Hard Probes in Heavy Ion Collisions

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Hard Probesin Heavy Ion Collisions
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Contents
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1. Introduction
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Contents
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Direct photons pp and dAu

• NLO pQCD works very well.

PHENIX, PRL '07 QM06
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Direct photons AuAu

• NLO pQCD works very well for high pT (Turbide et
  al '05 Fries et al, '05
• Zakharov '04 Arleo '06). Low pT might demand
  something else. v2direct ?0.

PHENIX, QM06
PHENIX, PRL '07 QM06
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Dileptons DY, nuclear pdf's

• NA60 confirms the NA50 excess (factor 2-3) of
  intermediate mass
• dileptons in InIn. They are prompt not charm
  decays thermal?
• DY in the forward region test of parton
  densities (initial state eloss,
• saturation ideas,...) at low x.
• Small masses subject to considerable
  uncertainties e.g. factorization,
• higher twists,... DY may become an important
  background for electrons.

Armesto et al '06
Qiu, Zhang '02
Double/single scatt.
single electrons
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Contents
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Radiative eloss

• Medium-induced gluon radiation dominant over
  elastic scattering at high parton E ( pT at
  y0)
• Degrades the energy of the leading hadron jet
  quenching.
• Broadens the associated parton shower.
• Increases the associated hadron multiplicity.

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Light hadrons
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Non-photonic electrons

• pQCD (FONLL) underestimates pp (2-5).
• AuAu data compatible with 100 charm.

Armesto et al '05 06
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qhat
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Jet quenching in AdS/CFTLiu, Rajagopal,
Wiedemann, hep-ph/0612168

• Define qhat through th expectation value of a
  Wilson loop.
• Use the AdS/CFT correspondence to compute it at
  strong coupling.
• It is in the right order of magnitude 1
  GeV2/fm, T3 but not to the dof.
• N4 SYM is not QCD no confinement or quarks,...
  but the result looks
• solid with respect to deformations of the metric,
  introduction of flavors,...
• Parallel developments for the drag force of a
  low momentum Q.
• This (AdS/QCD) is (mainly) a model for
  confinement, and it has motivated
• re-thinking e.g. screening lengths and qhat in a
  flowing medium, expansion.

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Other physics mechanisms
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Medium-jet interplay

• High associated pT
• tangential emission
• (Dainese et al '05, Zhang et al
• '07), medium/flow
• interplay (Voloshin '03,
• Armesto et al '04, Renk,
• Ruppert 05), radiation
• (Vitev '06, Polosa, Salgado
• '06),...

pT(assoc) gt 0.15 GeV/c

• Low associated pT sonic boom
• (Stocker '04, Casalderrey, Shuryak '04,
• Muller, Ruppert, Renk '05),
• Cherenkov radiation (Dremin '05,
• Koch, Majumder, Wang '05),
• radiation, flow effects,...

STAR '05
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Problems with finite E constraints
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Contents
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Baseline

• ee- 60-80 of J/psi produced with more charm
  (Belle, BaBar) higher
• orders in NRQCD?, additional mechanisms (Kaidalov
  '03).
• pp polarization puzzle goes on NRQCD? (Nayak,
  Qiu, Sterman '05).
• pA smaller absorption at RHIC than at SPS )
  unexpected from theory!,
• negative Feynman-x (HERA-B).

HERA-B at HP06
?
PHENIX '05
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Sequential suppression

• In the last 5 years, lattice results and
  potential model calculations support
• a sequential melting of quarkonium in the QGP.

• Sequential melting provides an alternative
  mechanism (others comovers,
• percolation,...) to explain data (Karsch,
  Kharzeev, Satz '05).

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Recombination

• At RHIC/LHC, 10/100 ccbar pairs per collision
  regeneration?

• To be tested by rapidity distributions, pT
  broadening, flow LHC?

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5. Conclusions

• Hard processes in HIC have a twofold interest
• Extension of pQCD to new domains new
  theoretical tools.
• Relation with other domains (hard and high
  density QCD, strings).
• Characterization of the produced medium.
• Together with v2 and the (anti)baryon to meson
  anomaly, they have been
• key to establish the production of high density
  matter in HIC at RHIC.
• Photons and dileptons offer a well defined
  benchmark at high enough pt.

• Single particles yields understood, open
• issues in single e and differential probes.
• Much work ahead in charmonium.
• Lesson from RHIC control experiments
• (pp, dAu) must be an integral part of the
• HIC program to get clear conclusions.
• LHC large yields of hard processes will
• be available this subject will play a central
• role in the heavy ion program.

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BDMPS/GLV
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AMY/MW
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The role of length
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The role of expansion
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The BW proposal (hep-ph/0506218)
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Medium-modified Sudakovs(NA, Cunqueiro,
Salgado, Xiang, in preparation)
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Medium-modified ff (ACSX in preparation)
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RAA (ACSX in preparation)

• Different choices of scales in the
• ff lead to the same description of pp
• but to different medium results for
• transverse momentum lt 10 GeV/c.
• qhat 1 GeV2/fm (L6 fm fixed).