1. Photons and Dileptons at LHC Rainer Fries Texas A&M University & RIKEN BNL Heavy Ion Collisions at the LHC: Last Call for Predictions CERN, June 1, 2007

2. LHC Predictions: Photons & Dileptons2 Rainer Fries Outline Electromagnetic Probes Direct Photons & Dileptons Jet-Medium Photons; Elliptic Flow Calculations for RHIC and LHC Work* done by S. Turbide, C. Gale, D. K. Srivastava and R. J. Fries *based on earlier work involving B. Muller, G. Moore, S. Jeon

3. LHC Predictions: Photons & Dileptons3 Rainer Fries Electromagnetic Probes Evolution of High Energy Nuclear Collisions Measuring bulk hadrons:  Information about the fireball at chemical/thermal freeze-out  Extrapolate back to much earlier times. Biased by models? Measuring jets/hadrons from jets:  Information about the first 4-6 fm/c.  Information about dN g /dy, c s (?), maybe more  Information indirect = multiple integrations involved. QGP

4. LHC Predictions: Photons & Dileptons4 Rainer Fries Electromagnetic Probes Evolution of High Energy Nuclear Collisions Photons and dileptons (virtual photons)  Penetrating probes, some from very early times.  Direct  + background Sensitive to temperature (or temperature history) Approximate hierarchy: larger P T or M ~ larger sensitivity to early times

5. LHC Predictions: Photons & Dileptons5 Rainer Fries Photons at high P T Prompt photons from initial hard processes  e.g. initial Compton q + g  q +   No final state effects at all. Fragmentation/vacuum bremsstrahlung  Possible large contribution from q, g   + X  Sensitivity to medium effects in the final state ~ similar to hadrons For both: sensitivity to shadowing + initial state nuclear effects  Needs to be studied in detail in p+A; still not done at RHIC.  Definition of R AA ?

6. LHC Predictions: Photons & Dileptons6 Rainer Fries Photons at Low and Intermediate P T Thermal/bulk photons (QGP + hadronic phase)  Measure temperature T  Difficult measurement, need to subtract background at low P T Photons from jet-medium interactions [RJF, Müller, Srivastava; Zakharov]  Jet-photon conversion  Induced photon bremsstrahlung

7. LHC Predictions: Photons & Dileptons7 Rainer Fries Photons from Jet-Medium Interactions Mechanism is novel but very natural  RHIC: jets do interact with the medium strongly But: no unambiguous signature for these photons so far.  YAPS: yet another photon source Rates sensitive to fireball physics  Temperature measurement with GeV Photons?  Sensitivity to energy loss: different integral over fireball history  Complementary information, but how feasible are measurements?

8. LHC Predictions: Photons & Dileptons8 Rainer Fries Jet-Photon Conversion Annihilation and Compton scattering in the fireball Cross sections forward/backward peaked:  “Conversion”: very effective process. Yields approximately proportional to the jet distributions  Sensitivity to early time jet distributions  E.g. annihilation:

9. LHC Predictions: Photons & Dileptons9 Rainer Fries Jet-Photon Conversion How bright are these new sources? First estimate:  As important as other direct photon sources for P T ~ 4-6 GeV at RHIC energies, even more important at LHC.  P T slope steeper than initial hard photons. FMS PRL 90 (2003)

10. LHC Predictions: Photons & Dileptons10 Rainer Fries Elliptic Flow Jet-medium photons show azimuthal anisotropy  Hadrons: longer path leads to more suppression  Conversion photons: longer path leads to increased production! Prediction: conversion photons have negative v 2 ! Other photon sources:  Initial hard processes: v 2 = 0  Fragmentation: v 2 > 0 (like hadrons from jets)  Thermal photons: v 2 > 0 (  U. Heinz talk) Hadrons Conversion Photons

11. LHC Predictions: Photons & Dileptons11 Rainer Fries Elliptic Flow Bad news:  No clear signal at RHIC  Strong dependence on details of the calculation Good news: sensitive to initial conditions ?? PHENIX Turbide, Gale, Fries

12. LHC Predictions: Photons & Dileptons12 Rainer Fries Dileptons Very similar for dileptons Same sources:  Thermal emission  Jet-induced emission  Initial hard scattering (Drell-Yan) Background from charm & bottom

13. LHC Predictions: Photons & Dileptons13 Rainer Fries Techniques LO prompt photon and jets with K = 1.5 at LHC NLO Drell-Yan EKS shadowing included. Jet-photon conversion  Define f jet x f thermal

14. LHC Predictions: Photons & Dileptons14 Rainer Fries Energy Loss: AMY AMY formalism [Arnold, Moore, Yaffe]  Rate equation for jet energy loss  Induced photon emission included. One parameter: coupling  s   s = 0.3 describes the  0 and photons at RHIC

15. LHC Predictions: Photons & Dileptons15 Rainer Fries Modeling & Parameters Fireball:  1-dimensional Bjorken expansion & cooling  T c = 160 MeV Initial conditions:  RHIC: temperature T i = 370 MeV at  i = 0.26 fm/c.  LHC: temperature T i = 845 MeV at  i = 0.088 fm/c. Little dependence on  i for  i T i 3 ~ dN/dy held const.

16. LHC Predictions: Photons & Dileptons16 Rainer Fries Predictions

17. LHC Predictions: Photons & Dileptons17 Rainer Fries Photons Photon spectra and R AA in central collisions. S. Turbide, PRELIMINARY

18. LHC Predictions: Photons & Dileptons18 Rainer Fries Dileptons Dilepton P T spectrum at small mass, central collisions. [Turbide,Gale, Srivastava, RJF]

19. LHC Predictions: Photons & Dileptons19 Rainer Fries Dileptons Dilepton mass spectrum at high P T, central collisions. [Turbide,Gale, Srivastava, RJF]

20. LHC Predictions: Photons & Dileptons20 Rainer Fries Photon – Jet Correlations Jet-medium photons change jet-photon correlations! Away-side spectra of photons / jets triggered by 20 GeV jet / 15 GeV photons Toy model calculation!

21. LHC Predictions: Photons & Dileptons21 Rainer Fries Summary Electromagnetic probes at large and intermediate PT: new and complementary information about the fireball. Jet-photon conversion photons/induced photon bremsstrahlung from the medium: important effect at LHC, has to be established. Negative v 2 for conversion, maybe observable Jet-photon conversion might be important for photon- tagged jets.