Dunlop, WW 2006 1

What More Can Be Learned from High Pt Probes at RHIC?

James DunlopBrookhaven National Laboratory

Dunlop, WW 2006 2

Suppression

Suppression an established probe of the density of the mediumThe medium is dense: what more can be said?

J. Adams et al, Phys. Rev. Lett. 91 (2003) 072304

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Binary collision scaling p+p reference

Dunlop, WW 2006 3

Central RAA Data

Increasing density

The Limitations of RAA: “Fragility”

Surface bias leads effectively to saturation of RAA with density

Challenge: Increase sensitivity to the density of the medium

K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511

A. Dainese, C. Loizides, G. Paic, Eur. Phys. J. C38(2005) 461

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Black and White

• Medium extremely black to hadrons, limiting sensitivity to density • Medium transparent to photons (white): no sensitivity• Is there something grey?

S.S. Adler et al, Phys. Rev. Lett. 94, 232301 (2005)

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Baryon enhancement

• Large enhancement in baryon/meson ratios in central Au+Au collisions– Maximum at pT~3 GeV/c, after

which approach towards p+p

• Indication of dominant non-fragmentation contribution

• At what pT is this contribution no longer dominant?

STAR QM05, Barannikova

/K

0 s

Au+Au 0-10%

p+p

Au+Au 0-10%

p+p

Au+Au 0-5%

p+p

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Identified Particle RCP

All particles consistent for pT>~5 GeV: dominance of fragmentation?

• In principle, different contributions from gluons and quarks: where is the 9/4?

• My speculation: quarks also deep into saturation of RAA, no sensitivity

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STAR Preliminary

s-quark

0-5% Au+Au

p+p

RAA of Strange Particles

A note: RAA ≠ RCP

While RCP common across baryons, clear separation in RAA, increasingwith increasing strangeness content

Related to canonical strangeness suppression in p+p? Other mechanisms?

In any case, not known if this disappears at high pT or not

STAR QM05, S. Salur, nucl-ex/0509036

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Charm and Beauty

• In principle, single electrons are sensitive to charm and beauty– But, relative fraction of b/c uncertain, and calculations do not reproduce p+p data

• Hope is that heavier mass implies less energy loss (of all kinds)

Figure from N. Armesto et al, nucl-ex/0511257

Calculation from M. Cacciari et al, Phys. Rev. Lett. 95, 122001 (2005) Wicks et al, nucl-ex/0512076

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Suppression of single electrons

• Despite expectations, electrons suppressed at the same level as hadrons

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Suppression of single electrons

Current set of theoretical calculations moving towards data – Extreme densities (“violating entropy”) or addition of ignored elastic component – Uncertainty in b/c contribution limits strongly conclusions that can be made

• Side note: elastic contribution may have different “fragility”• Whatever solution, clear that the c quark must behave as u,d quarks: last hope for

more weakly interacting probe is the b quark

N. Armesto et al, nucl-ex/0511257

b

c

Wicks et al, nucl-ex/0512076

Data from QM05 STAR, J. Bielcik nucl-ex/0511005 PHENIX S.S. Adler et al, nucl-ex/0510047

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Beyond single particle spectra

• Overlap zone has ellipticity– path length dependence of suppression creates v2

– path length dependence can be probed with dihadron correlations

• Dihadron correlations introduce different geometric biases – Surface bias in trigger hadrons: longer pathlengths– No surface bias in trigger photons: full pathlength distribution?

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A note on v2: “non-flow”

p+p jet+jet (STAR@RHIC)

nucleon nucleonparton

jet

If one naively measures v2 inp+p collisions, how big a

signal do you see?(Hint: it’s not 0)

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

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v2 at “High” pT

• At intermediate pT, v2 far too strong for any quenching model

• At higher pT and for lighter systems, strong dependence on methods: large systematic errors (though see D. Winter for PHENIX’s view)

QM05 STAR, G. Wang, nucl-ex/0510034

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

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Dihadrons vs. Reaction Plane

• Indications of path length dependence in dihadron correlations

• To do: extend to higher pT (and better understand non-flow)

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

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Dijets from dihadrons

At high trigger pT, high associated pT:

clear jet-like peaks seen on near and away side in central Au+Au

STAR QM05, D. Magestro, nucl-ex/0510002

8 < pT(trig) < 15 GeV/c

STAR Preliminary

pT(assoc)>6 GeV

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Yields of away-side peaks

Away-side yield strongly suppressed

to level of RAA

No dependence on zT in measured rangeHow does this relate to surface/tangential

biases?

STAR QM05, D. Magestro, nucl-ex/0510002

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STAR Preliminary

Changing the probe: towards -jet in Au+Au

• Direct does not couple to medium or fragment into jets– remove from trigger both surface bias, fragmentation uncertainty in Q2

• Correlations triggered on clear near and away-side peaks • Strong contamination remains from 0 decay daughters

– Work in progress to separate out direct STAR QM05, T. Dietel, nucl-ex/0510046

From 30 ub-1

Full year Au+Au run with finalcalorimeters, RHIC+luminosity: ~0.5-1 nb-1

RHIC II: 30 nb-1/year

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Future programs

• RHIC: go rarer and rarer, in search for a weak probe– Beauty: last hope for a “grey”

probe; needs detector upgrades to both STAR and PHENIX to isolate from charm

– -jet: needs higher luminosity

• LHC: new energy frontier– Cross-sections much higher

but -jet backgrounds extend to higher pT; running time and luminosity lower

– Initial-state effects may be stronger

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Conclusions

• The medium is dense

• Progress occuring towards more quantitative statement– Strong quenching implies sensitivity dominated by geometry– Predictions of weaker quenching for single electrons from charm

and bottom not borne out by the data: active theoretical investigation

• Moving towards full tomography in the future– Significant samples of -jet events available with future

luminosities at RHIC and energies at LHC – Separation of charm, and most importantly for tomography,

beauty with upgraded RHIC detectors, and at the LHC