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Thermal Photons at RHIC and LHC

Thermal Photons at RHIC and LHC. Ju Hwan Kang (Yonsei University) APCTP 2010 LHC Physics Workshop at Korea Aug. 10 - 12, 2010, Konkuk Univ., Seoul, Korea. OUTLINE. De-confinement & RHIC Most famous QGP signal at RHIC High p T Suppression (Jet-Quenching) Why thermal photons?

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Thermal Photons at RHIC and LHC

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  1. Thermal Photons at RHIC and LHC Ju Hwan Kang (Yonsei University) APCTP 2010 LHC Physics Workshop at Korea Aug. 10 - 12, 2010, Konkuk Univ., Seoul, Korea

  2. OUTLINE • De-confinement & RHIC • Most famous QGP signal at RHIC High pT Suppression (Jet-Quenching) • Why thermal photons? • Thermal photons at RHIC PHENIX Results • Thermal photons at LHC ALICE & Transition Radiation Detector (TRD)

  3. RHIC’s Experiments STAR

  4. Deconfinement & RHIC Lattice Calculations: Tc = 170 ± 15% MeV (~2 x 1012 K) • QCD : established theory of the strong interaction • Quarks and gluons deconfined at high temperatures, at least from Lattice QCD • RHIC : Relativistic Heavy Ion Collider (√s = 200 GeV/nucleon for Au+Au) • To produce a hot QCD matter by colliding heavy ions

  5. High pT particle production At RHIC, most of high pT particles are fromjets. hadrons jet • proton-proton collision: • hard scattered partons • fragment intojets of hadrons hadrons • nucleus-nucleus collision : parton energy loss if partonic matter supprssion of high pThadrons no suppression of high pTphotons

  6. Evidence for Parton Energy Loss? No energy loss for g‘s Energy loss for quark and gluon jets 0’s and ’s are suppressed, direct photons are not:Evidence for parton energy loss (jet quenching, indicating production of deconfined state or QGP)

  7. Jet on the “other” side? Jet correlations in proton-proton reactions. Strong back-to-back peaks. Azimuthal Angular Correlations

  8. Jet on the “other” side? Jet correlations in central Gold-Gold. Away side jet disappears for particles pT > 2 GeV Leading hadrons Medium Azimuthal Angular Correlations

  9. Why thermal photons? • Jet-quenching (or high pT suppression of hadrons) is widely accepted as QGP signal at RHIC. • The other famous result at RHIC is Elliptic Flow which is also from the measurement of hardons. • Since hadrons interact strongly with the matter, they can not carry the information deep inside the matter (Jet-quenching tells only the amount of disappearing jets). • The temperature of the matter measured with thermal photons would give a direct information on the properties of the matter (QGP).

  10. Electromagentic probes (photon and lepton pairs) e+ e- g* g • Photons and lepton pairs are cleanest probes of the dense matter formed at RHIC • These probes have little interaction with the matter so they carry information deep inside of the matter • Temperature of QGP by measuring pT spectrum of direct photons • Thermal photons from QGP is the dominant source of direct photons for 1<pT<3 GeV/c

  11. Photons in nucleus-nucleus collisions g q g q p p r g Time Thermalizedmedium (QGP!?), T0 > Tc ,Tc 170 - 190 MeV(QGP thermal ) Phase transitionQGP → hadron gas (Low pT) Freeze-out (hadron decay ) Initial hard parton-partonscatterings (hard ) g p g

  12. Many source of photons p p r g pQCD direct photons from initial hard scattering of quarks and gluons quark gluon g Thermal photonsfrom hot quark gluon plasma Thermal photonsfrom hadron gasafter hadronization background g Decay Photons from hadrons (p0, h, etc) p g

  13. Thermal photons (theory prediction) g q g q p p r g Hadron decay photons S.Turbide et al PRC 69 014903 • High pT (pT>3 GeV/c) pQCD photon • Low pT (pT<1 GeV/c) photons from hadronic gas • Themal photons from QGP is the dominant source of direct photons for 1<pT<3 GeV/c • Measurement is difficult since the expected signal is only 1/10 of photons from hadron decays

  14. Virtual photons to improve S/B • Source of real photon should also be able to emit virtual photon • At m0, the yield of virtual photons is the same as real photon • Real photon yield can be measured from virtual photon yield, which is observed as low mass e+e- pairs • Advantage: hadron decay background can be substantially reduced. For m>mp, p0 decay photons (~80% of background) are removed • S/B is improved by a factor of five

  15. Virtual Photon Measurement 0 Dalitz decay Compton Any source of real g can emit g* with very low mass. Relation between the g* yield and real photon yield is known. Process dependent factor • Case of hadrons (p0, h) (Kroll-Wada) • S = 0 at Mee > Mhadron • Case of direct g* • IfpT2>>Mee2 S = 1 • For m>mp, p0 background (~80% of background) is removed • S/B is improved by a factor of five Direct g p0 h

  16. Enhancement of almost real photon pp Au+Au (MB) PRL, 104, 132301 (2010) • Low mass e+e- pairs (m<300 MeV) for 1<pT<5 GeV/c • p+p: • Good agreement of p+p data and hadronic decay cocktail (, , , ’, ) • Au+Au: • Clear enhancement visible above mp =135 MeV for all pT • Excess  Emission of almost real photon Mp Mp 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV

  17. Fraction of direct photons Fraction of direct photons Compared to direct photons from pQCD p+p Consistent with NLO pQCD (gluon compton) Au+Au Clear excess above pQCD (thermal g?) PRL, 104, 132301 (2010) r = direct g/inclusive g p+p Au+Au (MB) μ = 0.5pT μ = 1.0pT μ = 2.0pT NLO pQCD calculation with 3 different scales by Werner Vogelsang

  18. Direct photon spectra exp + TAA scaled pp PRL, 104, 132301 (2010) • Direct photon measurements • real (pT>4GeV) • virtual (1<pT<5GeV) • pQCD consistent with p+p down to pT=1GeV/c • Au+Au = “scaled p+p” + “expon”: Fit to pp NLO pQCD (W. Vogelsang) The inverse slope TAuAu > Tc ~ 170 MeV

  19. Theory comparison • Hydrodynamical models are compared with the data D.d’Enterria &D.Peressounko T=590MeV, t0=0.15fm/c S. Rasanen et al. T=580MeV, t0=0.17fm/c D. K. Srivastava T=450-600MeV, t0=0.2fm/c S. Turbide et al. T=370MeV, t0=0.33fm/c J. Alam et al. T=300MeV, t0=0.5fm/c F.M. Liu et al. T=370MeV, t0=0.6 fm/c • Hydrodynamical models are in qualitative agreement with the data

  20. Comparison with models Direct photon yield in p+p is consistent with pQCD, but direct photon yield in Au+Au is much larger. If direct photons in Au+Au are of thermal origin, the inverse slope is related to the initial temperature Tinit . Hydrodynamical models with Tinit=300-600MeV at the plasma formation time t0=0.6-0.15 fm/c are in qualitative agreement with the data. Tinit is about 1.5 to 3.0 time Tavg due to the space time evolution. Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV

  21. On the Map “free” Gas We are here “free” Gas 500 Plasma 400 T (MeV) Quark Gluon Plasma “Perfect” Liquid 300 “Perfect” Liquid 200 Hadrons Tc ~ 170 MeV; e ~ 1 GeV/fm3 100 At these temperature, QGP is “perfect” liquid. At higher temperature, it can become “gas”

  22. ALICE (A Large Ion Collider Experiment) at CERN LHC To study even hotter QCD matter…

  23. LHC SPS ALICE

  24. TRD (Transition Radiation Detector) • |η|<0.9, 45°<θ<135° • 18 supermodules in Φ sector • 6 Radial layers • 5 z-longitudinal stack  total 540 chambers  750m² active area  28m³ of gas • In total 1.18 million read-out channels • TRD can measure electrons, and also low pT photons (e+e- ) to study thermal photons at LHC

  25. (Plastic fiber + Air) Transition radiation (TR) is produced if a highly relativistic (γ>900) particle traverses many boundaries between materials with different dielectric properties. Electrons can be identified using total deposited charge, and signal intensity as function of drift time.

  26. 1 Event signals of Electron and Pion purse height electron t 0 3μs purse height Pion t 0 3μs

  27. Hopefully, in near future, the initial temperature of the QGP produced at LHC can be also measured to be compared with that of RHIC. • Expected to be higher initial temperature at LHC…

  28. Backups

  29. Elimination of backgrounds Photon conversion minimized by a helium bag (~0.4% of a radiation length). Combinatorial background was removed with a mixed event technique. Elimination of unphysical correlations arising from overlapping tracks or hits. Background from photon conversions and cross pairs is removed with the cut on mass and opening angle. To check the background subtraction, some data with extra of 1.68% radiation length (X0) to increase the background by factor of 2.5.

  30. Virtual photon emission rate Real photon yield Turbide, Rapp, Gale PRC69,014903(2004)

  31. Direct Photons in Au+Au Blue line: Ncoll scaled p+p cross-section Direct photon is measured as “excess” above hadron decay photons Measurement at low pT difficult since the yield of thermal photons is only 1/10 of that of hadron decay photons PRL 94, 232301 (2005) Au+Au data consistent with pQCD calculation scaled by Ncoll

  32. Motivation : Direct  production • Leading order diagram in perturbation theory • Direct  production in p+p •  One of the best known QCD process… Really? Hard photon : Higher order pQCD Soft photon : Initial/final radiation, Fragmentation function

  33. TRD working principle The total energy loss by TR for charged particle : depends on its Lorentz factor γ = E / mc2 Usefullness of TR : For discrimination the electron from hadons in the momentum range between 1GeV/c and 100GeV/c. Cut through one side of a TRD readout chamber FROM P. Shukla -- ICPA-QGP'05, Kolkata, 8-12

  34. “Current” plan for LHC Please find below the outcome of a meeting to define the LHC running schedule for the next few years. We will have a long run spanning 2010 and most of 2011 at 7 TeV (presumably with a short technical stop again during Christmas 2010, but this has still to be decided), followed by a long shutdown starting mid to end 2011 to bring the machine up to its design Energy. A long run now is the right decision for the LHC and for the experiments. It gives the machine people the time necessary to prepare carefully for the work that’s needed before allowing 14 TeV, or 5.5 TeV/nucleon .

  35. direct / 0 in Au+Au at s = 200 GeV Au+Au p0 + X (peripheral) Au+Au p0 + X (central) Strong suppression Au+Au direct + X Blue lines:Ncoll scaled pQCD p+p cross-section Peripheral spectra agree well with p+p (data & pQCD) scaled by Ncoll Data exhibits suppression: RAA=red/blue < 1

  36. Electron pair measurement in PHENIX g p DC e+ e- PC1 magnetic field & tracking detectors PC3 designed to measure rare probes:+ high rate capability & granularity + good mass resolution and particle ID - limited acceptance • 2 central arms: electrons, photons, hadrons • charmonium J/, ’ -> e+e- • vector meson r, w,  -> e+e- • high pTpo, p+, p- • direct photons • open charm • hadron physics Au-Au & p-p spin

  37. LMR-I = quasi-real virtual photon Dilepton spectrum as a function of m_ee & pT from a simulation of hadron decays. m<300 MeV, 1<pT<5 GeV/c • LMR I(pT >> mee)quasi-real virtual photon region. Low mass pairs produced by higher order QED correction to the real photon emission LMR II : dilepton production is expected to be dominated by the hadronic gas phase(mass modification?)

  38. Input hadron spectra for cocktail Fitting with a modified Hagedorn function for pion, for all other mesons assume m_T scaling by replacing p_T by

  39. e+e- mass spectra in pT slices p+p Au+Au arXiv:0912.0244 • p+p in agreement with cocktail • Au+Au low mass enhancement concentrated at low pT Excess has a similar shape to the cocktail and the level of the excess is approximately constant.

  40. Photon Probe of Nuclear Collisions g g p p f Time p K freeze-out time expansion Hadron Gas quark-gluon plasma hard parton scattering Space Au Au p p K

  41. Extraction of the direct  signal r = direct g*/inclusive g* fdirect : direct photon shape with S = 1 • Interpret deviation from hadronic cocktail (, , , ’, ) as signal from virtual direct photons • Fit in 120-300MeV/c2(insensitive to p0 yield) arXiv:0804.4168 arXiv:0912.0244 A. Adare et al., PRL accepted

  42. Initial temperature Tave(fit) = 221 MeV TC from Lattice QCD ~ 170 MeV Tini at t0 (thermalization time) is 1.5 to 3 times Tavg due to the space-time evolution From data: Tini > Tavg = 220 MeV From models: Tini = 300 to 600 MeV t0 = 0.15 to 0.6 fm/c Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV

  43. Summary of RHIC thermal photons • e+e- pairs for m<300MeV and 1<pT<5 GeV/c were measured • Excess above hadronic background is observed • Excess is much greater in Au+Au than in p+p • Treating the excess as internal conversion of direct photons, the yield of direct photon is dedued. • Direct photon yield in p+p is consistent with NLO pQCD • Direct photon yield in Au+Au is much larger. • Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±19(stat)±19(sys) MeV • Hydrodynamical models with Tinit=300-600MeV at t0=0.6-0.15 fm/c are in qualitative agreement with the data. • Lattice QCD predicts a phase transition to quark gluon plasma at Tc ~ 170 MeV

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