dunlop, ww 2006 1 what more can be learned from high pt probes at rhic? james dunlop brookhaven...
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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
Dunlop, WW 2006 4
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)
Dunlop, WW 2006 5
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
Dunlop, WW 2006 18
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