1. Medium properties and jet-medium interaction from STAR
Jana Bielcikova
for the STAR Collaboration
NPI ASCR and Center for physics of ultra-relativistic
heavy-ion collisions, Prague, Czech Republic
XLIII Rencontres de Moriond “QCD and High Energy Interactions”

2. XLIII Rencontres de Moriond
Outline:
Introduction
“Jet”-medium interaction via 2- and 3-particle correlations
- conical emission ?
- long range pseudo-rapidity correlations (“ridge”)
Summary
Jana Bielcikova
XLIII Rencontres de Moriond

3. Probing QCD matter with jets
Au+Au
p+p
p+p
A+A
flow
near-side
away-side
What happens to high-pT particles/jets
which pass through the medium?
Are they similar to p+p or modified by the medium?
Full jet reconstruction in A+A collisions at RHIC
difficult due to underlying background
 use azimuthal correlations of high-pT particles
trigger
associated 
Jana Bielcikova
XLIII Rencontres de Moriond

4. Jet-like correlations at RHIC
4 <pT(trig)<6 GeV/c
2 GeV/c <pT(assoc)<pT(trig)
0.15<pT(assoc)<4.0 GeV/c
STAR, PRL 95 (2005)
STAR, PRL 91 (2003)
central Au+Au 200 GeV:
disappearance of away-side correlations observed at intermediate pT
d+Au and p+p similar -> jet suppression is a final state effect
lowering associated pT resurrects correlated yield
- enhanced yield at near and away side + shape modification
Jana Bielcikova
XLIII Rencontres de Moriond

5. Conical emission in A+A collisions?
2.5 < pTtrig< 4 GeV/c and 1< pTassoc < 2.5 GeV/c
Mach cone in heavy-ion physics introduced
in1970’s (Hofmann, Stöcker, Heinz, Scheid, Greiner)
a supersonic parton creates shock waves:
- hydrodynamics
Stöcker et al., NPA750 (2005) 121
Casalderrey-Solana et al., NPA774 (2006) 577
Renk, Ruppert, PRC73 (2006)
- colored plasma
Ruppert, Mueller, PLB618 (2005) 123
- AdS/CFT
Gubser, Pufu, Yarom, PRL100, (2008)
Čerenkov gluon radiation by a superluminal parton
Dremin, NPA750 (2006) 233
Koch et. al., PRL96 (2006)
40-60%
0-12%
Preliminary
M. Horner (STAR), J.Phys.G34, S995,2007
To distinguish from other
mechanisms 3-particle
correlation studies needed
Jana Bielcikova
XLIII Rencontres de Moriond

6. Conical flow or deflected jets? (I)
Medium
away
near
di-jets
cartoons of 3-particle
azimuthal correlations
(1 trigger + 2 associated particles) π
di-jets
away
near
Medium
mach cone
conical emission
Medium
away
near
deflected jets
deflected jets
(by the collective movement, “flow”, of the expanding medium)
Armesto , Salgado, Wiedemann,
PRL 93, (2004)
Jana Bielcikova
XLIII Rencontres de Moriond

7. Conical flow or deflected jets? (II)
Bedanga Mohanty
STAR uses 2 methods:
1. Jet+flow background
- model dependent analysis
evidence for conical emission
2. Cumulant method
C. Pruneau (STAR),J.Phys.G34 (667),2007;
C. Pruneau, PRC 74 (2006)
unambiguous evidence
for 3-particle correlations
strength and shape of away-side
structures depend on magnitude
of flow coefficients
no conclusive evidence for
conical emission
d+Au
central Au+Au
STAR Preliminary
STAR Preliminary
Note: Large and complicated
backgrounds
J. Ulery (STAR), arXiv:
3 < pT,trig < 4 GeV/c
1 < pT,assoc < 2 GeV/c
central Au+Au C3 
Subtraction of v2v2v4 terms
using v2 = 0.06
Subtraction of v2v2v4 term
using v2 = 0.12
STAR Preliminary
STAR Preliminary 

8. Mach cone or Čerenkov gluons?
Bedanga Mohanty
Mach cone or Čerenkov gluons?
Cone angle (radians)
pT (GeV/c)
STAR preliminary
Mach cone
cone angle independent of pTassoc
Čerenkov gluon radiation:
cone angle decreases with pTassoc
pT dependence of cone angle favors Mach cone over Čerenkov gluon emission
Koch, Majumder, Wang, PRL (2006)
Jana Bielcikova
XLIII Rencontres de Moriond

9. Momentum conservation in correlation analyses
C3 cumulant pT conservation
3-particle correlation
Calculation for:
<pT,trig > = 3.2 GeV/c
<pT,assoc >= 1.2 GeV/c
N. Borghini, PRC 75 (2007)
Calculation by N. Borghini:
momentum conservation  sizable correlation between pairs or triplets
of high-pT particles in central Au+Au collisions at RHIC
 C3(pT) ~ C3(flow)
 jet “distorts” the event
needs to be evaluated in the data
2-particle correlation
after v2 subtraction
N. Borghini, arXiv:
Jana Bielcikova
XLIII Rencontres de Moriond

10. A closer look at the near-side peak …
pTtrig=3-6 GeV/c, 2 GeV/c <pTassoc< pTtrig
d+Au, 200 GeV
STAR preliminary
Au+Au, 200 GeV
“jet”
ridge
Additional near-side
correlation in 
observed in central Au+Au collisions
What is the ridge?
1) Medium heating and parton recombination
Chiu & Hwa PRC 72 (2005)
2) Radial flow + high-pT trigger particle
Shuryak, Phys.Rev.C76 (2007)
Voloshin, nucl-th/ NPA 749, (2005) 287
3) Parton radiation and its coupling
to the longitudinal flow
Armesto, Salgado, Wiedemann, PRL 93 (2004)
4) Momentum broadening in anisotropic QGP
Romatschke, PRC 75 (2007)
5) Longitudinal broadening of quenched
jets in turbulent color fields
Majumder, Mueller, Bass, PRL 99 (2007)
6) Momentum kick imparted on partons in medium
Wong, PRC PRC 76 (2007)
Jana Bielcikova
XLIII Rencontres de Moriond

11. Centrality and system size dependence of near-side yield
Bedanga Mohanty 
ridge  
jet
jet+ridge
after v2
subtraction
“jet” yield independent of colliding system,
Npart and trigger particle type
ridge yield increases with Npart
Au+Au: J. B. (STAR), QM 2006
Cu+Cu: C. Nattrass (STAR), QM2008
Jana Bielcikova
XLIII Rencontres de Moriond

12. “Jet” and ridge: pT dependence
STAR preliminary
pT assoc > 2 GeV/c
Ridge: solid symbols
Jet: open symbols
J. Putschke (STAR), J.Phys.G34:S679 (2007)
J. Putschke (STAR), J.Phys.G34:S679 (2007)
STAR preliminary
Jet:
T(jet) > T(bulk)
T(jet) increases with pTtrig
Ridge:
T(ridge) ~ T(bulk)
T(ridge) ~ independent of pTtrig
ridge persists up to pTtrig ~ 10 GeV/c
“jet” slope
ridge slope
inclusive slope
“jet” slope
ridge slope
inclusive slope
h-h correlations
pTassociated>2GeV/c
Jana Bielcikova
XLIII Rencontres de Moriond

13. Particle composition in “jet” and ridge
Bedanga Mohanty
C. Suarez (STAR), poster, QM2008
Ridge vs. Inclusive
“Jet” Cone vs. Inclusive
STAR Preliminary
Au+Au
Baryon/meson ratios:
“jet”: smaller than inclusive
ridge: similar to inclusive
Cu+Cu: C. Nattrass (STAR), QM2008
Au+Au: J.B. (STAR), WWND07
Jana Bielcikova
XLIII Rencontres de Moriond

14. Near-side “jet” and ridge w.r.t. event plane
Bedanga Mohanty
trigger
in-plane
out-of-plane
3< pTtrig < 4 GeV/c, pTassoc = GeV/c
Au+Au 200 GeV
20-60%
STAR Preliminary
“jet” part, near-side
ridge part, near-side
0-5%
jet part, near-side
ridge part, near-side
Au+Au 200 GeV
STAR Preliminary
A.Feng (STAR), QM2008
ridge: decreases its magnitude
with φS
“jet”: slight increase with φS
magnitude consistent
with d+Au
out-of-plane fS=90o
in-plane fS=0o
Jana Bielcikova
XLIII Rencontres de Moriond

15. 3-particle DhxDh correlations
2) In medium radiated
gluons diffused in 
1) Jet fragmentation
in vacuum
In medium radiated gluons still collimated
Data:
STAR Preliminary
d+Au
Au+Au
3<pTtrig<10 GeV/c, 1<pTassoc<3 GeV/c, ||<0.7
STAR Preliminary
Uniform overall excess of associated particles observed at intermediate pT (more data needed to look at higher pT)
P. Netrakanti (STAR), QM 2008
Jana Bielcikova
XLIII Rencontres de Moriond

16. XLIII Rencontres de Moriond
Summary
Strong modification of correlation patterns (not present in d+Au collisions) observed in central A+A collisions at RHIC:
broadening of away-side peak with angular substructure
inconsistent with Čerenkov gluon radiation
possible evidence for conical emission ?
(need to evaluate effects of momentum conservation)
medium responds through “ridge” formation in pseudo-rapidity
bulk-like properties (spectra, particle composition)
medium density/path length effects (dominated in the event plane)
further studies of 3-particle correlations to understand ridge underway
THANK YOU!
XLIII Rencontres de Moriond
Jana Bielcikova

17. XLIII Rencontres de Moriond
BACKUP
XLIII Rencontres de Moriond
Jana Bielcikova

18. Di-hadron correlations w.r.t. event plane
Bedanga Mohanty
Di-hadron correlations w.r.t. event plane
trigger
in-plane
out-of-plane
3< pTtrig < 4 GeV/c, pTassoc= GeV/c
out-of-plane fS=90o
in-plane fS=0o
A.Feng (STAR), QM2008
Au+Au 200 GeV
STAR Preliminary
d+Au
20-60%
0-5%
RMS
20-60% : away-side evolves from single (φS=0o) to double peak (φS=90o)
0-5% : double peak shows up at smaller φS
at large φS there is little difference between two centrality bins
Centrality and φS dependence:
XLIII Rencontres de Moriond
Jana Bielcikova

19. Di-hadron correlations: pathlength effects
Bedanga Mohanty
Di-hadron correlations: pathlength effects
3< pTtrig < 4 GeV/c, pTassoc= GeV/c
STAR Preliminary
v2{4}
v2{EP}
v2 syst. error
Au+Au 200 GeV
in-plane:
similar to d+Au in 20-60%
broader than d+Au in central collisions
out-of-plane:
small difference between the two
centralities
The RMS increases with phiS.
A.Feng (STAR), QM2008
Away-side features reveal
pathlength effects
20-60%
0-5%
XLIII Rencontres de Moriond
Jana Bielcikova

20. Extracting near-side “jet” and “ridge” yields
pTtrig=3-4 GeV/c, pTassoc>2 GeV/c
J = “jet”, R= “ridge”
const bkg. subtracted
(J)
||<0.7 2 
const bkg. subtracted
(J)
||<0.7 1
J. Putschke (STAR),QM’2006
(J+R)
||<1.7
(J+R)
||<1.7
v2 subtracted
(J)
no v2 subtraction needed
(J+R)
- (R)
XLIII Rencontres de Moriond
Jana Bielcikova

21. What is the origin of the ridge? (I)
1) Medium heating and parton recombination
Chiu & Hwa Phys. Rev. C72:034903,2005
hard parton enhances thermal parton distribution
(ΔT=15 MeV)
 recombination of thermal partons forms
a pedestal (ridge)
enhanced baryon/meson ratio
2) Parton radiation and its coupling
to the longitudinal flow
gluon bremsstrahlung of hard-scattered parton
radiated gluon contributes to broadening
Armesto et al, PRL 93 (2004)
Bormio 2008
Jana Bielcikova

22. What is the origin of the ridge? (II)
3) Longitudinal broadening of quenched
jets in turbulent color fields
A. Majumder, B. Mueller, S.A.Bass, hep-ph/
plasma instabilities in expanding medium
 non-thermal color fields
 broadening of jet cone
 wide ridge in rapidity at low pTassoc
Bormio 2008
Jana Bielcikova

23. What is the origin of the ridge? (III)
4) Correlations between jet and radial flow
S. Voloshin, nucl-th/ , Nucl.Phys. A749, 287 (2005)
E. Shuryak, arXiv: [nucl-th]
radial expansion + jet quenching  correlation
ridge is independent of jet
particle spectra in ridge:
points of origin are biased towards surface
 ‘a bit’ stiffer slope than that of bulk
5) Momentum kick
C.-Y. Wong , arXiv: , arXiv:
medium partons acquire ‘kick’ from propagating jet
T=470 MeV, q1 (mom. kick) and σy (rapidity distribution)
narrow peak in Df depends mainly on momentum kick
ridge in Dh depends on initial parton y distribution
More quantitative theoretical
predictions are needed!
Bormio 2008
Jana Bielcikova