Direct Photon Measurements: initial conditions of heavy ion reactions at RHIC Alberica Toia for the PHENIX Collaboration Stony Brook University / CERN IV Workshop on Particle Correlations and Femptoscopy Krakow, September

Evolution of the Universe Nucleosynthesis builds nuclei up to He Nuclear Force…Nuclear Physics Universe too hot for electrons to bind E-M…Atomic (Plasma) Physics E/M Plasma Too hot for quarks to bind!!! Standard Model (N/P) Physics Quark- Gluon Plasma?? Too hot for nuclei to bind Nuclear/Particle (N/P) Physics Hadron Gas Solid Liquid Gas Today’s Cold Universe Gravity…Newtonian/General Relativity

The “Little Bang” in the lab High energy nucleus-nucleus collisions: –fixed target (SPS: √s=20GeV) –colliders RHIC: √s=200GeV LHC: √s=5.5TeV QGP formed in a tiny region ( m) for very short time ( s) –Existence of a mixed phase? –Later freeze-out Collision dynamics: different observables sensitive to different reaction stages 2 counter-circulating rings, 3.8 km circumference Top energies (each beam): –100 GeV/nucleon Au-Au. –250 GeV polarized p-p. –Mixed Species (e.g. d-Au)

Probing Heavy Ion Collisions time hard parton scattering Au hadronization freeze-out formation and thermalization of quark-gluon matter? Space Time expansion Jet cc  e   p K    Photons and dileptons: radiation from the media –direct probes of any collision stages (no final-state interactions) –large emission rates in hot and dense matter –according to the VMD their production is mediated in the hadronic phase by the light neutral vector mesons (ρ, ω, and φ) which have short life-time  Changes in position and width: signals of the chiral transition? Direct photon sources: – Compton scattering qg   q – Annihilation qq   g – Bremsstrahlung from inelastic scattering of incoming or thermalized partons

Energy density in heavy ion collisions T.D.Lee: “In HEP we have concentrated on experiments in which we distribute a higher and higher amount of energy into a region with smaller and smaller dimensions.” [Rev. Mod. Phys. 47 (1975) 267] Energy density: “Bjorken estimate” (for a longitudinally expanding plasma): Transverse Energy central 2% PHENIX 130 GeV  int ~ 100x  nucleus ~ 10x  critical

The matter is so dense that it melts(?) J/  (and regenerates it ?) RHIC The matter is so opaque that even a 20 GeV  0 is stopped The matter is so strongly coupled that even heavy quarks flow The matter is so dense that it modifies the shape of jets PHENIX preliminary strongly interacting Quark-Gluon Plasma (sQGP) in HI collisions at RHIC The matter is so dense that even heavy quarks are stopped What does it emit? What is the temperature?

Photon Emission Quark Gluon Plasma –De-confined phase of quarks and gluons should emit thermal radiation Direct photons are an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions. – Penetrate the strong interacting matter – Emitted from every stage of collisions Hard photons (High p T ) – Initial hard scattering, Pre-equilibrium Thermal photons (Low p T ) – Thermodynamic information from QGP and hadron gas  measure temperature of the matter –Dominant source for 1<p T <3 GeV/c –Measurement is difficut since the expected signal is only 1/10 of photons from hadron decays hard: thermal: Decay photons (  0 → ,  → , …) S.Turbide et al PRC

Direct photons in p+p and d+Au p+p Test of QCD –direct participant in partonic interaction –Less dependent on FF than hadron production Reduce uncertainty on pQCD photons in A+A good agreement with NLO pQCD Important baseline for Au+Au d+Au: –initial-state nuclear effects –no final-state effects (no medium produced) –Study initial-state effects 2 Extended in RUN5 data

Blue line: N coll scaled p+p cross-section Direct Photons in Au+Au Au-Au data consistent with pQCD calculation scaled by N coll THERMAL PHOTONS? Measurement at low p T (where an excess above the know sources may hint to thermal photon production) difficult because of detector resolution

Alternative: Virtual Photons Any source of real  can emit  * with very low mass. If the Q 2 (=m 2 ) of virtual photon is sufficiently small, the source strength should be the same The ratio of real photon and quasi-real photon can be calculated by QED  Real photon yield can be measured from virtual photon yield, which is observed as low mass e + e - pairs Kroll-Wada formula S : Process dependent factor Case of Hadrons Obviously S = 0 at M ee > M hadron Case of  * If p T 2 >>M ee 2 Possible to separate hadron decay components from real signal in the proper mass window. q  g q e+e+ e-e-

Signal Extraction arXiv: arXiv: Au+Aup+p Real signal – di-electron continuum Background sources – Combinatorial background – Material conversion pairs – Additional correlated background Visible in p+p collisions Cross pairs from decays with 4 electrons in the final state Pairs in same jet or back-to-back jet

Hadronic Cocktail Calculation arXiv: Remaining pairs after background subtraction – Real signal + Hadron decay components Estimate hadron components using hadronic cocktail Mass distributions from hadron decays are simulated by Monte Carlo. –  0, ,  ’, , , , J/ ,  ’ Effects on real data are implemented. – PHENIX acceptance, detector effect, efficiencies … Parameterized PHENIX  0 data with assumption of  0 = (  + +  - )/2 Hadronic cocktail was well tuned to individually measured yield of mesons in PHENIX for both p+p and Au+Au collisions.

Cocktail Comparison arXiv: arXiv: Au+Aup+p – Excellent agreement with cocktail Au+Au – Large enhancement in low mass region – Integrated yield in 150MeV < m ee < 750MeV data/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model)

0 < p T < 8 GeV/c0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c1.5 < p T < 8 GeV/c PHENIX Preliminary p T -Sliced Mass Spectra ○ Au+Au ● p+p Normalized by the yield in m ee < 100MeV The low mass enhancement decreases with higher p T No significant indication that this low mass enhancement contribute to m 1 GeV/c We assume that excess is entirely due to internal conversion of direct 

Low mass & High p T region p+pAu+Au (MB) 1 < p T < 2 GeV/c 2 < p T < 3 GeV/c 3 < p T < 4 GeV/c 4 < p T < 5 GeV/c p+p –Good agreement between real and cocktail –Small excess at higher p T Au+Au – Good agreement in M ee < 50MeV/c 2 – Enhancement is clearly seen above 100MeV/c 2.

Determination of  * fraction, r r : direct  * /inclusive  * Direct  * /inclusive  * is determined by fitting the following function for each p T bin. M ee (GeV/c 2 ) Reminder : f direct is given by Kroll-Wada formula with S = 1. Fit in MeV/c 2 gives – Assuming direct  * mass shape  2 /NDF=11.6/10 – Assuming  shape instead of direct  * shape  2 /NDF=21.1/10 Twice as much as measured  yield Assumption of direct  * is favorable.

direct  * /inclusive  * μ = 0.5p T μ = 1.0p T μ = 2.0p T Base line Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang. p+p Consistent with NLO pQCD – better agreement with small µ Au+Au Clear enhancement above NLO pQCD p+pAu+Au

Direct Photon Spectra The virtual direct photon fraction is converted to the direct photon yield. p+p – First measurement in 1-4GeV/c – Consistent with NLO pQCD and with EmCal method – Serves as a crucial reference Au+Au – Above binary scaled NLO pQCD – Excess comes from thermal photons? NLO pQCD (W. Vogelsang) Fit to pp exp + TAA scaled pp scaled ppexponential

1 st measurement of Thermal Radiation Au+Au = pQCD + exp.  T = 221  23 (stat)  18 (sys) Initial temperatures and times from theoretical model fits to data: –0.15 fm/c, 590 MeV (d ’ Enterria et al.) –0.17 fm/c, 580 MeV (Rasanen et al.) –0.2 fm/c, MeV (Srivastava et al.) –0.33 fm/c, 370 MeV (Turbide et al.) –0.6 fm/c, 370 MeV (Liu et al.) –0.5 fm/c, 300 MeV (Alam et al.) From data: T ini > 220 MeV > T C From models: T ini = 300 to 600 MeV   = 0.15 to 0.5 fm/c D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)

Dilepton Spectra p+pAu+Au p+p –Agreement with cocktail Au+Au –p T >1GeV/c: small excess  internal conversion of direct photons –p T <1GeV/c: large excess  q-q,  - , …? SLOPE ANALYSIS Single exponential fit: –Low-p T : 0<m T <1 GeV –High-p T : 1<m T <2 GeV 2-components fits –2exponentials –m T -scaling of  0 + exponential Low p T : –inverse slope of ~ 120MeV –accounts for most of the yield

Previous measurements The enhancement is concentrated at low p T CERES measured an excess of dielectron pairs, confirmed by NA60, rising faster than linear with centrality attributed to in- medium modification of the  spectral function from  annihilation. CERES NA60

Summary We have measured e+e- pairs in p+p and Au+Au collisions at √s NN =200 GeV –Large excess above hadronic background is observed For m<300MeV/c 2 and 1<p T <5 GeV/c –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 deduced. Direct photon yield in p+p is consistent with a NLO pQCD Direct photon yield in Au+Au is much larger. –Spectrum shape above T AA scaled pp is exponential, with inverse slope T=221 ±23(stat)±18(sys) MeV Hydrodynamical models with T init = MeV at  0 = fm/c are in qualitative agreement with the data. Additional excess in Au+Au at p T <1GeV/c –Inverse slope T~120 MeV  Additional source of virtual g around T crit, responsible of most of the inclusive dilepton yield, so far not explained by theories…

Backup

Centrality Dependency submitted to Phys. Rev. Lett arXiv:  0 region: Agreement with cocktail LOW MASS Low Mass: yield increases faster than proportional to N part  enhancement from binary annihilation (ππ or qq) ? Intermediate Mass: yield increase proportional to N coll  charm follows binary scaling

Understanding the p T dependency Comparison with cocktail Single exponential fit: –Low-p T : 0<m T <1 GeV –High-p T : 1<m T <2 GeV 2-components fits –2exponentials –m T -scaling of  0 + exponential Low p T : –inverse slope of ~ 120MeV –accounts for most of the yield

Theory Comparison II Freeze-out Cocktail + “random” charm +  spectral function Low mass M>0.4GeV/c 2 : some calculations OK M<0.4GeV/c 2 : not reproduced Intermediate mass Random charm + thermal partonic may work Low-p T slope not reproduced Gluon Compton q  g q e+e+ e-e- q q HADRONIC PARTONIC  -  annihilation q-q annihilation R.Rapp + H.vanHees K.Dusling + I.Zahed E.Bratkovskaja + W.Cassing

Extract 2 components 2 EXPONENTIALSHAGEDORN + EXPONENTIAL We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) We fit Hagedorn to M ee <100MeV (  0 -dominated) Then we fit (a*m T -scaling + exponential) to the other mass bins Because of their different curvature, m T -scaling and the exponential account for more or less of the yield in the low-p T region.

Yields and Slopes Intermediate p T : inverse slope increase with mass, consistent with radial flow Low p T : inverse slope of ~ 120MeV accounts for most of the yield SLOPESYIELDS Total yield (DATA) 2expo fit m T -scaling +expo fit Low-p T yield

Theory Comparison II Calculations from R.Rapp & H.vanHees K.Dusling & I.Zahed E.Bratovskaja & W.Cassing (in 4  )

Questions 1. Enhancement at M<2M  If pions are massless can  annihilation produce ee with M<300MeV? 2. Enhancement at low p T, with T~120 MeV and now flow Is the same low-p T enhancement seen at SPS never reproduced by theory? Different initial temperature Different system evolution Do we miss something in the system evolution which may have different relevance at SPS and at RHIC? RHIC SPS

PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) 2 central arms: electrons, photons, hadrons –charmonium J/ ,  ’  e  e  –vector meson   e  e  –high p T       –direct photons –open charm –hadron physics 2 muon arms: muons –“onium” J/ ,  ’,      –vector meson      –open charm combined central and muon arms: charm production DD  e  global detectors forward energy and multiplicity –event characterization Au-Au & p-p spin PC1 PC3 DC magnetic field & tracking detectors e+e+ ee   designed to measure rare probes: + high rate capability & granularity + good mass resolution and particle ID - limited acceptance

Photon conversion rejection Conversion removed with orientation angle of the pair in the magnetic field Photon conversion   e+e- at r≠0 have m≠0 (artifact of PHENIX tracking: no tracking before the field) effect low mass region have to be removed Beampipe MVD support structures r ~ m ee Inclusive Removed by phiV cut After phiV cut z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay

Photon conversion cut No cut M<30 MeV &  V <0.25 & M<600 MeV &  V <0.04 M<600 MeV &  V <0.06 M<600 MeV &  V <0.08 M<600 MeV &  V <0.10 M<600 MeV &  V <0.12 M<600 MeV &  V <0.14 M<600 MeV &  V <0.20 M<600 MeV &  V <0.40

Physical background Semi-correlated Background π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+ X  0   *  e+e- e+e- “jets” z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay Photon conversion   e+e- at r≠0 have m≠0 (artifact of PHENIX tracking) Conversion removed with orientation angle of the pair in the magnetic field Background is charge-independent Calculate the shape with MC Normalize to the like-sign spectra  Good description of the data arXiv:

Combinatorial Background PHENIX 2 arm spectrometer acceptance: dN like /dm ≠ dN unlike /dm different shape  need event mixing (like/unlike differences preserved) Use Like sign as a cross check for the shape and to determine normalization Small signal in like sign at low mass  N++ and N–- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV) Normalization: 2√N++ N--  Uncertainty due to statistics of N++ and N--: 0.12% Correction for asymmetry of pair cut K=k+-/√k++ k-- = Systematic error (conservative): 0.2% TOTAL SYSTEMATIC ERROR = 0.25% LIKE SIGN SPECTRA Use same event topology (centrality, vertex, reaction plane) Remove every unphysical correlation

Comparison of BG subtraction Methods 36 to determine syst. uncertainty: spread of two extreme cases (blue & orange): 5-10% Monte Carlo method Like sign method (with some variations) give consistent results over the full invariant mass range

Acceptance charge/pT 00 00 z vertex Define acceptance filter (from real data) Correct only for efficiency IN the acceptance “Correct” theory predictions IN the acceptance Single electron pT > 200 MeV  Pair mT > 400 MeV Not an analysis cut, but a constrain from the magnetic field mass pTpT

Cross check Converter Method We know precise radiation length (X 0 ) of each detector material The photonic electron yield can be measured by increase of additional material (photon converter was installed) The non-photonic electron yield does not increase Photonic single electron: x 2.3 Inclusive single electron :x 1.6 Combinatorial pairs :x 2.5 Photon Converter (Brass: 1.7% X 0 ) N e Electron yield Material amounts:  0 0.4%1.7% Dalitz : 0.8% X 0 equivalent radiation length 0 With converter W/O converter 0.8% Non-photonic Photonic converter