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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

Electromagnetic Probes at RHIC-II. G. David Physics Department BNL Jan. 15, 2008. Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL. This talk. Roots: Nov. 2004  Physics Working Group organized to explore

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Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

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  1. Electromagnetic Probes at RHIC-II G. David Physics Department BNL Jan. 15, 2008 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  2. This talk Roots: Nov. 2004  Physics Working Group organized to explore physics possibilities offered by RHIC-II (actually: what is RHIC-II?) Trigger: arXiv: nucl-ex/0611009 (RHIC-II electromagnetic probes working group write-up, soon to be published, and Rachid read it… ) Easy: preaching to the choir (hopefully ) Hard: timing is awkward (QM’08 is around the corner  but several new results cannot be shown now…) The story line: - same facility, same experiment(s)  reduced systematics - exploration phase  precision, precision, precision - rare probes, differential quantities - role of electromagnetic probes in mapping the phase diagram - the dual use of luminosity: high statistics and/or species/energy scan Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  3. The promise (and trap) of electromagnetic probes Penetrating probes emitted at all stages then surviving unscathed ( ae << as) “Historians” of the heavy ion collision: encode all sub-processes at all times But for the very same reason their message is hard to decipher! Cartoon only: sources of g, mean pT vs time (dashed: hadrons) hard scatt pT (GeV) jet Brems. jet-thermal jet fragmentation All we can see is the projection to this axis with the dashed sources as background dominated up to medium pT by this sQGP hadron gas hadron decays log t 1 10 107 (fm/c) Overlapping signals from different sources, separated in time Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  4. From discovery to exploration I’ll save you (most of) the pep-talk what great discoveries we made. Instead, let me state what we are missing so far ( “exploration”) This is a partial list, with strong bias towards electromagnetic probes. We didn’t map out the phase diagram, find (or disprove the existence of) the critical point We don’t know what makes it thermalize incredibly fast We don’t know what happens to vector meson masses and yields (is chiral symmetry (partially) restored?) We don’t have a statement yet on thermal photons and initial temperature We are just getting sensitive enough to differentiate between jet energy loss models and constrain their free parameters We couldn’t disentangle yet different sources of medium pT direct photons We must get more precise (while staying accurate…  ) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  5. CERES – PB+Au, r mass Excess spectrum: c2 analysis (ndf = 12, stat. errors only): c2 (i.m.h.) = 9.4 (P = 66.8%) c2 (d.ρ-m.) = 34.6 (P = 0.0005%) CERES Pb-Au inclusion of syst errors (MC method) yields: P (i.m.h.) = 84.2% P (d.ρ-m.) = 10.9% Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  6. r NA60: centrality dependence of spectral shape peak: R=C-1/2(L+U) continuum: 3/2(L+U) nontrivial changes of all three variables at dNch/dy>100 ? Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  7. NA60: the rise and fall of radial flow of thermal dimuons Disentangle pT spectra of thermal continuum (from peak), get slopes (steeper than peak) Flow is not directly measured (yet?) pT spectra do not depend on centrality, but do depend on mass Combining M and pT of thermal dileptons to break hadron-parton duality? Strong rise of Teff with dimuon mass even persists for the pure in-medium part. Sudden drop for M>1 GeV now even more sharply defined Rise consistent with radial flow of a hadronic source (here pp→r→mm) Drop signals sudden transition to low-flow source, i.e. source of partonic origin ? (here qq→mm) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  8. H. Stocker calls it mRHIC… Mantra: “same experiment, same systematics” “acceptance at y=0 unchanged, multiplicities grow very slowly” “evolution (ratios) have smaller errors than individual points…” Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  9. Energy/species scan is on everybody’s mind Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  10. Energy/species scan is on everybody’s mind (M. Gazdzicky) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  11. FAIR • Compressed Baryonic Matter @ FAIR – high mB, moderate T: • searching for the landmarks of the QCD phase diagram • first order deconfinement phase transition • chiral phase transition (high baryon densities!) • QCD critical endpoint • in A+A collisions from 2-45 AGeV starting in 2015 (CBM + HADES) • The equation-of-state at high B • collective flow of hadrons • particle production at threshold energies (open charm) • Deconfinement phase transition at high B • excitation function and flow of strangeness (K, , , , ) • excitation function and flow of charm (J/ψ, ψ', D0, D, c) • charmonium suppression, sequential for J/ψ and ψ' ? • QCD critical endpoint • excitation function of event-by-event fluctuations (K/π,...) • Onset of chiral symmetry restoration at high B • in-medium modifications of hadrons (,, e+e-(μ+μ-), D) • mostly new measurements • CBM Physics Book (theory) in preparation (Claudia Hoehne) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  12. Possible sweet spots for electromagnetic probes (QGP) IMR (1.5-3.0GeV) pT integrated dileptons 1.5-3.0GeV direct photons low mass, qT>2GeV/c dileptons (compilation by R. Rapp) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  13. Initial temperature, thermalization scale Observables come from: production mechanisms folded with system evolution You have to know/assume Ti, Tc, Tf, v1, v2, EOS, t… (all interdependent to some level) Case in point: PHENIX preliminary low pT direct photon spectra reasonably described by models where ti ranges 0.15-0.5 (1.3!)fm/c, and Ti ranges 300-660MeV An interesting proposal: while both real and virtual photon spectra depend on Ti, v0, Tf, EOS (Tc dep. negligible) their ratio may depend only on Ti (?!) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  14. Direct measurement of initial temperature? The claim is quite interesting (and relatively “easy” to measure in PHENIX): Rem is sensitive only to the initial temperature, but (largely) independent of flow, Tc, ti, form of the EOS, … Note that this independence doesn’t seem to have a “deep” underlying reason (or at least none the authors are aware of…) Alam, Sinha, arXiv:0705.1591 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  15. Thermal (?) photons by internal conversion Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  16. Direct photons – high and low pT PRL 94 (2005) 232301 QM’05 QM’06 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  17. World data vs own data The point: Same accelerator, same experiment, similar systematic errors  more precise mapping of the evolution (even if errors are relatively large) p0 RAA, 62GeV Au+Au: p0 points are the same, but the reference changed from fit to world data to our own p+p measurement New p0 RAA, 62GeV Au+Au compared to suppression in 200GeV Au+Au If the new result survives, the physics message changes quite a bit! Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  18. RAA with pQCD RAA with p+p data PHENIX – “Isospin Effect” (?) The isospin effect (charge difference between uud and udd) SHOULD be there, but is this (and only this “trivial effect”) what we see? Or do we see in addition some genuine photon suppression? Only “primordial” photons should be unaltered, “medium-induced” photons can be enhanced or suppressed F. Arleo, JHEP09 (2006) O15 W. Vogelsang, NLO pQCD + isospin Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  19. What is the p+p reference? calculation vs data Photons in 200GeV p+p (Run-5) 0.5pT favored, but even this misses the shape Black circles: Run-5 data divided by an empirical fit. Blue lines: NLO pQCD (different both in magnitude and shape) 20-30% deviation (only!) would be a reason to celebrate 5-6 years ago, but now we are trying to confirm / refute additional signals at that level (like jet-photon conversion or isospin effect) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  20. Photon RAA (200 GeV) with fit to p+p data Remember: integrated RAA with NLO pQCD Most central collisions Is the high pT suppression real? Is it suppression at all? Are p+p data the right thing to normalize photon RAA? Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  21. Isospin Effect – xT scaling Unfortunately the suppression is seen in a region where we are very sensitive to detector bias (cluster merging). Also, so far it was seen only in one of the detectors (the one more prone to merging) xT scaling to the rescue? The reason: certain known detector imperfections (like shower merging, nonlinearity…) are smaller! Yes, we do our best to correct for them but nothing beats not having the problem in the first place… The catch: sources at intermediate pT (like jet conversion) that are so far of unknown magnitude, come into play, too! Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  22. So: is it real? Stay tuned! Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  23. But this hinges upon a reliable measurement of R and you must go out quite far in pT Also it cries for statistics and/or better reaction plane resolution PHENIX Preliminary sNN=200GeV Au+Au Photon flow (PHENIX) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  24. jet fragment photon v2 > 0 annihilation compton scattering jet v2 > 0 Medium induced (inc.energy loss) v2 < 0 The promise of flow and isolation Fragmentation: non-isolated Bremsstrahlung: non-isolated Jet-photon conversion: isolated “Primordial”: isolated Note: assuming no energy loss  fragmentation g is isotropic  jet-g conversion dominates v2 v2<0 1/ Get the NN part (including isospin effect) 2/ Get the jet-conversion (jet-th) part from isolated, v2<0 3/ Get the fragmentation from non-isolated, v2>0 4/ …  TALL ORDER, TO SAY THE LEAST So if something like this were the truth, in principle you could try to disentangle the components like this: Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  25. Source size – photon HBT (planned) Thermal (<600MeV) Hard scattering (2GeV) STAR proposal PRL 93, 162301 (2004) Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  26. A step forward: p0 RAA vs reaction plane Double-differential RAA reveals strong pTand reaction plane (geometry) dependence  stronger constraint on energy loss models But requires more statistics (RXPN  better detector is equivalent to higher statistics) Does this mean the era of bulk RAA is over? Not quite! PRC 76 (2007) 034904 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  27. Getting quantitative: statistical analysis Final results (Run-4) on p0 RAA (PHENIX) Does this bulk (f-integrated) quantity really tell you something? Would it tell you something if the errors on the last points were reduced? Important: often increase in statistics not only reduces your statistical error, but opens up new ways to reduce systematic errors as well! arXiv 0801.1665 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  28. Quantitative constraints on opacity (PQM) Experimental uncertainties only! PQM predictions (one specific implementation) for various <q> (red curve: best fit) Note: <q> is not cast in stone, it’s implementation dependent; theoretical uncertainties (much) bigger than experimental ones PQM: radiative loss, static medium, no IS mult. scat., no mod. PDF. arXiv 0801.1665 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  29. Quantitative constraints on gluon density (GLV) Experimental uncertainties only! GLV predictions for various dNg/dy (red curve: best fit) GLV: <L>, opacity exp., Bj. exp. medium, radiative only, IS mult. scat., mod. PDF. arXiv 0801.1665 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  30. Quantitative constraints on gluon density (WHDG) Experimental uncertainties only! WHDG predictions for various dNg/dy (red curve: best fit) WHDG: <L>, opacity exp., Bj. exp. medium, radiative and collisional, no IS mult. scat., no mod. PDF. arXiv 0801.1665 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  31. p0 RAA fitted with a simple straight line Slope consistent with zero: m = 0.0017 +/-0.0035 (+/- 0.0070) c/GeV (1 and 2s) 1, 2, 3s uncertainty contours With present experimental uncertainties the statement that single high pTp0 is “fragile” to opacity is not supported (more uncertainty in theories). This of course doesn’t mean that multi-differential observables should not be pursued. But they also come at a price! arXiv 0801.1665 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  32. A case for (much) more statistics Five highest points contribute 70% of the total c2. If the fits are limited to 5-10GeV/c, p-values increase to 55% (PQM), 36% (GLV) 17% (WHDG), 75% (linear fit) Theoretical uncertainties are much bigger: the ball seems to be in the theorists’ court! arXiv 0801.1665 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  33. Summary RHIC-II is apparently on its way, but in today’s world we shouldn’t let our guard down Dual use of luminosity: energy/species scan and rare probes we are well positioned to find the CEP first we are equally well positioned to nail down jet energy loss mechanisms and much, much more Statistics not only extend range, reduce statistical errors, make rare probes accessible, but often helps reduce systematic errors as well  precision We are in the exploration phase and the name of the game is precision But precision isn’t a goal for itself, either: the goal is to constrain (or refute) theories. Sometimes it is best done with increased precision, other times with multi-differential observables We are in this boat together. So far cooperation of theorists and experimentalists has been stellar. Let’s keep it this way! Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  34. What fraction of pQCD photons are from the primordial hard scattering? Testable in p+p – prompt photons are isolated (little or no energy deposit in the neighborhood) Important validation of pQCD (and feasible in p+p) Direct photons: prompt vs. fragmentation Red curve: prompt g Blue curve: fragmentation (30% of all at high pT) Caveats: - questionable at low pT - purely theoretical (exp. cuts not applied) - real-life cuts always push the red curve higher Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  35. Isolation cut 0.1*E > Econe(R=0.5) By M.Werlen, JETPHOX -.35<y<.35 =pT BFG set2, CTEQ6M By W.Vogelsang, R=0.4 =pT, CTEQ6M Isolated vs. non-isolated photons in p+p Primary (hard scattering) photons vs. photons associated with jets (fragmentation, FS hadron decay) Calculable, and reasonable agreement with data even if the usual 0.5 cone is near PHENIX’s limit Fraction of isolated/all photons, p+p, 200 GeV Clean: no additional source from jet-medium interaction Biased: finite acceptance Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  36. Direct g and p0 nuclear modification factor RdA in d+Au Both g and p0 consistent with 1 Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  37. Nuclear Modification Factor 100 xT Consistent with 1  No modification within the error This is first measurement of ‘EMC effect’ for gluons “EMC effect” for gluons Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

  38. Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL

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