1. Hard Processes in Heavy Ion Collisions N. Armesto XXXIII International Conference on High Energy Physics ICHEP'06 Moscow, July 26th-August 2nd 2006 Néstor Armesto Departamento de Física de Partículas and Instituto Galego de Física de Altas Enerxías Universidade de Santiago de Compostela 1

2. Contents N. Armesto Hard Processes in Heavy Ion Collisions 1. Introduction. 2. Benchmark processes: direct photons and dileptons. 3. Jet quenching. 4. Quarkonium suppression. 5. Conclusions. General references: Yellow Report on Hard Probes in Heavy Ion Collisions at the LHC; Hard Probes 2006 (http://hp2006.lbl.gov/). See the talks by Arkhipkin, Buesching, Dremin, Gay Ducati, Hallman, Levai, Lokhtin, Safarik, Sarycheva, Wang and Woehri. 2

3. 1. Introduction: N. Armesto Hard Processes in Heavy Ion Collisions 3 Jet quenching/heating Quarkonium suppression Control of the benchmark: prompt photons and dileptons ● Hard processes (probes of the medium created in a HIC): those whose benchmark (result of the probe in cold nuclear matter) is computable within perturbative QCD, for which a hard scale is required (p T, m Q,...>>1/R h ). ● Strategy: no medium (pp) and cold nuclear matter (pA) understood in pQCD define the benchmark for the probe; results in hot medium (AB) and their difference with expectation provide a (pQCD or not) characterization.

4. Direct photons: N. Armesto Hard Processes in Heavy Ion Collisions: 2. Benchmark: direct  and dileptons 4 ● NLO pQCD works very well for high p T (Turbide et al '05; Fries et al, '05). Low p T in AuAu demands something else. v 2 direct  ~0. D’Enterria, Peressounko '05

5. Dileptons: DY, nuclear pdf's N. Armesto Hard Processes in Heavy Ion Collisions: 2. Benchmark: direct  and dileptons 5 ● NA60 confirms the NA50 excess (factor ~ 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. Qiu, Zhang '02 Double/single scatt. single electrons Armesto et al '06

6. Contents N. Armesto Hard Processes in Heavy Ion Collisions 3. Jet quenching: * Radiative energy loss. * Light hadrons. * Non-photonic electrons at RHIC. * qhat, or how opaque is the medium? * Other physical mechanisms. * Medium-jet interplay. 6

7. Radiative eloss: N. Armesto Hard Processes in Heavy Ion Collisions: 3. Jet quenching 7 Medium-induced gluon radiation dominant over elastic scattering at high parton E (~ p T at y=0): ● Degrades the energy of the leading hadron: jet quenching. ● Broadens the associated parton shower. ● Increases the associated hadron multiplicity., The BDMPS (GLV, ZW, AMY) formalism describes this process: interference of production and re-scatterings of the radiated gluon, Transport coefficient Fragmentation outside the medium (p T > 7 GeV?; HF; LHC?)

8. Light hadrons: N. Armesto Hard Processes in Heavy Ion Collisions: 3. Jet quenching 8 p T parton >5 GeV Detailed modeling of geometry. (Quark Matter 05) Dainese, talk at PANIC05 D'Enterria '05 Dainese et al '04

9. Non-photonic electrons: N. Armesto Hard Processes in Heavy Ion Collisions: 3. Jet quenching 9 color charge mass effect Genuine prediction of radiative eloss:  E (g) >  E (q) >  E (Q) Armesto et al '05; ‘06 ● pQCD (FONLL) underestimates pp (3-5). ● AuAu data compatible with 100 % charm.

10. qhat and medium opacity: N. Armesto Hard Processes in Heavy Ion Collisions: 3. Jet quenching 10 ● pQCD (Baier '02) favors qhat < 1 GeV 2 /fm. ● Strong coupling computations using the AdS/CFT correspondence are now under debate (Liu et al ’06). ● Surface bias (Muller '03). ● Where the discrepancy between models comes from?: ● AMY (Jeon et al '05) gives qhat ~ 2 GeV 2 /fm: expansion, no interference. ● MW gives qhat ~ 3-4 GeV 2 /fm: expansion, interference. ● Eskola et al, Dainese et al gives qhat ~ 4-14 (X3) GeV 2 /fm: no expansion, interference. ● GLV gives qhat < 1 GeV 2 /fm: expansion, interference. Reasons?: value of  s, expansion, interference, quenching weights, treatment of the length,... Eskola et al '04 Salgado, Wiedemann '03  s =1/3-1/2

11. Other physical mechanisms: N. Armesto Hard Processes in Heavy Ion Collisions: 3. Jet quenching 11 If R AuAu e <0.4 in the range 5<p T <10 GeV: ● Strong interaction of Q with the medium; hadronization inside, elastic scattering?, three body processes (Djordjevic et al '06, Hees et al '05, Teaney et al '05, Liu, Ko '06). ● Larger transport coefficient? (but upper bound to come, hopefully, from correlations (PHENIX '05; STAR '06) ).

12. Medium-jet interplay: N. Armesto Hard Processes in Heavy Ion Collisions: 3. Jet quenching 12 STAR'06 p T (assoc) > 0.15 GeV/c 4 < p T (trig) < 6 GeV/c STAR '05 ● High associated p T : tangential emission (Dainese et al '05), medium/flow interplay (Voloshin '03, Armesto et al '04, Renk, Ruppert ‘05), radiation (Vitev '06, Salgado '06),... ● Low associated p T : sonic boom (Stocker '04, Casalderrey, Shuryak '04, Muller, Ruppert, Renk '05), Cherenkov radiation (Dremin '05, Koch, Majumder, Wang '05), flow effects,...

13. Contents N. Armesto Hard Processes in Heavy Ion Collisions 4. Quarkonium suppression: * Baseline: from e + e - to dAu. * Sequential suppression. * Recombination. 13

14. Baseline: N. Armesto Hard Processes in Heavy Ion Collisions: 4. Quarkonium suppression 14 ● e + e - : 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, negative Feynman-x (HERA-B):  PHENIX '05

15. Sequential suppression: N. Armesto Hard Processes in Heavy Ion Collisions: 4. Quarkonium suppression 15 ● 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) : p T broadening?

16. Recombination: N. Armesto Hard Processes in Heavy Ion Collisions: 4. Quarkonium suppression 16 ● At RHIC/LHC, ~ 10/100 ccbar pairs per collision: regeneration? ● To be tested by rapidity distributions, p T broadening, LHC?

17. 5. Conclusions: N. Armesto Hard Processes in Heavy Ion Collisions 17 ● Hard processes in HIC have a twofold interest: * Extension of pQCD to new domains: new theoretical tools, relation with other domains (hard QCD, high density QCD),... * Characterization of the produced medium. ● Together with v 2 and the (anti)baryon to meson anomaly, they have been key to establish the production of high density matter in HIC at RHIC. ● 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: if problems are solved, this subject will play a central role in the heavy ion program.