Hard Probes at LHC Theoretical Perspective

Hard Probes at LHCTheoretical Perspective Berndt Müller Hard Probes 2006 Monterrey June 9-16, 2006

The LH-I-C • Heavy ion physics at the LHC is only ~2 years away • The LHC program will be the last step of RHI physics into uncharted territory for a long time (forever?) • Are we (the theory community) ready for it? • What is our grand strategy for the RHIC + LHC era?

Physics goals for the LHC • If RHIC is a “discovery facility”, then the LHC should be the “confirmation / consolidation facility”: • Does the theoretical framework developed at RHIC hold up? • LHC will provide quantitative tests of the models developed to describe the RHIC data: • Saturation of the initial gluon density • (Almost) ideal hydrodynamic evolution of matter (v2) • Scaling of parton energy loss with ∫rt dt • Color screening, quark recombination • Major new probes: contained jets and b-quarks, permitting much improved control of theoretical predictions.

initial LHC state RHIC LHC QCD phase diagram

RHIC LHC ? QCD equation of state 170 340 510 MeV

sQGP or tQGP ? • Low viscosity and large parton energy loss indivate that QGP probed at RHIC is strongly coupled (sQGP). • Recent studies of effects of plasma instabilities suggest that “strong coupling” effects may be produced by turbulent behavior of an expanding weakly coupled QGP (tQGP). • Consequences for LHC are very different: • sQGP picture suggests that physics of weakly coupled QGP at LHC (at early times) could be totally different • tQGP effects persist under LHC conditions  picture suggests that medium produced at LHC will behave very similar to that produced at RHIC.

100 GeV jets similar to 2 GeV hadrons at RHIC Bulk physics probes 10-4 < x < 10-3 Forward/backward regions provide access to very small x  10-5 LHC kinematic range

The “soft” environment What kind of state do we expect to be produced in Pb+Pb collisions at the LHC ?

~ 1/Q2 Initial state: Saturated glue Nonlinear interactions among classical fields lead to the saturation of gluon density in the transverse plane. “Color Glass Condensate” Glassy gluons are liberated – quark pairs are produced rapidly. “Glasma” turns into a quark-gluon plasma..

LHC LHC 3 4 RHIC RHIC ECM dependence: dN/dy, dE/dy NLO pQCD with geometric parton saturation (Eskola et al. - EKRT) expansion

h+hbeam Limiting fragmentation RHIC data (PHOBOS) show universal dN/dh in fragmentation region with central plateau, which grows with ECM. Gelis, Staśto, Venugopalan - hep-ph/0605087

3 (Armesto et al. hep-ph/0407018) From fit to HERA e-p and NMC nuclear photoabsorption data. 2 ECM dependence of dN/dy Geometric scaling à la Golec-Biernat & Wüsthoff

LHC conditions: synopsis LHC kinematic range is much larger than RHIC’s: • Higher energy density e0 at earlier time t0. • Jet physics can be probed to pT > 100 GeV. • b, c quarks are plentiful, good probes. • Increased lifetime of QGP phase (10-15 fm/c) makes preequilibrium effects less important. • Even more rapid expansion reduces life-time of final-state hadron gas.

Hard Probes… • Hard Probes are Standard Model observables that can be predicted perturbatively, with the exception of some infrared sensitive parts that either: • Can be determined from other measurements or by means of reliable lattice simulations, • Or, are the quantities to be probed. • It is important to know, in which range of kinematic parameters a probe can be considered as “hard” in this sense. • It is important to understand how strongly the extraction of physics from a hard probe depends on a detailed understanding of (and sufficiently realistic modeling of) bulk matter properties.

List of (possible) hard probes • High-pT hadrons • High-pT di-hadrons (or g + hadron) • Single jets • g-jet correlations • Heavy quarkonia • High invariant mass lepton pairs • High pT photons • W and Z bosons accessible at RHIC and LHC accessible at LHC (but not at RHIC ?)

Hard yields at LHC • Charm (bottom) production predicted to increase from RHIC to LHC by factor 10 (100) • ~100 c pairs and ~5 b pairs per central Pb-Pb collision • 20 jets with Etot > 50 GeV per second • 1 W-boson per second • 1 Z-boson every 3 seconds But note that dN/dy increases by factor ~3, dET/dy by factor ~5.

Are we ready… …to make predictions of hard probes for the LHC ? • The ground work has been laid: • Hard probes in heavy ion collisions at the LHC • PDF’s, shadowing, and pA collisions – hep-ph/0308248 • Jet physics – hep-ph/0310274 • Heavy flavour physics – hep-ph/0311048 • Photon physics– hep-ph/0311131

State of theory Do we have a coherent theoretical framework ? • High-pT hadrons: YES • High-pT di-hadrons (or g + hadron): MAYBE • Single jets: YES • g-jet correlations: YES • Heavy quarkonia: NO • High invariant mass lepton pairs: YES • High pT photons: MAYBE • W and Z bosons: NO

Ubi sumus? An illustrative example: High pT hadrons (aka. Jets)

D(1) Q2 Medium modified fragmentation functions: L q q g Measured medium property: D(2) PQCD framework: Jets F(1) F(2) Factorization:

The devil is in the details • A general theoretical framework is not enough: • Extraction of q^ requires detailed modeling of reaction geometry (scattering vertices, density distribution, path lengths, expansion, etc.). • Value of q^ extracted from data is correlated with assumptions about path length L and expansion pattern. • Extracted values range from q^ = 0.5 GeV2/fm  15 GeV2/fm. • Prediction of RAA for heavy quarks has failed: • Observed suppression of D mesons can be (barely) reconciled with help of additional elastic collisional energy loss mechanism,… • …if down-feeding from b-quarks is ignored. • Contribution from quark recombination at intermediate pT was not expected: • Domain of dominance depends sensitively on transverse flow.

Vitev et al (GLV) Armesto et al (ASW) LHC Flat or rising RAA ? Extrapolations to LHC energy vary widely due to modeling differences:

Prod. points in (x,y) for partons giving hadrons with pT > 5 GeV: increasing s Why RAA is flat • Most partons produced in the center are totally absorbed • Long path lengths are exploited only by high momentum partons • RAA does not increase at high pT • What holds for singles, does not hold for coincidences… Loizides, nucl-ex/0501017

Di-hadrons probe interior Deeper penetration of higher-pT probe leades to increased “punch-through” of away side jet. Detector T. Renk Vertex distribution for trigger hadrons of pT – 25 GeV/c in Pb+Pb @ LHC

B/h ratio at LHC • For 10 < pT < 20 GeV, dominant effect due to b mass. • RB/h enhancement at “low” pT probes mass dependence of (radiative) energy loss

R.J. Fries, BM, nucl-th/0307043 includes parton energy loss br = 0.85 br = 0.75 br = 0,65 Hadron production at LHC • Study of RAA can be extended to full measurement of modified Dph(z). • Full energy loss distribution becomes relevant. Suggestion to theorists: Publish w(DE) integrated over emission points, angles, for comparison among models. Recombination: Increased flow at LHC pushes domain of reco dominance to higher pT. Thermal-shower reco may even dominate for baryon production to very large pT (Hwa et al.) frag reco

Soft background: Distributed over ~300 particles. “Jet quenching” Jet energy is not lost, but just redistributed inside the jet cone to larger kt than in vacuum fragmentation (LPM effect). Separation of fully evolved jet from background will become possible at LHC for large jet energies. kT q Medium modifications of jet shape can tell us about the mechanism of energy loss of the initiating parton. Are jets induced by b-quarks modified differently than those induced by light quarks/gluons?

But if the gluon gets absorbed, how can we distinguish the process from collisional energy loss, which involves absorption of a virtual gluon by the medium? q q g Is there a Mach cone? abs. rad. coll. Jet-medium interactions Medium not only leads to modification of jet due to induced radiation of gluons (due to elastic interactions with the radiated gluons), but also interacts inelastically with the radiation: Radiated gluons can deposit energy and momentum into the medium. Differentiation of collisional and radiative energy loss must be subsumed into an analysis of the elastic and inelastic interactions of the developing jet with the medium. The theoretical approach needs to be developed to match onto experimentally accessible kinematic observables and/or cuts.

Other hard probes

NRQCD CEM CEM Quarkonium production pQCD framework: Either NRQCD factorization including color octet components of the (Q-Qbar) state and feed-down from excited states or color evaporation model. J/Y  NRQCD

(a) Spectral function: LQCD simulations, with analytic continuation to real time, suggest Td~ 1.5 - 2Tc for J/Y and . But width G(T) unknown. (b) Inelastic dissociation: pQCD calculations of gluon absorption give substantial rates for LHC conditions. Datta et al.  J/Y Quarkonium emission No comparable theoretical framework exists, in which to treat medium modifications to (Q-Qbar) spectral function, (in-)elastic dissociation by the medium, recombination in a unified manner.

Recombination is unavoidable, if c-quarks are “dense” and thermalized. (Semi-)realistic calculation with full geometry and flow are needed, but the contribution is potentially large, and growing with c.m. energy. J/Y RHIC LHC Quarkonium emission II Ionization by thermal gluons: If gluons are responsible for parton energy loss, they must also dissociate J/Y. Are the predictions really compatible with the J/Y data at RHIC energy?

Electromagnetic probes • Well developed theoretical framework • Importance of sophisticated collision modeling recognized • Direct probes of thermal medium emission • Low mass dileptons enhanced by long lifetime and pushed to large pT by flow • Credible predictions possible after NA60 data • Thermal photons probe initial T • EM probes will increasingly be useful in conjunction with other hard probes: • J/Y and  • Photon tagging of jets • Jet-to-photon conversion as part of the modified Dpg(z)

Weak(er) probes ? • W and Z have widths O(2 GeV)  decay on time scale of QGP formation and thermalization at LHC. • Does the medium influence the hadronic decay channels? [Probably not significantly, because M counts, not G.] • But the medium will affect the final-state propagation of the (hadronic) decay products, e.g. Z  b-bbar. • Can this be used as a probe of the medium?

Summary • Theory of different hard probes for LHC energies in different stages of development: • EM probes in rather good shape • Hard parton / jet probes require improved (realistic) treatment of evolving fireball geometry/flow; inelastic interaction between jet and medium requires conceptual clarification • Unified QCD based framework for quarkonium probes missing • Do we anticipate a transition from sQGP (at RHIC) to pQGP (at LHC – at early times)? • Is the RHIC medium a strongly coupled plasma or just a turbulent plasma with anomalous transport properties? • How can hard probes help to decide between these pictures? • Are there new “surprises” which we can anticipate?

Hard Probes at LHC Theoretical Perspective