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Electromagnetic Measurements at RHIC

Electromagnetic Measurements at RHIC. Hideki Hamagaki Center for Nuclear Study University of Tokyo. Prologue. EM probe = penetrating probe? correct EM probes provide information on when and where they are produced

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Electromagnetic Measurements at RHIC

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  1. Electromagnetic Measurements at RHIC Hideki Hamagaki Center for Nuclear Study University of Tokyo

  2. Prologue • EM probe = penetrating probe? • correct • EM probes provide information on when and where they are produced • EM probe? = leptons or photons; promptly produced without intermediate state (hadron resonances); thermal photons/pairs, direct photons, Drell-Yan pairs, pairs from annihilation (quark-level and hadron-level), • Physics quantities with EM measurements have many other sources • hadron’s leptonic or photonic decays (as I will show you later) • some of hadrons work as penetrating probes, depending on their life-time and when and where they are produced; "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  3. Prologue -- cont. • J/y • production: initial stage, and hadronization stage (recombination) • life-time: ct = 2264 fm; decay far outside the source • long life time: p0, h, D and B • low-mass vector mesons; r, w, f • production: creation in hadron phase + coalescence in the freeze-out stage • life-time: ct(r) = 1.3 fm, ct(w) = 23.3 fm, ct(f) = 46.2 fm • a part of w and f decays inside the source (modifications), and the rest outside the source (free decay) • r  pp r (recreation; thermalized): r  ee (escape from the source) is dominated by the decay inside, can be used as a fast clock, and is a “pseudo-penetrating probe” "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  4. Outline of My Talk • Photon measurements • Lepton measurements • Single electron measurements  leptonic decay of open quarks • J/y measurements • Thermal pairs and vector mesons • Summary and Outlook "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  5. direct photons initial hard scattering non-thermal pre-equilibrium QGP thermal Hadron gas photons from hadron decay Direct photons • Direct photon is a unique probe, which provides direct information of its birth, because of penetrating property • photons can come from every stage of collisions, and can have various origins • direct photons = not from “hadron decay” "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  6. thermal decay hard Quick Look of Various Photon Sources • for thermal and hard photon measurements, hadron decay is a non-trivial background source • strong suppression of high pT hadrons would improve the ratio, in particular, for hard direct photons • a window for QGP thermal photons; pT = 1 ~ 3 GeV/c "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  7. Success in the 1st round • Direct photon in high pT region in Au+Au collisions • suppression of high pT p0 yield makes the g/p0 ratio larger • comparable to a pQCD calculation (Ncoll scaling), which means;  no strong initial state effect; modification of structure function  p0 suppression is the final state effect • 2nd round  thermal photons "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  8. Is thermal photon yield suppressed? • Comparison of the present result with calculations with kT broadening and jet QGP bremsstrahlung • fast quarks passing through QGP, which is a significant photon source for pT < 6 GeV/c • At present, it is too early to claim anything significant • Gluon plasma (GP) and photon yield • chemically non-equilibrium state; GP  QGP • hotter than QGP ( smaller degeneracy) • hot glue scenario? • If GP stays long, it will have impact to photon production in hot matter; photon yield will be related to the evolution from GP to QGP • Other way to say: photon suppression is the signature of GP; native no-QGP hadron picture can be ruled out "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  9. Single electrons are from many sources • Photonic • Photon conversions • from decay mesons • from direct photons • Dalitz decays of p0, h, h’, w, f • Non-photonic • leptonic decay of D & B • Thermal lepton-pairs • Di-electron decays of r, w , f • Kaon decay (Ke3 p0en) • Leptonic decay of D & B is dominant among the “signals” • Today’s signal is tomorrow’s BG • Subtraction is a tricky business "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  10. without Conv. with Conv. Entries/Nevt Ne 1.7% 1.1% 0.8% With a converter in N(tot)/N(photonic) W/O converter Conversion at the detectors Dalitz : 0.8% X0 equivalent Entries/Nevt Non-photonic 0 0 Mininum Bias Au+Au in sNN=200GeV pT[GeV/c] Extraction of non-photonic components "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  11. Centrality dependence of electron yield • Ncoll scaling seems to work well • NAuAu(pT;b) = Npp(pT)*Ncolla a = 0.938+-0.075(stat) +- 0.018(sys) a = 0.958+-0.035(stat) with p-p data included • Binary scaling seems to work well • keen interest has been in the behavior of heavy quarks in dense matter • thermalization? • energy loss? "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  12. Hints of communication • v2 for non-photonic electron • prediction based on a quark coalescence model v2(D) ≈ v2(light) + v2（charm） • “thermal” is preferred • RAA(pT) for non-photonic single electrons in Au+Au • significant reduction at high pT • suggest sizable energy loss!  Year-4 RUN results with high statistics are definitely needed to confirm these "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  13. Quarkonium in HI collisions • Novel idea of J/y suppression • by Matsui and Satz(1986; before experimental results) • a good probe of deconfinement • suppresion due to Debye screening in deconfined phase • History in Brief • observation of suppression in S + A  turned out to be similar to p + A • anomalous suppression observed in Pb + Pb  a result to evident QGP at SPS "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  14. Recent Progress in Theory (I) • Lattice-QCD told us; • confining potential starts to disappear at low temperature far below TC • J/y does not melt easily • M. Asakawa, T. Hatsuda; Phys. Rev. Lett. 92 (2004) 012001 • Impact to the scenarios • Is dissociation dynamical or static? "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  15. Recent Progress in Theory (II) • Enhancement of J/y yield • basic idea is to add recombination of charms to scenario of the direct production with subsequent suppressions • statistical hadronization model • coalescence in the final stage • L. Grandchamp et al., Nucl.Phys. A715 (2003) 545 • A. Andronic et al., Nucl.Phys.A715 (2003) 529 • kinetic formation model • reproduction in QGP + coalescence in the final stage • R.L. Thews et al., Phys. Rev. C 63 (2001) 054905 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  16. J/y in Au-Au collisions at RHIC • results from Year-2 RUN • very poor • but is already inconsistent with large enhancement scenarios • e.g. kinetic formation model, cf. PRC 63, 054905 (2001) • In Run-4, 240 mb-1 recorded with improved detector performance • ~100 times more J/y signals expected than in Run-2 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  17. Thermal radiation and low-mass vector mesons • Still missing at RHIC • Experimentally, combinatorial background is very large and must be subtracted properly • Large physics background comes from charm. • Charm production is measured with ~15% accuracy by single electron measurements. A prediction R. Rapp, nucl-ex 0204003 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  18. R. Rapp (Nucl. Phys A661(1999) 238c Loss-mass pairs • Famous CERES results • enhancement in the pair yield • A probe of Chiral symmetry restoration and/or medium modification • Only hadronic decays so far at RHIC • rp+p- • STAR (Phys. Rev. Lett. 92, 092301) • f K+K- • PHENIX (nucl-ex/0410012) • STAR (nucl-ex/0406003) • Lepton-pair measurement is demanding • should provide information on space-time evolution at RHIC, which might be very different from SPS • mass shift vs broadening "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  19. Combinatorials is overwhelming Real and Mixed e+e- Distribution Real - Mixed with systematic errors • Combinatorial background is determined with ~1% accuracy in Run3 and Run2 using a mixed event method • Higher statistics from Run4 data should help, but it may not be enough for low-mass vector mesons Run2 AuAu Minimum Bias 0 1 2 3 [GeV/c2] 0 1 2 3 [GeV/c2] "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  20. “combinatorial pairs” total background S/B ~ 1/500 Irreducible charm background all signal charm signal How to measure low-mass pairs • At PHENIX, R&D efforts to develop a Dalitz rejecter, in order to manage combinatorial backgrounds • HBD (hadron Blind Detector) is a strong candidate • UV photon detector • with CsI cathode • CF4 gas radiator • Ne(Cherenkov) > Ne(ionization) "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  21. Summary and Outlook • Direct photons • thermal photons may not be abundant • Single electron measurement in Au+Au collisions • dominant contributions from charm and bottom • yield at mid-rapidity scales with Ncoll  no significant effect from initial state • possible finite v2 and high pT suppression • flow and stopping effect -- to be determined • J/y result for HI collisions at RHIC • inconclusive result from Year-2 RUN • good statistics in Year-4 RUN  stay tuned • Thermal pairs and low-mass vector mesons • combinatorials, charms, ... • HBD is to buster combinatorials "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  22. Heavy flavor production • Charm (or bottom) production • Leading order at low x = ’’gluon fusion’’ A good probe of; • Initial state (probably OK) • Parton distribution functions • kT broadening • Early stage of evolution (?) • Parton energy loss? • Thermalization & Flow? • Thermal production? • How to measure? • exclusive • leptonic decay  measure electrons "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  23. The result implies; • no significant initial state effect in the mid-rapidity at RHIC energy • gluon shadowing & anti-shadowing are not large • no anomalous enhancement due to thermal production • No energy loss? • No thermalization?  next slide Pb / p "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  24. D from PYTHIA D from Hydro B from PYTHIA B from Hydro e from PYTHIA e from Hydro Is Heavy quarks immune to Hot matter? • The answer is “not definite” • Electron pT distribution is not sensitive to modification of pT distribution of the parent, due to decay kinematics • Need to extend to higher pT, in order to see “flow’’ or “energy loss’’ effects  Year-4 RUN • A caveat S. Batsouli et al.: Phys.Lett. B557 (2003) 26 "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

  25. Limitation of inclusive e measurement • High pT region in e • c decay  b decay • not high pT/m for parent c and b • Exclusive measurement • D0 K+ + p- ... bySTAR • OK in p+p & d+Au • combinatorial will be a killer in Au+Au  need vertex determination "Electromagnetic measurements at RHIC"@ICPAQGP 05 Hideki Hamagaki

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