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Thermal Radiation Mapping the Space-Time Evolution

Thermal Radiation Mapping the Space-Time Evolution. Thermal radiation from hadronic collisions : An old but still hot idea! Mapping the time evolution Experimental results on “thermal” radiation State of the Art experiment: NA60 Energy frontier: PHENIX Future perspectives

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Thermal Radiation Mapping the Space-Time Evolution

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  1. Thermal Radiation Mapping the Space-Time Evolution • Thermal radiation from hadronic collisions: • An old but still hot idea! • Mapping the time evolution • Experimental results on “thermal” radiation • State of the Art experiment: NA60 • Energy frontier: PHENIX • Future perspectives • PHENIX VTX and HBD upgrades • Lessons learned and future opportunities Axel Drees, WWND 2012, Dorado del Mar, Puerto Rico Promise to help unravel time evolution Time evolution not understood; Hints for new physics? Unique possibilities for future

  2. Thermal Radiation from QGP Axel Drees

  3. J/ QGP Drell-Yan Shuryak PLB 78B (1978) 150 Shuryak 1978: Birth of the Quark Gluon Plasma • Data from 400 GeV p-A at FNAL • e+e- for high mass PRL 37 (1976) 1374 • m+m- for high mass PRL 38 (1977) 1331 p-A 400 GeV Ultimately the wrong explanation, but this paper was landmark and kicked off the search for the QGP and its radiation! Key lesson: Know your backgrounds! In particular charm and bottom! Axel Drees

  4. Thermal Radiation from Expanding Source • Radiation from longitudinally and radially expanding fire ball in “local equilibrium” • Real and virtual photon momentum spectrum at mid rapidity: • Temperature information • Integrated over space time evolution • due to T4 dependence sensitive to early times • Collective expansion • Radial expansion results in blue and red shift • Longitudinal expansion results in red shift • Virtual photon mass spectrum • Temperature information • Not sensitive to collective expansion Planck spectrum: yield  T4 , mean  T boosted by collective motion Mass and momentum dependence allows to disentangle flow from temperature contributions!! Axel Drees

  5. e- p r* g* e+ p g q e- q g q p p q e+ r g Microscopic View of Thermal Radiation Production process: real or virtual photons (lepton pairs) hadron gas: photons low mass lepton pairs QGP: photons medium mass lepton pairs Key issues: In medium modifications of mesons pQCD base picture requires small as But as can not be small for dNg/dy~ 1000 (i.e. in a strongly coupled plasma) Equilibrium of strong interaction! Equilibrium not a necessary condition! Experimentally observed yield integrated over full time evolution! Axel Drees

  6. Experimental Issue: Isolate Thermal Radiation , * from A+A Direct Need to subtract decay and prompt contributions Non-thermal Thermal Pre-equilibrium Hadron Gas “Prompt” hard scattering Quark-Gluon Plasma Hadron Decays Sensitive to space-time evolution 1 10 107 log t (fm/c) Axel Drees

  7. Map Out Time Evolution with Thermal Radiation • Experimental method • Measure inclusive g and l+l- • Subtract experimental background e.g. combinatorial pair background • Subtract “cocktail” of known sources, i.e. hadron decays after freeze out • Isolate thermal radiation • Real photons cocktail: • p0g, hg, wp0g, ... • More than 90% of photon yield • Lepton pair cocktail: • p0e+e-, h, wp0e+e- and direct decaysr,w,fe+e-, J/ye+e- ... • Semileptonic decays of heavy flavor • Drell Yan • Dileptons have mass  remove contribution from p0  more sensitive to thermal radiation than photons Dileptons are more sensitive than photons Axel Drees

  8. 2.5 T dipole magnet muon trigger and tracking beam tracker vertex tracker magnetic field target hadron absorber Muon Other State of the Art Measurements with NA60 Next slides mostly derived from talks given by SanjaDamjanovic • NA60 features • Classic muon spectrometer • Precision silicon pixel vertex tracker • tagging of heavy flavor decay muons • Reduction of combinatorial background by • vetoing p, K decay muons • Double dipole for large acceptance (low mass) • High rate capability NA60 can isolate “thermal” contribution Axel Drees

  9. Continuum Excess Measured by NA60 Fully acceptance corrected • Planck-like mass spectrum, falling exponentially (T > 200 MeV) • For m>mr good agreement with three models in shape and yield • Main Sources m < 1 GeV • p+p- r  m+m- • Sensitive to medium spectral function • Main sources m > 1 GeV • qq m+m- • p a1 m+m-(Hess/Rapp approach) 300 MeV ~ 1/m exp(-m/T) 200 MeV Eur. Phys. J. C 59 (2009) 607; CERN Courier 11/2009 Evidence for thermal dilepton radiation Axel Drees

  10. Sensitivity to Spectral Function Not acceptance corrected • Models for contributions from hot medium (mostly pp from hadronic phase) • Vacuum spectral functions • Dropping mass scenarios • Broadening of spectral function • Broadening of spectral functions clearly favored! pp annihilation with medium modified r works very well at SPS energies! Phys.Rev.Lett. 96 (2006) 162302 Hadronic contributions explored and exhausted Axel Drees

  11. Dominance of partons for m>1GeV Phys. Rev. Lett. 100 (2008) 022302 • Schematic time evolution of collision at CERN energies • Partonic phase early emission: high T, low vT • Hadronic phase late emission: low T, high vT • Experimental Data: thermal radiation • Mass < 1 GeV from hadronic phase • <Tth>= 130-140 MeV < Tc • Mass > 1 GeV from partonic phase • <Tth>= 200 MeV >Tc Teff~ <Tth>+ M <vT>2 • partonic • qq→m+m- • hadronic • p+p-→r→m+m- Dileptons for M >1 GeV dominantly of partonicorigin Eur. Phys. J. C 59 (2009) 607 Axel Drees

  12. Status: Thermal Radiation at SPS energies • State of the art dilepton experiment: NA60 • Isolate thermal radiation • Planck like exponential mass spectra • exponential mTspectra • zero polarization • general agreement with models of thermal radiation • Emission sources of thermal dileptons(from m-pT): • hadronic(p+p-annihilation) dominant for M<1 GeV • partonic(qq annihilation) visible M>1 GeV • In-medium r spectral function identified: • no significant mass shift of the intermediate  • only broadening. Axel Drees

  13. DC PC1 PC2 magnetic field & tracking detectors PC3 Thermal radiation at RHIC Energies: PHENIX Disclaimer: ongoing analysis from STAR analysis Photons, neutral pion p0g g e+e- pairs E/p and RICH Calorimeter e- g g e+ No background rejection! dilepton S/B < 1:150 HBD upgrade! Axel Drees

  14. Dilepton Continuum in p+p Collisions PHENIX Phys. Lett. B 670, 313 (2009) Data and Cocktail of known sources represent pairs with e+ and e- PHENIX acceptance Data are efficiency corrected Excellent agreement of data and hadron decay contributions with 30% systematic uncertainties Consistent with PHENIX single electron measurement sc= 567±57±193mb Axel Drees

  15. Dilepton Continuum in d+Au Collisions PHENIX preliminary Consistent with known sources Data will constrain known sources with high precision In particular bottom cross section Axel Drees

  16. Au+Au Dilepton Continuum Excess 150 <mee<750 MeV: 3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model) hadron decay cocktail tuned to AuAu PHENIX Phys. Rev. C 81 (2010) 034911 PHENIX VTX upgrade Charm from PYTHIA filtered by acceptance sc= Ncoll x 567±57±193mb Charm “thermalized” filtered by acceptance sc= Ncoll x 567±57±193mb Intermediate-mass continuum: consistent with PYTHIA since charm is modified room for thermal radiation Axel Drees

  17. Low Mass Dilepton Puzzle Large low mass enhancement • Models calculations with broadening of spectral function: • Rapp & vanHees • Central collisions scaled to mb • + PHENIX cocktail • Dusling & Zahed • Central collisions scaled to mb • + PHENIX cocktail • Bratkovskaya & Cassing • broadening pp annihilation with medium modified r insufficient to describe RHIC data! PHENIX Phys. Rev. C 81 (2010) 034911 Excess not from hadronic phase!! Axel Drees

  18. Soft Low Mass Dilepton Puzzle acceptance corrected • mTspectrum of excess dileptons • Subtract cocktail • Correct for pair acceptance • Fit two exponentials in mT –m0 • Eludes any theoretical interpretation • Hint also in NA60 data • Insufficient date for more detailed information 300 < m < 750 MeV 258  37  10 MeV Soft component below mT~ 500 MeV: Teff ~ 100MeV independent of mass more than 50% of yield 92  11  9 MeV PHENIX Phys. Rev. C 81 (2010) 034911 Excess has 2 components : (1) Thermal radiation (next slides); (2) Soft “exotic” source, red shift, glasma, color B-field Axel Drees

  19. First Measurement of Thermal Radiation at RHIC PHENIX Phys. Rev. C 81 (2010) 034911 * (m→0) =  ; m << pT • Direct photons from real photons: • Measure inclusive photons • Subtract p0and h decay photons at S/B < 1:10 for pT<3 GeV • Direct photons from virtual photons: • Measure e+e- pairs at mp < m << pT • Subtract h decays at S/B ~ 1:1 • Extrapolate to mass 0 T ~ 220 MeV g* (m0) g First thermal photon measurement: Tini > 220 MeV > TC pQCD Need to consider radial flow! Axel Drees

  20. Calculation of Thermal Photon Yield PHENIX Phys. Rev. C 81 (2010) 034911 • Reasonable agreement with data • factors of two to be worked on .. • Initial temperatures and times from theoretical model fits to data: • 0.15 fm/c, 590 MeV (d’Enterria et al.) • 0.2 fm/c, 450-660 MeV (Srivastava et al.) • 0.5 fm/c, 300 MeV (Alam et al.) • 0.17 fm/c, 580 MeV (Rasanen et al.) • 0.33 fm/c, 370 MeV (Turbide et al.) • Observations comparing models: • Correlation between T and t0 • Yield typically to low • Yield not correlated with Tini Tini= 300 to 600 MeV t0= 0.15 to 0.5 fm/c Low yield  earlier emission (yield  T4) increase factor 2 with <20% change in T Axel Drees

  21. Thermal Photons also Flow • How to determine elliptic flow of thermal photons? • Establish fraction of thermal photons in inclusive photon yield • Predict hadron decay photon v2 from pion v2 • Subtract hadron decay contribution from inclusive photon v2 PHENIX arXiv:1105.4126 Large v2 of low pT thermal photon Axel Drees

  22. Thermal Photon Puzzle Models fail to describe simultaneously photon yield, T and v2! Hees/Gale/Rapp Phys.Rev.C84:054906,2011. Tini ~ 325MeV Large flow requires late emission! Apparent contradiction with yield, which points towards early emission! R. Chatterjee and D. K. Srivastava PRC 79, 021901(R) (2009) PRL96, 202302 (2006) Axel Drees

  23. Status: Thermal Radiation at RHIC energies • PHENIX e+e- and g from √sNN = 200 GeV • Soft low mass dilepton puzzle • larger excess beyond contribution from hadronic phase with medium modified r-meson properties … not from hadronic phase • soft momentum distribution … not from hot partonic phase • Thermal photon puzzle • Large thermal yield with T > 220 MeV (20% of decay photons) … suggests early emission • Large elliptic flow (v2) … suggests late emission PHENIX data on E&M probes seems INCONSISTENT with standard hydro space-time evolution! And exhibits UNKOWN additional sources! Speculation: look between impact (t=0) and t0 Axel Drees

  24. Near Future: PHENIX HBD upgrade Window less CF4 Cherenkov detector GEM/CSI photo cathode readout Operated in B-field free region Improve S/B by rejecting combinatorial background 10% central, 62 GeV: • HBD fully operational: • Single electron ~ 20 P.E. • Conversion rejection ~ 90% • Dalitz rejection ~ 80% • Improvement of S/B factor 5 to published results efficiency 60% Rejection 90% p+p with HBD uncorrected p+p data in 2008/9 Au+Au data in 2009/10 Au+Au background subtraction needs to be finalized, results at QM Axel Drees

  25. Near Future: PHENIX VTX upgrade Vertex resolution in Y Tracking with 4 layers of silicon vertex detector 300mm • Promise to tag e+e pairs from ccbar • Opens opportunity to measure thermal radiation above M = 1 GeV • Drawback • added material, increased background • Not compatible with HBD, no rejection FVTX in 2011/12 49.6mm 24.8mm VTX in 2010/11 29.2mm (sim) (cm) DCA resolution Online display of Au+Au collision sDCA ~ 80 mm Impact on dilepton measurement unclear Axel Drees

  26. Summary of Findings • We have discovered “thermal” radiation from heavy ion collisions • Dileptons allow to disentangle space-time evolution of collision • NA60 established method with m+m- from In-In at 158 AGeV • PHENIX e+e- and g from √sNN = 200 GeV • Soft low mass dilepton puzzle • Thermal photon puzzle • Data inconsistent with “standard hydrodynamic space-time evolution • Next steps towards state of the art experiments (at RHIC) • PHENIX HBD & VTX data • STAR with full detector upgrades • Significant progress will requires a new experiment at RHIC dedicated to thermal radiation measurements! Axel Drees

  27. Backup Slides Axel Drees

  28. Short Detour on Cosmic Background Radiation • Discovered by chance in 1962 • Perfect Black Body spectrum with T=2.37 K in 1992 (COBE) • WMAP power spectrum 2006 • First data from Planck Satellite search for finger print of Inflation probing early evolution at t < 3 10-12fm Homogeneity of background radiation Requires inflation phase! Axel Drees

  29. STAR p+p Dilepton Data STAR arXiv:1204.1890 PHENIX cocktail in STAR acceptance MC@NLO for heavy flavor resolution not tuned for STAR STAR charm cross section s = 920 mb Axel Drees

  30. Lesson learned to Pursue Thermal Radiation Strong Physics Program Large Discovery Potential • Build dedicated thermal radiation experiment • Map thermal radiation in phase space • Deconvolve temperature and flow • Map time evolution of system • Focus on Dileptons • e+e- preferred for collider and y=0 • g in coincidence is a must to tag background • m+m- good at forward rapidity might be nice addition at y=0 • Measure heavy flavor simultaneously • Open and closed heavy flavor and much more as by product Axel Drees

  31. Comment on RHIC vs SPS vs LHC • RHIC is at a sweet spot • System is well in partonic phase • Proof of principle to measure thermal radiation exists • Many unsolved puzzle – which are not small! • large unknown source, large partoniccontribution, rapid thermalization, time evolution? • SPS at to low energy • Dominated by hadronic phase • Little to learn about early phase • Program at its end (or already beyond) • LHC at to high energy • System created at very similar condition compared to RHIC temperature • Dilepton continuum inaccessible due to background • Charm cross-section so high that irreducible background (both physics and random) becomes prohibitive for precision measures • Thermal photons may be possibly via low mass high pT virtual photons? • Detectors not setup for dilepton measurements Strong physics program at RHIC with little competition from LHC Axel Drees

  32. Thermal Radiation Experiment • Design requirement (educated guess) • Large acceptance (2p ; Dy=2) • For high statistics and better systematics • Charged tracking • Good electron id (1:1000 p rejection) • Excellent momentum resolution (dp/p < 0.2% p) • Combinatorial background rejection • Passive: minimize material budget (in particular before first layer) • Active: solid Dalitzrejection scheme • Heavy flavor detection • Low mass precision vertex tracker (<10-20mm DCA) • Photon measurement • Sufficient energy resolution (<10%/√E; small constant term) • High DAQ rate (all min bias you can get ~ 40 kHz) Do not compromise on requirements! Axel Drees

  33. Transverse Mass Distributions of Excess Dimuon transverse mass: mT = (pT2 + m2)1/2 Phys. Rev. Lett. 100 (2008) 022302 Eur. Phys. J. C 59 (2009) 607 • All mT spectra exponential for mT-m > 0.1 GeV • Fit with exponential in 1/mT dN/mT ~ exp(-mT/Teff) • Soft component for <0.1 GeV?? • Only in dileptons not in hadrons (speculate red shift???) Axel Drees

  34. Intermediate Mass Data for 158 AGeV In-In • Experimental Breakthrough • Separate prompt from heavy flavor muons • Isolate prompt continuum excess Intermediate Mass Range prompt continuum excess 2.4 x Drell-Yan Eur.Phys.J. C 59 (2009) 607 Axel Drees

  35. Interpretation as Direct Photon Relation between real and virtual photons: Extrapolate real g yield from dileptons: Virtual Photon excess At small mass and high pT Can be interpreted as real photon excess no change in shape can be extrapolated to m=0 Axel Drees

  36. Search for Thermal Photons via Real Photons • PHENIX has developed different methods: • Subtraction or tagging of photons detected by calorimeter • Tagging photons detected by conversions, i.e. e+e- pairs • Results consistent with internal conversion method The internal conversion method should also work at LHC! internal conversions Axel Drees

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