What’s Missing in our Current Picture from High pT

Measurements at RHIC?

Saskia Mioduszewski

Texas A&M University

23 March, 2007

Single-Particle Spectra – What have we learned?

Hadrons are suppressed, photons are not – photons serve as the “control” experiment

High-pT suppression comparison to theory: GLV dNg/dy ~ 1000 I. Vitev and M. Gyulassy, PRL 89 (2002) 252301

Initial energy density: 0 ~ 15 GeV/fm3

PHENIX, Phys. Rev. Lett. 96, 202301 (2006)

What can we learn about Energy Loss?

Fractional effective energy loss: Sloss (MJT)

“Effective” because of surface bias when analyzing single particle spectra

PHENIX 0 SpectrumRenk and Eskola, hep-ph/0610059

8 < pT < 15 GeV/c

PHENIX, nucl-ex/0611007

New Understanding of Energy Loss through Heavy Flavor?

GLV calculation requires collisional energy loss to describe electrons from heavy-flavor decays

perhaps collisional energy loss not negligible as previously assumed

STAR: nucl-ex/0607012, PHENIX (QM2006): nucl-ex/0611018

S. Wicks, QM 2006

Heavy flavor suppression– Challenge to Theory

Electrons from heavy-flavor decays are more suppressed than expected, in particular with non-zero contribution from bottom

p+p collisionsX. Lin, QM 2006

pedestal and flow subtracted

4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig

Di-Jets through Hadron-Hadron Correlations

“Disappearance of away-side jet” in central Au+Au collisions

0-5%

Escaping Jet“Near Side”

Lost Jet“Far Side”

STAR, PRL 90 (2003) 082302

IAA (Jet-correlated Yield in AA) / (Jet-correlated Yield in pp)

Evolution of Jet Structure

At higher trigger pT (6 < pT,trig < 10 GeV/c), away-side yield varies with pT,assoc

For lower pT,assoc (1.3 < pT,assoc <1.8 GeV/c), away-side correlation has non-gaussian shape becomes doubly-peaked for lower pT,trig

pedestal and flow subtracted

4 < pT,trig< 6 GeV/c, 2 < pT,assoc< pT,trig

M. Horner, QM 2006

STAR preliminary

0-12% 200 GeV Au+Au

Hard-soft correlations

Hard-soft: away-side spectra approaching the bulk.

Inclusive in top 5%?

Three-particle correlation – N.N. Ajitanand, J. Ulery

Mediumaway

near

deflected jets

away

near

Medium

mach cone

STAR, PRL 95,152301 (2005)

4 < pT,trig< 6 GeV/c

Near-side Correlation

Additional long-range correlation in

Au+Au 20-30%

the “ridge”

Coupling of high pT partons to longitudinal expansion - Armesto et al, PRL 93 (2004)QCD magnetic fields- Majumder et al, hep-ph/0611035In recombination framework: Coupling of shower partons to thermal partons undergoing longitudinal expansion- Chiu & Hwa Phys. Rev. C72:034903,2005Radial flow + trigger bias –S.A. Voloshin, Nucl. Phys. A749, 287 (2005)

J. Putschke, QM 2006Au+Au 0-10%

STAR preliminary

(J+R)

||<1.7

J = near-side jet-like corrl.

R = “ridge”-like corrl.

v2 modulated bkg. subtracted

(J+R)

||<1.7

flow (v2)corrected

Extracting near-side “jet-like” yields

1

Au+Au 20-30%

2

2

(J+R)- (R)

con

st b

kg.

sub

tra

cte

d

(J

)

||

<0.

7

(J)

no bkg. subtraction

const bkg. subtracted

(J)

||<0.7

J. Putschke, QM 2006

STAR preliminary

“Jet” spectrum vs. “Ridge” spectrum

“jet” slope“ridge” slopeinclusive slope

efft Tptt epdpdN //

J. Putschke, QM 2006

STAR preliminary STAR preliminary

Ridge Yieldpt,assoc. > 2 GeV

STAR preliminary

Ridge yield persists up to highest trigger pT and approximately constant yield

J. Putschke, QM 2006

“Reappearance of away-side jet”

With increasing trigger pT, away-side jet correlation reappears

4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig

STAR, Phys. Rev. Lett. 97 (2006) 162301

Surface Bias of Di-Jets?

Renk and Eskola, hep-ph/0610059

8 < pT,trig< 15 , 4< pT,assoc< 6 GeV/c

8 < pT,trig< 15 GeV/c

STAR, Phys. Rev. Lett. 97 (2006) 162301

Comparison of IAA to RAA

IAA = Yield(0-5% Au+Au) Yield(d+Au)

In the di-jets where trigger pT is 8-15 GeV/c, the suppression is same as for single particles as a function of pT

= Near-side IAA

= Away-side IAA

8 < pT(trig) < 15 GeV/c

D. Magestro, QM 2005

Near-side Yields vs. zT

After subracting the Ridge M. Horner, QM 2006

Away-side Yields vs. zT

M. Horner, QM 2006

Away-side suppression as a function of pT,trig

M. Horner, QM 2006

Away-side IAA

Away-side suppression reaches a value of 0.2 for trigger pT > 4 GeV/c, similar to single-particle suppression

IAA (Jet-correlated Yield in AA) / (Jet-correlated Yield in pp)

What other handles do we have?

Centrality, trigger and associated pT,…..

….Reaction plane

In-plane

Out-planeSTAR

4 < pT,trig< 6 GeV/c, 2 < pT,assoc< pT,trig

STAR, Phys. Rev. Lett. 93 (2004) 252301

Another handle: -jet

q

Photon-jet measurement is, in principle, sensitive to full medium

Bias to where away-side jet is close to surface?

Together with di-jet measurement for comparison

Another differential observable

Increasing ratio of direct photons to decay photons with centrality due to hadron suppression at high pT

PHENIX, Phys. Rev. Lett. 94, 232301 (2005)

Wang et al., Phys.Rev.Lett. 77 (1996) 231-234

1

/Ntr

igdN

/d

(rad)

Another handle: -jet

Current Results from Run-4 Au+Au collisions:

J. Jin, QM 2006T. Dietel, QM 2005

q

Summary

• Limited information extracted from single-particle pT spectra – Effective fractional energy loss reaches 20% for most

central collisions– Initial energy density ~ 15 GeV/fm3 from radiative

energy loss models

• Di-Jets (those that are observed) may have less surface bias

• Photon-Jet Measurement will complement the di-jet for more complete probe

• Heavy-flavor suppression not consistently described by theoretical models with light meson suppression – need elastic energy loss

Conclusions What is missing from our picture?

• Need a consistent theoretical approach to describe different observables

• Need more quantitative model predictions for “ridge” explanation

• Need more quantitative descriptions of jet modifications from experimenters– Particle species in ridge vs. in jet (J. Bielcikova’s talk)– Path-length dependence of jet-correlated yields (using

reaction plane dependence)

• Need a great deal of statistics for -jet measurement (J. Dunlop’s talk)

STAR preliminaryJet+Ridge ()

Jet ()

Jet)

yiel

d

,)

Npart

3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV

Sloss

• S(pT )/pT = S0,• is a constant for all pT > 3 GeV/c, which also results in a• constant ratio of the spectra, RAA(pT ). For the constant• fractional shift, the Jacobean is simply dS(pT )/dpT = S0

• RAA(pT ) = (1 + S0)−n+2

• RAA(pT )1/(n−2) =1/(1 + S0)• The effective fractional energy loss, Sloss, is related to• the fractional shift in the measured spectrum, S0. The• hadrons that would have been produced in the reference• p + p spectrum at transverse momentum pT + S(pT ) =• (1 + S0)pT , were detected with transverse momentum,• pT , implying a fractional energy loss:• Sloss = 1 − 1/(1 + S0) = 1 − RAA(pT )1/(n−2)