Jet Chemistry and Contributions to EM Signals Rainer Fries Texas A&M University & RIKEN BNL Quantifying Properties of Hot QCD Matter, INT, Seattle July 14, 2010

INT Rainer Fries Overview Photons and the case for photons from jets “Flavor” Conversion of Jets Elliptic Flow and Correlations with Photons Fluctuations, Tomography and Higher Harmonics with Hard Probes (optional) [With W. Liu, Phys.Rev.C77:054902,2008 Phys.Rev.C78:037902,2008] [ ] [ ] [With R. Rodriguez, E. Ramirez, arXiv: [nucl-th]]

INT Rainer Fries Photons from Jets

INT Rainer Fries Classifying Photon Sources Identify all important sources and develop a strategy to measure them individually. Transverse momentum spectra of single direct photons  Hierarchy in momentum  Reflects hierarchy in average momentum transfer (or temperature) in a cooling and diluting system) More sophisticated strategies:  Elliptic Flow  Correlations of photons with hadrons and jets EE Hadron Gas Thermal T f QGP Thermal T i “Pre-Equilibrium”? Jet Re-interaction √(T i x√s) Hard prompt

INT Rainer Fries Initial Hard Photons Prompt photons from initial hard scattering of partons in the nuclei. Calculable in factorized QCD perturbation theory p+p collisions: important baseline to understand prompt photons in heavy ion collisions despite somewhat different initial state. Compton Annihilation PDF Parton cross section PDF Parton processes at leading order:

INT Rainer Fries Fragmentation Photons Photons can also fragment off jets created in initial collisions (Bremsstrahlung)  Described by photon fragmentation function  Factorization: At NLO, prompt hard and fragmentation photons can be treated consistently. Possible problem in nuclear matter:  Final state suppression for fragmenting photons but not for prompt photons?  Induces uncertainty in direct photon baseline. Parton process: PDF Parton cross section PDF FF

INT Rainer Fries Initial Hard Photons Prompt photon data in p+p well described by NLO calculations. This seems like a safe baseline! Photon world hadron colliders [Aurenche et al., PRD (2006)]

INT Rainer Fries Initial Hard Photons: Nuclear Effects Do we have control over initial state effects for prompt photons in nuclear collisions?  Isospin: correct blend of protons and neutrons in colliding nuclei is important (  u = 4  d !)  Shadowing and EMC effect: usually taken into account by modified parameterizations for nuclear PDFs (EKS …); source of some uncertainty!  Cronin effect: initial state scattering leading to broadening. Final state effects for fragmentation photons: most calculations assume final state parton is quenched until the photon is created.

INT Rainer Fries Thermal Photons Annihilation, Compton and bremsstrahlung processes also occur between thermalized partons in a QGP. Hope to measure the temperature T (or its time-average), confirm existence of deconfined quark-gluon phase Resummation program (hard thermal loop) + collinear radiation (AMY) A hot hadron gas shines as well.  Annihilation, creation and Compton-like processes with pions  + vector mesons, baryons … [Arnold, Moore & Yaffe, JHEP (2001, 2002)] [Kapusta, Lichard & Seibert (1991)] [Baier et al. (1996)] [Aurenche et al. (1996, 1998)]

INT Rainer Fries Summary So Far Thermal + hard photons Sufficient to give a decent description of RHIC data. [Turbide, Rapp & Gale, PRC (2004)] [d’Enterria & Peressounko (2006)]

INT Rainer Fries There Must Be More! Any process that radiates gluons should be able to radiate real and virtual photons. Final state interactions of jets can give us additional photons. Compton, annihilation and Bremsstrahlung processes can also occur between a fast parton in a jet and a medium parton.

INT Rainer Fries There Must Be More! Elastic conversion cross sections peak forward and backward.  Yield from these jet-to-photon conversions: Induced photon bremsstrahlung  jet  jet   [RJF, Müller & Srivastava, PRL (2002)] [Zakharov, JETP Lett. (2004)] x vac

INT Rainer Fries Jet-Medium Photons Features:  Spedtrum sensitive to leading jet particle distrubtions at intermediate times.  Strongly dependent on temperature.  An independent thermometer? How bright is this new source?  Can be as important as initial hard photons at intermediate p T ! FMS PRL 90 (2003) [Zakharov, JETP Lett. (2004)]

INT Rainer Fries Jet-Medium Photons Pitching a wider tent:  Classify particles as either thermal or belonging to a (mini)jet:  Photons from these particles in kinetic theory: Jets will lose only partially energy before conversions  Conversion photons provide additional constraints for jet quenching models. Most comprehensive scheme on the market: expanded AMY  Induced gluon + photon radiation  Rate equations for jets  Elastic conversions included thermal photons conversion photons Did we forget these? No, irrelevant at present collider energies [Jeon & Moore]

INT Rainer Fries Adding Jet-Medium Photons Complete phenomenological analysis including simultaneous fit of pion quenching  Extended AMY (+ hadronic gas); hydro fireball; initial state effects Good description of RHIC single inclusive direct photon spectra. But: little sensitivity to individual sources. How strong are conversion photons? [Turbide, Gale, Frodermann & Heinz (2007)] [Qin, Ruppert, Gale, Jeon & Moore (2009)]

INT Rainer Fries Adding Jet-Medium Photons More Sensitivity: Nuclear Modification R AA Jet-medium photons roughly make up for the loss through jet quenching, except for very large P T. [Qin, Ruppert, Gale, Jeon & Moore (2009)]

INT Rainer Fries Jet-Medium Dileptons Jets can convert into virtual photons Dileptons w/o hadronic sources: Possible signals at high transverse momentum. [Turbide, Gale, Srivastava & RJF, PRC 74 (2006)] [Srivastava, Gale & RJF, PRC 67 (2003)]

INT Rainer Fries “Flavor” Conversions

Simplest possible application: opacity of the medium  Drag force on QCD jets or hadrons = jet quenching  Most models: energy loss of the leading parton. Sensitive to transport coefficient = momentum transfer squared per mean free path. Several calculations on the market using different sets of assumptions, e.g. INT Rainer Fries I F AMY BDMPS ASW GLV DGLV Higher Twist AMY Perturbative plasma in the high temperature limit Extrapolated from DIS off large nuclei (e+A  h+X) Hard Probes Revisited

INT Rainer Fries Hard Probes Revisited How else can we use hard probes? Track changes in flavor and chemistry in the medium! Identity of a parton can change when interacting with a medium. Here: general definition of “flavor”:  Gluons g  Light quarks q = u,d  Strange quarks s  Heavy quarks Q = c,b  Real photons, virtual photons (dileptons)  Measure flavor conversions  jet chemistry I F Example: Schäfer, Wang, Zhang; HT formalism

INT Rainer Fries Jet Chemistry Flavor of a jet here = identity of the leading parton.  Flavor of a jet is NOT a conserved quantity in a medium.  Only well-defined locally! The picture here:  Parton propagation through the medium with elastic or inelastic collisions  After any collision: final state parton with the highest momentum is the new leading parton (“the jet”) Hadronization: parton chemistry  hadron chemistry  Hadronization washes out leading parton signals  Changing multiplicities in jets in medium might also change hadron chemistry: changed hadronization [Sapeta, Wiedemann]

INT Rainer Fries What Can Chemistry Tell Us? Measure equilibrium or rate of approach to equilibrium. Low P T : Intermediate P T : recombination, ridge vs jet etc. inclusive Au+Au: M. Lamont (STAR) SQM06 Cu+Cu: C. Nattrass (STAR), QM2008 Au+Au: J.B. (STAR), WWND07

INT Rainer Fries Why Could It Be Exciting? For chemistry, momentum transfer is not important (unless there are threshold effects) Rather: flavor conversions are sensitive to the mean free paths of partons in the medium. Complementary information to :  Many interactions with small momentum transfer?  Few scatterings with large momentum transfer? Measurements will be challenging  Need particle identification beyond 6-8 GeV/c at RHIC, outside of the recombination region.

INT Rainer Fries Quark-Gluon Conversions Gluon  (light) quark conversions Available in some jet quenching schemes (HT, AMY, …) Relative quenching of gluons and quarks: color factor 9/4  Not explicitly observed in data  Shouldn’t be there in a system with short mean free path! [Ko, Liu, Zhang; Schäfer, Zhang, Wang; …] Ko et al: elastic g  q conversions  Lose 30% of quark jets at RHIC  enhance p/  ratio; need elastic cross sections  4 to get p+p values  Dependence on fragmentation functions!

INT Rainer Fries Two Examples for Rare Probes Example 1: excess production of particles which are rare in the medium and rare in the probe sample  Example: photons  Need enough yield to outshine other sources of N rare. Example 2: chemical equilibration of a rare probe particle  Example: strangeness at RHIC  Coupling of jets (not equilibrated) to the equilibrated medium should drive jets towards chemical equilibrium. jet photon g s

INT Rainer Fries Conversion Rates Coupled rate equations for numbers of jet particles (flavors a, b, c, …) in a fireball simulation. Here: reaction rates from elastic 2  2 collisions Need to compare to 2  3 processes. Non-perturbative mechanisms? Photons and dileptons; inverse reaction negligible Heavy quarks production? Quark / gluon conversions

INT Rainer Fries Results: Protons Use the model by Ko, Liu and Zhang:  Rate equations plus energy loss.  Elastic channels; cross sections with K-factor  Longitudinally and transversely expanding fireball RHIC: T i = fm/c LHC: T i = fm/c Use double ratios to cut uncertainties from fragmentation functions. [Ko, Liu, Zhang] [Liu, RJF] Recombination region [Liu, RJF, PRC (2008)]

INT Rainer Fries Results: Strangeness Kaons: see expected enhancement at RHIC  Measure above the recombination region! No enhancement at LHC  Too much initial strangeness!  Maybe it works with charm at LHC? Recombination region

INT Rainer Fries Numerical Results: Heavy Quarks Additional threshold effect At RHIC: additional heavy quark production marginal LHC: not at all like strangeness at RHIC; additional yield small  Reason: charm not chemically equilibrated at LHC  Results in small chemical gradient between jet and medium charm  Also: threshold effect LHC [Liu, RJF, PRC (2008)]

INT Rainer Fries Recent Results from STAR STAR at QM 2009  Kaon enhancement seen between 6 and 10 GeV/c.  A proper signal of conversions?  Caution: p enhancement too big. Blast from the past: strangeness enhancement!

INT Rainer Fries Elliptic Flow at High P T

INT Rainer Fries Elliptic Flow v 2 Azimuthal anisotropy for finite impact parameter. Three different mechanisms: x y z Initial anisotropy Final anisotropy Elliptic flow v 2 Bulkpressure gradient collective flowv 2 > 0 saturated hard probe path lengthquenchingv 2 > 0 rare hard P T probe path lengthadditional production v 2 < 0 [Turbide, Gale & RJF, PRL 96 (2006)]

INT Rainer Fries Photon Elliptic Flow Have to add other photon sources with vanishing or positive v 2.  Almost perfect cancellation, |v 2 | small Status:  Large negative v 2 excluded by experiment.  Large uncertainties from fireball model? [Liu & RJF, PRC (2006)] [Turbide, Gale & RJF (2006)] [Chatterjee, Frodermann, Heinz, Srivastava; …]

INT Rainer Fries Strangeness Elliptic Flow Strangeness as non-equilibrated probe at RHIC: additional strange quarks have negative v 2. Expect suppression of kaon v 2 outside of the recombination region. [Liu & RJF (2008)] w/ conversions w/o conversions Recombination taken into account

INT Rainer Fries Correlations at High P T

INT Rainer Fries Correlations with Photons Photon-hadron and photon-jet correlations can provide a handle on the initial energy of a jet before quenching. “Gold Plated Measurement” for energy loss. Caution: additional photon sources + radiative corrections complicate the picture. [Wang, Huang & Sarcevic (1996)] 

INT Rainer Fries Correlations with Photons Dilution of kinematic correlation through different photon sources! NLO effects important. [Qin, Ruppert, Gale, Jeon, Moore,(2008); (2009)] [Arleo et al. (2004)]

INT Rainer Fries Spatial Fluctuations and Tomography with Hard Probes

INT Rainer Fries Spatial Structures and Hard Probes Fluctuations in the initial state are important for bulk observables. Do we expect an impact of spatial fluctuations on hard probes? They are sensitive to early times! Can hard probes tell us about the spatial structure of the fireball, i.e. can we do something akin to tomography?  Seemingly hopeless: we sum over many events and only see an average fireball. b=3.2 fm b=11 fm

INT Rainer Fries Quenching with Fluctuations Density integral along the path of a parton created at point r. The relevant quantity for energy loss is the emission probability weighted integral. With fluctuating emission and background densities: Relevant information contained in the correlation function between emission and background densities. R |r2-r1||r2-r1|

INT Rainer Fries Quenching in a Fluctuating Background Simple 2-component model for R : Fluctuation signal on energy loss:  Shows potential cancellation between stronger quenching in regions of stronger emission and less quenching around those regions.  Sign depends on details of R. Elliptic flow signal in a fireball with short and long axes X and Y resp.  Expect less v 2 in this simple model.

INT Rainer Fries Numerical Study: R AA Numerical study using event-by-event jet quenching. Events from GLISSANDO Glauber model using collision densities Two quenching models (simple ~L 2 deterministic energy loss [sLPM], Armesto-Salgado-Wiedemann [ASW]). Both models give less quenching at all centralities and momenta. [Broniowski, Rybczynski & Bozek, CPC (2009)] b = 3.2 fm

INT Rainer Fries Numerical Study: R AA R AA can be refitted across all centralities and momenta after adjusting the quenching strength. Additional uncertainty to extraction of from geometry. smoothEvent-by- event c sLPM c ASW

INT Rainer Fries Residual Signatures Elliptic Flow reduced After refitting: small residual suppression. Di-hadron pair suppression reduced. After refitting: potentially larger suppression. Spatial structures do leave a finger print in hard probe observables. Enough so to be useful? Have not studied time-evolution. b = 11 fm

INT Rainer Fries Higher Harmonics Bulk physics: initial state fluctuations lead to non-vanishing v 3, possibly a larger v 4 etc. Same should be true for hard probes.  If observable in experiment, tests for energy loss models.  More information about the initial state. Here: interesting case of v 1. v1v1 v1v1 v2v2 v2v2 v3v3 v3v3 v4v4 v4v4 Smooth event Asymmetric event

INT Rainer Fries Higher Harmonics Clear v 1 signal in engineered events. Survives on the percent level in more realistic event sample from GLISSANDO. Must be compensated by recoil at low P T. Look for it in bulk events with large momentum triggers?

INT Rainer Fries Summary and Outlook Hadro-chemistry for hard probes  Flavor changing processes are present in jet-medium interactions.  Jet chemistry contains information complementary to jet quenching measurements.  Predict strangeness enhancement at high P T. Photons and dileptons from jets  Compatible with data but still not unambiguously confirmed by experiment.  New approaches using elliptic flow and photon-jet correlations. Fluctuations in the fireball are important for hard probes physics.  Just another uncertainty or a chance to measure the inhomogeneity of the fireball?  Other harmonics besides v 2 are there!