High-p T results from STAR – what did we learn?

High-pT results from STAR – what did we learn?

Jana Bielcikova for the STAR Collaboration

NPI ASCR and Center for physics of ultra-relativistic heavy-ion collisions, Prague, Czech Republic

Jana Bielcikova 4th workshop "High-pT at LHC", Prague 2

Outline:

• Introduction• Baryon/meson enhancement, quark vs gluon energy loss • Di-hadron correlations: system size dependence • “Jet”-medium interaction via 2- and 3-particle correlations - conical emission - ridge

- path length effects• Emerging and future studies: -jet, full jet reconstruction • Summary

Jana Bielcikova 3

Probing QCD matter with high-pT particles and jets

What happens to high-pT particles/jets which pass through the medium?Are they similar to p+p or modified by the medium?

p+pAu+Au

4th workshop "High-pT at LHC", Prague

Tools:• inclusive pT spectra• di-hadron correlations• multi-hadron correlations• jets, -jets

C. Natrass

O. Barannikova

J. PutschkeA. Hamed

See also talks by:


• large enhancement of B/M ratio in Au+Au relative to p+p collisions - reaches maximum at pT~3 GeV/c

• particle production via parton coalescence/recombination

Baryon/meson ratios at RHIC

d+A

u, Au+

Au: S

TA

R, P

RL 97 (152301) 2006

p+p: P

. Fachini (S

TA

R), Q

M2008

Au+Au: p/ ~ 1 Λ/K0S ~ 1.8

p+p: p/ ~ 0.3 Λ/K0S

~ 0.6e++e-: p/ ~ 0.1-0.2

M. Lam

ont (ST

AR

), SQ

M2006

Gluon jet contribution factor increases from , K, p towards: e.g. pT = 8 GeV/c: 50% for 90% for p

If

and

At high pT for same beam energy, system and centrality:

RCP() > RCP(p) RCP(K) > RCP()

Gluon vs quark energy loss (I)

4

9~

E

E

q

g

2 ˆ LqCE Rs

B. Mohanty (STAR), arXiv: 0705.0953

AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)

Jana Bielcikova 54th workshop "High-pT at LHC", Prague

High-pT (pT > 6 GeV/c): RCP()~RCP(p)

Gluon vs quark energy loss (II)

no color factor effect of 9/4 observed up to pT ~ 12 GeV/c ?

I. Vitev, QM2008

non-asymptotic limit: q and g losses are closer to each other


B.

Moh

anty

, P

. F

achi

ni (

ST

AR

), Q

M20

08

System size dependence of di-hadron yields6 < pT

trig < 10 GeV/c, pTassoc >3 GeV/c

Parton Quenching Model (PQM): BDMPS energy loss, Glauber geometry, bin scaling C. Loizides, Eur. Phys. J. C 49, 339 (2007)

Modified Fragmentation Model: NLO, higher twist, hard sphere geometry, part scaling Zhang, Owens, Wang, Wang, PRL 98, 212301

More model calculations needed to disentangle effects fromenergy loss and density scaling/profile

O. C

atu

(ST

AR

), QM

08

Near-side peak Away-side peak

zT=pTassoc/pTtrig


Jana Bielcikova 8

Conical emission in A+A collisions?2.5 < p

Ttrig< 4 GeV/c and 1< p

Tassoc < 2.5 GeV/c

40-60%

0-12%

Preliminary

• 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) 011901

- colored plasma Ruppert, Mueller, PLB618 (2005) 123

- AdS/CFT Gubser, Pufu, Yarom, PRL100, (2008) 012301

• Čerenkov gluon radiation by a superluminal parton Dremin, NPA750 (2006) 233 Koch et. al., PRL96 (2006) 172302

To distinguish from other mechanisms 3-particle correlation studies needed


M. H

orn

er (S

TA

R), J.P

hys.G

34

, S9

95

,20

07

4th workshop "High-pT at LHC", Prague 9

Conical flow or deflected jets? (I)

Mediumaway

near

deflected jets

away

near

Medium

mach cone

Medium

away

near

di-jets 00

π

π

Jana Bielcikova

deflected jets (by the collective movement, “flow”, of the expanding medium)

Armesto , Salgado, Wiedemann, PRL 93, (2004) 242301

conical emission

di-jets

cartoons of 3-particle azimuthal correlations(1 trigger + 2 associated particles)

courtesy to J. Ulery

10

Conical flow or deflected jets? (II)

STAR uses 2 methods:1. Jet+flow background - model dependent analysis- evidence for conical emission

2. Cumulant methodC. Pruneau (STAR),J.Phys.G34 (667),2007;C. Pruneau, PRC 74 (2006) 064910

- 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- an improved analysis with rotating EP shows conical structures

3 < pT,trig

< 4 GeV/c

1 < pT,assoc < 2 GeV/c

3 < pT,trig

< 4 GeV/c

1 < pT,assoc < 2 GeV/c

Abelev et al (STAR), arXiv:0805.0622

d+Au central Au+Au

central Au+AuC3

Subtraction of v2v2v4 termsusing v2 = 0.06

Subtraction of v2v2v4 term using v2 = 0.12

STAR Preliminary

central Au+Au

Note: Large and complicated backgrounds

STAR Preliminary

11

pT dependence of cone angle

Cone angle

independent of pTassoc


Čerenkov gluon emission: cone angle decreases with pT

assoc

BUT! Hydro: expects pT dependence as well (G. Torrieri, priv.comm.)

Con

e an

gle

(rad

ians

)

pT (GeV/c)

STAR preliminary

Koch, Majumder, Wang, PRL96 172302 (2006)


Momentum conservation in correlation analysesC3 cumulant pT conservation 3-particle correlation

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 Data: possibly a fraction of stat. conservation: assuming bkg follows exp. drop a small effect for pT

trig~3-4 GeV/c

N. Borghini, PRC 75 (2007) 021904

2-particle correlation after v2 subtraction

Calculation for:<p

T,trig > = 3.2 GeV/c

<pT,assoc >= 1.2 GeV/c

Calculation for:<p

T,trig > = 3.2 GeV/c

<pT,assoc >= 1.2 GeV/c

N. Borghini, arXiv:0710.2588

out-of-plane S=90oin-plane S=0o

3< pTtrig < 4 GeV/c, pT

assoc=1.0-1.5 GeV/c

trigger in-plane

trigger out-of-plane

• 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:A.Feng (STAR), QM2008

Au+Au 200 GeV STAR Preliminary

STAR Preliminary

d+Au20-60%

0-5% Au+Au 200 GeV

d+Au

Away-side peak: path-length effects?

RMS


shoulder

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

Away-side features reveal path-length effects

STAR Preliminary

v2{4}

v2{EP}

v2 syst. error

Di-hadron correlations: path-length effects


assoc=1.0-1.5 GeV/c

Au+Au 200 GeV

A.Feng (STAR), QM2008


A.Feng (STAR), QM2008

Yield in shoulder region independent of s

Yield in peak region decreases with s

“shoulder”

A closer look at the near-side peak …

Jana Bielcikova 15

Additional near-sidecorrelation in observed in central Au+Au collisions

pTtrig=3-4 GeV/c, 2 GeV/c <pT

assoc< pTtrig

“jet”ridge

d+Au, 200 GeV Au+Au, 200 GeV

The ridgeology …1) Medium heating and parton recombination Chiu & Hwa PRC 72 (2005) 034903

2) Radial flow + high-pT trigger particle Shuryak, Phys.Rev.C76 (2007) 047901

Voloshin, nucl-th/0312065 NPA 749, (2005) 287 Gavin, Pruneau, Voloshin , NPA 802, 107 (2008)

3) Parton radiation and its coupling to the longitudinal flow Armesto, Salgado, Wiedemann, PRL 93 (2004) 242301

4) Momentum broadening in anisotropic QGP Romatschke, PRC 75 (2007) 014901

5) Longitudinal broadening of quenched jets in turbulent color fields Majumder, Mueller, Bass, PRL 99 (2007) 042301

6) Momentum kick imparted on partons in medium Wong, PRC PRC 76 (2007) 054908

7) Glasma flux tubes Dumitru, Gelis, McLerran, Venugopalan, arXiv: 0804.3858

…


STAR preliminary


Ridge properties

Ridge yield: • increases steeply with centrality• ~ independent of pT

trigger

• baryon/meson ratios and pT -spectra are ‘bulk-like’

• system size and energy dependence see talk by C. Nattrass

STAR preliminary

Rid

ge y

ield

pTtrig (GeV/c)

pTassoc> 2 GeV/c

h-h, Au+Au 200 GeV Ridge: solid symbolsJet: open symbols

STAR preliminary

Ridge

Jet

inclusive

out-of-plane S=90oin-plane S=0o


assoc=1.0-1.5 GeV/c

Near-side peak: path-length effects?


trigger in-plane

trigger out-of-plane

Ridge component ||>0.7

“Jet” component

Au+Au 200 GeV

20-60%

STAR Preliminary

“jet” part, near-side

ridge part, near-side

ridge yield decreases with φS ridge larger in-plane “jet” yield: slight increase with φS magnitude similar

to d+Au

Jana Bielcikova 18

1) Jet fragmentation in vacuum

d+Au Au+Au

3<pTtrig<10 GeV/c, 1<pT

assoc<3 GeV/c, ||<0.7

STAR Preliminary

Uniform overall excess of associated particles observed at intermediate pT - more data needed - studies at higher pT

assoc

and pTtrigger

Note: Involves complicated background subtraction

3-particle xcorrelations2) In medium radiated gluons diffused in

3) In medium radiated gluons collimated

by longitudinal flow


Data:STAR Preliminary

P.

Ne

tra

kan

ti (S

TA

R),

QM

20

08

4) Combination of jet fragmentation and diffused gluons

The ‘soft’ ridge in non-triggered correlations


M. Daugherity (STAR), QM08

stat. errors only, syst.errors 9% (correlation amplitude)

M. D

augherity (ST

AR

), QM

08

• “Minijet” near-side peak amplitude and widthexperience a sharp transition at ~55% centralityfor 200 GeV and ~40% for 62 GeV Au+Au data. x• width is const. from mid-central to central Au+Au collisions

# correlated pairs/particle


Emerging and future high-pT studies

Full jet reconstruction in Au+Au collisions

The first full jet reconstruction in heavy-ion collisions!

J. Putschke (STAR)Hard Probes 2008


Jet spectrum and fragmentation function in Au+Au

• Jet spectrum: need to understand HT trigger bias

• Fragmentation function: no apparent modification with respect to p+p

Leading Order High Seed Cone(LOHSC)R=0.4, pT = 1 GeV/c, seed = 4.6 GeV/c

See talk by J. Putschke


-hadron correlations

• a “golden probe” of parton energy loss in the medium

•precise measurement of the in-medium modification of fragmentation function

See talk by A. Hamed


STAR preliminary

A. H

am

ed

(ST

AR

),QM

20

08

A. H

am

ed

(ST

AR

),Ha

rd P

rob

es 2

00

8

RHIC upgrade, stochastic cooling

Projected performance for-hadron measurement

TOF (Run 10): PID, clean e± μ± ID down

to low pT

DAQ1000 complete (Run 9):3x improvement in sampled luminosity for rare probes(e.g. jets in p+p)

Heavy Flavor Tracker (Run 13):direct measuruments of charm/bottom

STAR and RHIC upgrades



Summary• Constraining medium density using combined fits to RAA and IAA

large d+Au data sample will help to reduce exp. uncertainties

• Strong modification of correlation patterns in central A+A collisions at RHIC:

• broadening of away-side peak with angular substructurepossible evidence for conical emission

• medium responds through “ridge” formation in pseudo-rapidity bulk-like properties (spectra, particle composition) medium density/path length effects (dominated in the event plane)

• THE future measurements: -jets and full jet reconstruction are emerging

Stay tuned for new results coming from Run7 and Run8 + future STAR upgrades

High-p T results from STAR – what did we learn?

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