A Future for Measuring Thermal Radiation in Heavy Ion Collisions? Thermal radiation from hadronic collisions: An old but still hot idea! “thermal” radiation Experimental Challenges Experimental attempts to measure thermal radiation First successful experiment: CERES State of the Art experiment: NA60 Energy frontier: PHENIX Future perspectives Lessons learned and conclusions from them Dedicated experiment: Technical specifications Dedicated experiment: A strawman design

Thermal Radiation from QGP Axel Drees

Shuryak 1978: Birth of the Quark Gluon Plasma Shuryak PLB 78B (1978) 150 J/ Drell-Yan QGP 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! NA38 experiment was originally proposed to measure thermal radiation Key lesson: Know your backgrounds! In particular charm and bottom! Axel Drees

Naming Convention: Thermal?? 1 10 107 log t (fm/c) Photons from A+A Direct photons Photons from hadron decays “Prompt” hard scattering Pre-equilibrium Quark-Gluon Plasma Hadron gas Thermal Non-thermal Need to be more clear in what we mean by “thermal” and thermal equilibrium Axel Drees

Thermal Radiation Thermal photons a ~10% contribution to p0 photons Black body radiation Real Photons and virtual photons (lepton pairs) Static source back of envelope estimate: Thermal photons a ~10% contribution to p0 photons Range to look few 200 MeV to 2 GeV Axel Drees

An Expanding Source 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 Not sensitive to collective expansion Mass and momentum dependence allows to disentangle flow from temperature contributions!! Axel Drees

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) p r* g* e- e+ g p r q g q q e- e+ Additional issue: Need to know time evolution!! Axel Drees

Experimental Challenge Thermal radiation compete with “cocktail” of g and l+l- from hadron decays after freeze-out Real photons: p0g, hg, wp0g, ... More than 90% of photon yield Virtual photons: p0e+e-, h, wp0e+e- and direct decays r,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 Measure hadron decay contributions Focus on Dileptons, they are more sensitive than photons Axel Drees

Dilepton Experimental Challenges (I) Uncorrelated background: l+ and l- from different uncorrelated source Veto as many of the pairs actively by finding partner (rejection scheme) Remove remaining background statistically Like and unlike sign combinatorial pairs. Unlike sign background can be determined from like sign background, either measured (FG) or determined by event mixing (BG). Total number of pairs related by geometrical mean. True for e+e- since they are produced as pairs, even for different efficiencies. For m, produced as singles, strictly true only if produced with Poisson distribution. For different acceptance for singles or pairs need relative acceptance correction (obtained from mixed events) Axel Drees

Dilepton Experimental Challenges (II) Unphysical correlated background Limited double track/hit resolution False track match between detectors Not equal in like and unlike; Not reproducible by mixed events MUST be removed from event sample Correlated background: l+ and l- from same source but not signal “Cross” pairs  “jet” pairs Need case by case investigation with MC simulations and subtraction If background produces same numbers of ++ and - - pair same method as for different pair acceptance works X π0 e+ e- γ Axel Drees

Pioneering Dilepton Results form CERES/NA45 CERES PRL 92 (95) 1272 with 466 citations Discovery of low mass dilepton enhancement in 1995 p-Be and p-Au well described by decay cocktail Significant excess in S-Au (factor ~5 for m>200 MeV) Onset at ~ 2 mp suggested p-p annihilation Maximum below r meson near 400 MeV p r* g* e- e+ Launched massive theoretical investigation of meson properties in medium Axel Drees

Precision Measurements with NA60 Next slides mostly derived from talks given by Sanja Damjanovic 2.5 T dipole magnet beam tracker vertex tracker Muon Other hadron absorber muon trigger and tracking target magnetic field NA60 features Precision silicon pixel vertex tracker Classic muon spectrometer Double dipole for large acceptance (low mass) High rate capability Axel Drees

Low Mass Data Sample for 158 AGeV In-In NA60 can isolate continuum excess (including r meson) from decay background Phys. Rev. Lett. 96 (2006) 162302 Experimental advance High statistics Excellent background rejection Precision control of decay cocktail Example: NA60 can measure electromagnetic transition form factors for of η→μ+μ-γ and ω→μ+μ-π0 (Phys. Lett. B 677 (2009) 260) Axel Drees

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

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 qq  m+m- p a1  m+m- (Hess/Rapp approach) Main Sources m < 1 GeV p+p-  r  m+m- Sensitive to medium spectral function ~ 1/m exp(-m/T) 300 MeV 200 MeV Eur. Phys. J. C 59 (2009) 607; CERN Courier 11/2009 Evidence for thermal dilepton radiation Axel Drees

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! Axel Drees

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

Rise and Fall of Teff of Thermal Dimuons Mass < 1 GeV Linear increase of Teff with m Similar trend observed in hadrons Interpretation at SPS: Radial flow in hadronic phase! Dileptons sense flow of in-medium r p+p-→r→m+m- Mass > 1 GeV Sudden drop to Teff ~ 200 MeV Remains independent of mass Phys. Rev. Lett. 100 (2008) 022302 Teff ~ Tf + M <vT>2 Indication that source of thermal dileptons is different for low and large masses!! Axel Drees

Dominance of partons for m>1GeV 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 hadronic p+p-→r→m+m- partonic qq→m+m- Dileptons for M >1 GeV dominantly of partonic origin Axel Drees

Status: Thermal Radiation at SPS energies History Search started in 1986 First pioneering results on dileptons and photons (mostly limits) after 1995 Breakthrough with precise measurements (NA60) after 2006 Current status from dilepton experiment NA60 Planck like exponential mass spectra, exponential mT spectra, zero polarization and general agreement with thermal models consistent with interpretation of excess dimuons as thermal radiation Emission sources of thermal dileptons mostly hadronic (p+p- annihilation) for M<1 GeV, and mostly partonic (qq annihilation) for M>1 GeV In-medium r spectral function identified; no significant mass shift of the intermediate , only broadening. Axel Drees

Thermal radiation at RHIC Energies: PHENIX Disclaimer: ongoing analysis from STAR and potentially at LHC, but not finalized yet Photons, neutral pion p0  g g e+e- pairs E/p and RICH Calorimeter e- g g e+ PC1 PC2 PC3 DC magnetic field & tracking detectors Axel Drees

Dilepton Continuum in p+p Collisions 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 22

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 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 23

In Medium Mesons at RHIC??? Models calculations with broadening of spectral function: Rapp & vanHees Central collisions scaled to mb + PHENIX cocktail Dusling & Zahed Bratkovskaya & Cassing broadening broadening and dropping Au-Au mb with modified charm pp annihilation with medium modified r insufficient to describe RHIC data! Axel Drees

Contribution from Direct (pQCD) Radiation Measuring direct photons via virtual photons: any process that radiates g will also radiate g* for m<<pT g* is “almost real” extrapolate g*  e+e- yield to m = 0  direct g yield m > mp removes 90% of hadron decay background S/B improves by factor 10: 10% direct g  100% direct g* 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV pQCD q g hadron decay cocktail Small excess at for m<< pT consistent with pQCD direct photons Axel Drees

Direct Real Photons from Virtual Photons Significant direct photon excess beyond pQCD in Au+Au Axel Drees

First Measurement of Thermal Radiation at RHIC Direct photons from real photons: Measure inclusive photons Subtract p0 and 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* (e+e-)  m=0 g pQCD First thermal photon measurement: Tini > 220 MeV > TC Axel Drees

Calculation of Thermal Photons 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.) Correlation between T and t0 Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) Axel Drees

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 Large v2 of low pT thermal photon Axel Drees

Thermal Photon v2 Model Comparison Current models fail to describe direct photon v2 Hees/Gale/Rapp  Phys.Rev.C84:054906,2011. R. Chatterjee and D. K. Srivastava PRC 79, 021901(R) (2009) PRL96, 202302 (2006) Direct emission from hadronic phase insufficient! Axel Drees

Quark Scaling Behavior of v2 All hadrons flow collectively in common velocity field works for f and D mesons too favors a pre-hadronic origin Hadrons form from constituent quarks Flow builds up in partonic phase?! Common wisdom about space-time evolution may not be correct! Axel Drees

Summary of Findings We have discovered “thermal” radiation from heavy ion collisions NA60 m+m- from In-In at 158 AGeV Thermal source isolated experimentally Planck like mass-spectrum of thermal radiation Hadronic phase largest contributor (m < 1 GeV) Observe melting of r meson in medium Contribution from partonic phase (m > 1GeV) with <T> ~ 200MeV PHENIX e+e- and g from √sNN = 200 GeV Low mass excess larger than expected thermal contribution from hadron phase Thermal photons with <T> > 200 MeV (from g* extrapolated to m=0) Large elliptic flow (v2) of thermal photons which exceeds expected contribution from hadron phase More data to come in next years PHENIX HBD, STAR Expect significant progress requires new dedicated effort! Axel Drees

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 10-12 fm Much to learn from thermal radiation beyond temperature! Axel Drees

Lesson learned: Build a Dedicated Experiment Build dedicated thermal radiation experiment Map thermal radiation in phase space Deconvole 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 Strong Physics Program Large Discovery Potential Axel Drees

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 partonic contribution, 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

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: Dalitz rejection 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

Strawman Design active beam pipe Solenoid with ~2 T Dy = 2 ~30 cm vacuum pipe 2 cm 6 cm beam axis MAPS active pixel sxy < 20 mm X/X0 < 1% sDCA ~ 15 mm Solenoid with ~2 T Dy = 2 Silicon strip with sf ~100 mm dp/p < 0.1%p 1m active beam pipe GEM tracker, med. resolution vector with dE/dx, or RICH/HBD 0.7 m 0.1 m EMCal longitudinal segmented few 100ps resolution Rejection scheme: full track + tracklet mass cut tracklet: active beam pipe + inner GEM eID via dE/dx ~ 1/10 rejection few % p-resolution Electron ID: GEM tracker dE/dx EMCal TOF/ shower shape E/p Axel Drees

My Personal Conclusion Heavy ion physics at RHIC beyond PHENIX and STAR (>2015) should focus on “thermal” radiation Axel Drees

Backup Slides Axel Drees

Search for Thermal Photons via Real Photons The internal conversion method should also work at LHC! internal conversions 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 Axel Drees

Combinatorial Background: Like Sign Pairs Shape from mixed events Excellent agreements for like sign pairs also with centrality and pT Normalization of mixed pairs Small correlated background at low masses from double conversion or Dalitz+conversion normalize B++ and B- - to N++ and N- - for m > 0.7 GeV Normalize mixed + - pairs to Subtract correlated BG Systematic uncertainties statistics of N++ and N--: 0.12 % different pair cuts in like and unlike sign: 0.2 % --- Foreground: same evt N++ --- Background: mixed evt B++ Au-Au Normalization of mixed events: systematic uncertainty = 0.25% Axel Drees

Au-Au Raw Unlike-Sign Mass Spectrum arXiv: 0706.3034 Run with added Photon converter 2.5 x background Excellent agreement within errors! Unlike sign pairs data Mixed unlike sign pairs normalized to: Systematic errors from background subtraction: ssignal/signal = sBG/BG * BG/signal  up to 50% near 500 MeV 0.25% as large as 200!! Axel Drees

p-p Raw Data: Correlated Background Cross pairs Simulate cross pairs with decay generator Normalize to like sign data for small mass Like Sign Data Correlated Signal = Data-Mix Jet pairs Simulate with PYTHIA Normalize to like sign data Mixed events Unlike sign pairs same simulations normalization from like sign pairs Unlike Sign Data Alternative methode Correct like sign correlated background with mixed pairs Signal: S/B  1 Axel Drees

Centrality Dependence of Background Subtraction Compare like sign data and mixed background Evaluation in 0.2 to 1 GeV range For all centrality bins mixed event background and like sign data agree within quoted systematic errors!! Similar results for background evaluation as function pT Axel Drees

Background Description of Function of pT Good agreement Axel Drees

Fit Mass Distribution to Extract the Direct Yield: Example: one pT bin for Au+Au collisions 1/m Direct * yield fitted in range 120 to 300 MeV Insensitive to 0 yield Axel Drees

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

Interpretation as Direct Photon Relation between real and virtual photons: Virtual Photon excess At small mass and high pT Can be interpreted as real photon excess Extrapolate real g yield from dileptons: Example for one pT range: Excess*M (A.U). no change in shape can be extrapolated to m=0 Axel Drees