangeness in Quark Matter, 27 June - 1 July 2016
Nuclear Modification Factors Ø KS
0 RCP increases with decreasing beam energies -> the partonic energy loss effect less important in lower beam energy
Ø The cold nuclear matter effect (Cronin effect) starts to take over at lower energies
Ø RCP differences of particles becomes smaller at √sNN ≤ 14.5 GeV -> indication of different properties of the system compared higher energies
0 1 2 3 40
1
2
3Λ+Λ+
Ξ+-Ξ+
Ω+-Ω0sK
39GeV
STAR Preliminary0 1 2 3 40
1
2
3
27GeV
STAR Preliminary
0 1 2 3 40
1
2
3
19.6GeV
STAR Preliminary0 1 2 3 40
1
2
3
4
14.5GeV
STAR Preliminary
0 1 2 3 40
1
2
3
11.5GeV
STAR Preliminary0 1 2 3 40
1
2
3
7.7GeV
STAR Preliminary
(GeV/c)TP
(0-5
%/4
0-60
%)
cpR
Nuclear modification factor: partonic energy loss and recombination
17 Strangeness in Quark Matter, 27 June - 1 July 2016
Ø The separation of central (0-5%) and peripheral (40-60%) collisions in the ratio less obvious when collision energy ≤ 14.5 GeV: less baryon enhancement
-> possible change of medium property Ø Need more statistics at lower
beam energies
/KS0 Ratio
0 1 2 3 40
0.2
0.4
0.6
0.8
139GeV
STAR Preliminary0 1 2 3 40
0.2
0.4
0.6
0.8
127GeV
STAR Preliminary
0 1 2 3 40
0.2
0.4
0.6
0.8
119.6GeV
STAR Preliminary0 1 2 3 40
0.2
0.4
0.6
0.8
1 0-5%5-10%10-20%20-30%30-40%40-60%60-80%
14.5GeV
STAR Preliminary
0 1 2 3 40
0.1
0.2
0.3
11.5GeV
STAR Preliminary0 1 2 3 40
0.1
0.2
0.3
7.7GeV
STAR Preliminary
(GeV/c)TP
0 s/K
Λ
Enhancement of baryon at intermediate pT in central collisions: parton recombination
Λ
18 Strangeness in Quark Matter, 27 June - 1 July 2016
Ω/ϕ Ratio
Ø The ratios at 11.5 GeV seem to deviate from the trend observed at
higher beam energies for pT > 2.4 GeV/c Ø 40%-60% peripheral < 0-10% central for 19.6, 27 and 39 GeV Ø Need more statistics at lower beam energies
STAR: Phys. Rev. C 93, 021903 (2016)
19 Strangeness in Quark Matter, 27 June - 1 July 2016
NCQ Scaled Ω/ϕ Ratio
Ø One single strange quark distribution describes both Ω and ϕ spectra-> quark coalescence production
Ø Slope (T) from Boltzmann fit changes at 7.7 GeV. Centrality difference? Ø Decreasing s quark density below 19.6 GeV -> Possible phase transition
STAR: Phys. Rev. C 93, 021903 (2016)
20 Strangeness in Quark Matter, 27 June - 1 July 2016
Summary
Ø Multi-strange hadrons and ϕ meson v2 -> Partonic collectivity and coalescence hadronization ϕ mesons less sensitive to hadronic interactionsØ Energy dependence of v2 -> Further experimental investigations neededØ RCP increases with decreasing beam energies -> Partonic energy loss effect less importantØ The separation of central/peripheral /KS
0 less obvious below 19.6 GeV ->
Possible change of medium propertyØ Decreasing s quark density below 19.6 GeV -> Possible phase transition
Λ
21 Strangeness in Quark Matter, 27 June - 1 July 2016
iTPC upgrade: -1 < η < 1 -1.5 < η < 1.5; pT >125 MeV pT > 60 MeV/c.
EPD upgrade: Greatly improved Event Plane info; Alternative centrality definition
Beam Energy Scan II
Stay tuned for further experimental investigations with BES II focus on √sNN ≤ 20 GeV
ETOF upgrade: New charged hadron PID capabilities for 1.1<|η|<1.6
22 Strangeness in Quark Matter, 27 June - 1 July 2016
Backup
23 Strangeness in Quark Matter, 27 June - 1 July 2016
0 0.5 1 1.5 2 2.5 3
2v
0.05−
0
0.05
0.1
0.15 mesonφ
19.6 GeV14.5 GeV11.5 GeV7.7 GeV
(GeV/c)T
p
0-80%
ϕ Meson v2
Ø 14.5 GeV: Sizable ϕ meson v2, comparable to 19.6 GeV
Ø High statistics and more collision energies below 20 GeV needed!
STAR: Phys. Rev. C 93, 014907 (2016) Phys. Rev. C 88, 014902 (2013)