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High Energy Photons from Relativistic Heavy Ion Collisions

This research delves into the production of high-energy photons in relativistic heavy-ion collisions, focusing on processes like Compton scattering, annihilation, and bremsstrahlung. The study examines the interaction of partons and the effects of the Landau-Pomeranchuk-Migdal (LPM) suppression mechanism on photon yield. Additionally, it investigates the implications of photons in understanding collision dynamics and thermalization rates. The findings are crucial for advancing our understanding of these complex phenomena.

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High Energy Photons from Relativistic Heavy Ion Collisions

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  1. High Energy Photons from Relativistic Heavy Ion Collisions Dinesh Kumar Srivastava Variable Energy Cyclotron Centre Kolkata • Motivation • The PCM: Fundamentals & Implementation • Photon Production in the PCM • Medium Effects: Jet-Photon Conversion (FMS Photons) • Intensity Interferometry of Photons • Elliptic Flow of Thermal Photons WHEPP 2006, Bhubaneswar.

  2. Part #1: Photon Production in the PCM • Light from cascading partons in relativistic heavy-ion collisions • - S.A. Bass, B. Mueller and D.K. Srivastava, Phys. Rev. Lett. 90 (2003) 082301 • Semi-hard scattering of partons at SPS and RHIC: A study in contrast • -S. A. Bass, B. Mueller, and D. K. Srivastava, Phys. Rev. C 66 (2002) 061902 (R) • Intensity interferometry of direct photons in Au+Au collisions- S.A. Bass, B. Mueller and D.K. Srivastava, Phys. Rev. Lett. 93 (2004) 162301 • Dynamics of the LPM effect in Au+Au Collisions at 200 AGeV- T. Renk, S.A. Bass and D.K. Srivastava, nucl-th/0505059, Phys. Lett. B. (in press)

  3. Basic Principles of the PCM Goal: provide a microscopic space-time description of relativistic heavy-ion collisions based on perturbative QCD • Degrees of freedom: quarks and gluons • Classical trajectories in phase space (with relativistic kinematics) • Initial state constructed from experimentally measured nucleon structure functions and elastic form factors • An interaction takes place if at the time of closest approach dminof two partons • System evolves through a sequence of binary (22) elastic and inelastic scatterings of partons and initial and final state radiations within a leading-logarithmic approximation (2N) • Binary cross sections are calculated in leading order pQCD with either a momentum cut-off or Debye screening to regularize IR behavior • Guiding scales: initialization scale Q0, pT cut-off p0 / Debye-mass μD

  4. Initial State: Parton Momenta • flavour and x are sampled from PDFs at an initial scale Q0 and low x cut-off xmin • initial kt is sampled from a Gaussian of width Q0 in case of no initial state radiation • virtualities are determined by:

  5. Parton-Parton Scattering Cross-Sections • a common factor of παs2(Q2)/s2 etc. • further decomposition according to colour flow • Biswanath Layek working on including heavy quarks.

  6. Initial and Final State Radiation Probability for a branching is given in terms of the Sudakov form factors: Space-like branchings: Time-like branchings: Altarelli-Parisi splitting functions included: Pqqg , Pggg , Pgqqbar & Pqqγ

  7. Collision Rates & Numbers b=0 fm • Lifetime of interacting phase: ~ 3 fm/c • Partonic multiplication due to the initial & final state radiation increases the collision rate by a factor of 4-10 Are time-scales and collision rates sufficient for thermalization?

  8. Photon Production in the PCM • Relevant processes: • Compton: q g q γ • Annihilation: q qbar  g γ • Bremsstrahlung: q*q γ • Photon yield very sensitive to parton-parton rescattering

  9. What Can We Learn From Photons? • Photon yield directly proportional to the # of hard collisions • Primary-primary collision contribution to yield is < 10% • Emission duration of pre-equilibrium phase: ~ 0.5 fm/c • Photon yield scales with Npart4/3

  10. Photons: pre-equilibrium vs. thermal • pre-equilibrium contributions are easier identified at large pt: • window of opportunity above pt=2 GeV • at 1 GeV, need to take thermal contributions into account • short emission time in the PCM, 90% of photons before 0.3 fm/c • hydrodynamic calculation with τ0=0.3 fm/c allows for a smooth continuation of emission rate

  11. HBT Interferometry: Formalism • Correlation between two photons with momenta k1 and k2 is given by: • with S(x,k) the photon source function for a chaotic source • use Wigner function scheme (Hansa code by Sollfrank & Heinz) • emission vertices of a semiclassical transport are not valid Wigner fnct. • need to smear out emission vertices xi by ħ/pi • results are given in terms of outward, sideward & longitudinal correlators

  12. Photons: HBT Interferometry • pt=2 GeV: pre-thermal photons dominate, small radii • pt=1 GeV: superposition of pre- & thermal photons: increase in radii

  13. Landau-Pomeranchuk-Migdal Suppression The LPM effect accounts for the suppression of radiation due to coherence effects in multiple scattering f e b • the radiated parton e is assigned a formation time: kt a d c • if the radiating parton d suffers a collision before tformhas elapsed, then the radiation of parton e and it’s daughters does not take place • likewise for parton f with respect to e …

  14. LPM: Reaction Dynamics gluon pt distribution • high pt: harder slope, enhanced particle production • low pt: suppression of particle production

  15. Photon Production: LPM & Comparison to Data • PCM without LPM: • overprediction of photon yield • PCM with LPM: • photon yield for pt < 6 GeV strongly reduced • strong pt dependence of LPM suppression • good agreement with data

  16. Photons via Jet-Plasma Interactions R.J. Fries, B. Mueller, & D.K. Srivastava, PRL 90, 132301 (2003), R.J. Fries, B. Mueller, & D. K. Srivastava, PRC 72, 041902 (R) 2005, See also: S.Turbide, C. Gale, S. Jeon, and G. Moore, PRC 72, 014906 (2005). Dileptons via Jet-Plasma Interactions S. Turbide, C. Gale, D. K. Srivastava, & R. J. Fries, (to be published), hep-ph/0601042 Part #2:

  17. Photon Sources • Hard direct photons • EM bremsstrahlung • Thermal photons from hot medium • Jet-photon conversion Turbide, Gale & Rapp, PRC 69 014903 (2004)

  18. Jet-Plasma Interactions Plasma mediates a jet-photon conversion: • Jet passing through the medium: • large energy loss: jet quenching • electromagnetic radiation (real and virtual photons) from jet-medium interactions • suppressed by αEM; negligible as a source of additional jet quenching • can escape without rescattering • use as probe of energy loss? • visible among other sources of electromagnetic signals?

  19. QGP-induced EM Radiation • Annihilation and Compton processes peak in forward and backward directions: • One parton from hard scattering, one parton from the thermal medium; Cutoff p,min > 1 GeV/c. • Photon carries momentum of the hard parton. • Jet-photon conversion.

  20. Jet-photon Conversion: Rates • Annihilation and Compton rates: • Thermal medium:

  21. Photons from jet-plasma interaction

  22. FMS Results: Comparison to Data • Calibrate pQCD calculation of direct and Bremsstrahlung photons via p+p data: For pt<6 GeV, FMS photons give significant contribution to photon spectrum: 50% @ 4 GeV Fries, Mueller, & Srivastava, PRC 72, 041902 (R) 2005

  23. FMS: Centrality Dependence and Jet-quenching • Centrality dependence well described • Effect of energy-loss on jets before conversion ~ 20% (Turbide et al).

  24. FMS Photons: Further Confirmations S. Turbide and C. Gale, hep-ph/0512200

  25. Azimuthal Anisotropy of FMS Photons S. Turbide, C. Gale, and R. J. Fries, hep-ph/0508201.

  26. Jet-plasma or jet-thermal dileptons

  27. Application: Monitoring Jet Quenching Full jet reconstruction not possible at RHIC: • Measure suppression of single inclusive hadron spectra (compare to p+p baseline) • Better: photon-tagged (Wang & Sarcevic) or dilepton –tagged jets (Srivastava, Gale, & Awes): • q+g  q+: recoil photon knows the initial energy of the jet • Measure energy loss of quark as a function of quark energy E • Photons from jet-photon conversion provide a third, independent measurement. (FMS) • Better handle on the L dependence of energy loss • Jet-photon conversion is background for photon tagged jets

  28. Part #3: Elliptic flow of thermal photons Elliptic Flow of Thermal Photons in Relativistic Heavy Ion Collisions, Rupa Chatterjee, Evan S. Frodermann, Ulrich Heinz, and D.K. Srivastava, nucl-th/0511079. Photons are emitted from every point in space and time unlike hadrons which come out only from freeze-out surface, when the fluid element is at 100 MeV !

  29. Nuclear Fluid Dynamics • Transport of macroscopic degrees of freedom • Based on conservation laws:μTμν=0, μjμ=0 • For ideal fluid:Tμν= (ε+p) uμ uν - p gμν and jiμ = ρi uμ • Equation of State needed to close system of PDE’s:p=p(T,ρi) • assume local thermal equilibrium • Initial conditions (i.e. thermalized QGP) required for calculation • Simple case: scaling hydrodynamics • assume longitudinal boost-invariance • cylindrically symmetric transverse expansion • no pressure between rapidity slices • conserved charge in each slice

  30. Collective Flow: Overview • Directed flow (v1, px,dir) • spectators deflected from dense reaction zone • sensitive to pressure • Elliptic flow (v2) • asymmetry out- vs. in-plane emission • emission mostly during early phase • strong sensitivity to EoS • Radial flow (ßt) • isotropic expansion of participant zone • measurable via slope parameter of spectra (blue-shifted temperature)

  31. Elliptic Flow: Theory & Experiment • Data from STAR & PHENIX • Good agreement between hydro and pt-differential data • Data saturates for high pt Kolb & Heinz hep-ph/0204061 • Hydro+Micro: • strong sensitivity to QGP EoS • includes proper flavour dynamics • self-consistent freeze-out D. Teaney, J. Lauret, E. Shuryak nucl-th/0110037

  32. Does v2 Reflect Parton Flow? Recombination model suggests that hadronic v2 reflects parton v2 : P. Soerensen, UCLA & STAR @ SQM2003 • measurement of partonic v2 !

  33. Contours of Constant Energy Density

  34. For central collisions these will be identical.

  35. Eliiptic Flow of Thermal Photons

  36. Centrality Dependence : Elliptic Flow of Thermal Photons

  37. Summary • Photon Production in the PCM: • Photon yield very sensitive to parton rescattering • LPM effect needed for proper description of reaction dynamics • HBT experimentally challenging, but feasible with high statistics data sets • calculable in the framework of PCM and hydrodynamics • short emission duration in pre-equilibrium phase: small radii at high pt • larger source at later times due to emission of thermal photons Contd. ..

  38. Photon Production via Jet-Medium Interactions: • jet-photon conversion may contribute up to 50% @ 4 GeV to photon yield • results compatible with PHENIX data (centrality dependence, RAA) • analogous process for virtual photons: contribution to dilepton production • azimuthal asymmetry of the initial shape of hot & dense fluid. • Elliptic Flow of Thermal Photons: • Large p_T; QGP, small p_T; hadronic phase! First ever direct peep into QGP & Hadronic Phases

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