Electrons and Photons: a clock and a thermometer of Relativistic Heavy Ion Collisions Alberica Toia Stony Brook University l Dilepton Measurements at RHIC oSingle electron identification oCombinatorial Background oCocktail of hadrons oCharm contribution 200 GeV op+p oAu+Au l In medium modifications oChiral symmetry oSpectral function oThermal radiation l Heavy quarks energy loss

Dileptons in 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?

Dileptons in Heavy Ion Collisions Expected sources Light hadron decays –Dalitz decays    –Direct decays  and  Hard processes –Charm (beauty) production –Much larger at RHIC than at SPS Possible modifications suppression (enhancement) Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation charm modification exotic bound states 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?

HI low-mass dileptons at a glance time scale of experiments period of data taking ALICE (KEK E235) CERES DLS NA60 HADES CBM PHENIX energy scale of experiments (KEK E235)CERESDLS NA60HADESCBMPHENIX 10158[A GeV] 17[GeV]√s NN 200 // ALICE [A TeV] First RHIC results: –Au+Au:arXiv –p+p:arXiv 0802:0050 –Others coming soon…

Dielectron pairs –the history I low energy HADES (high acceptance, resolution, rate capability): first measurements Data: R.J. Porter et al.: PRL 79 (1997) 1229 BUU model: E.L. Bratkovskaya et al.: NP A634 (1998) 168 transport + in-medium spectral functions Giessen BUU DLS measured an excess of dielectron pairs over the expected yield Never fully explained excess over standard known sources compared with theory calculations Agreement with DLS Excess possibly explained with bremsstrahlung

Dielectron pairs –the history II high energy CERES measured an excess of dielectron pairs over the expected yield Attributed to in-medium modification of  spectral function from  annihilation NA60 First measurement of the  spectral function Clear excess above the cocktail , centered at the nominal  pole and rising with centrality DD _ DY NA50 Pb-Pb 158 GeV central collisions charm DY Thermal radiation: window 0.5 to 2.5 GeV direct photons (HELIOS, WA80/98) di-lepton pairs (NA38/50, NA60) Excess of dilepton yield measured above charm pairs

Predictions for RHIC LOW MASS: Vector Mesons INTERMEDIATE MASS: Charm & Thermal K. Gallmeister, B. Kämpfer, O. P. Pavlenko hep-ph/ (January 1998)  Strong enhancement of low-mass pairs persists at RHIC §Open charm contribution becomes significant R. Rapp nucl-th/ e- e+

The raw subtracted spectrum Same analysis on data sample with additional conversion material  Combinatorial background increased by 2.5 Good agreement within statistical error  signal /signal =  BG /BG * BG/signal large!!! 0.25% From the agreement converter/non- converter and the decreased S/B ratio scale error < 0.1% (well within the conservative 0.25% error we assigned) p+p Au+Au arXiv: arXiv:

Cocktail Tuning (p+p) Start from the π 0, assumption : π 0 = (π + + π - )/2 parameterize PHENIX pion data : Other mesons well measured in electronic and hadronic channel Other mesons are fit with: m T scaling of π 0 parameterization p T → √(p T 2 +m meson 2 -m π 2 ) fit the normalization constant  All mesons m T scale!!! PHENIX Preliminary arXiv:

p+p Cocktail Comparison Data absolutely normalized Excellent agreement data- cocktail Extract charm and bottom cross section Charm: integration after cocktail subtraction –  cc =544 ± 39 (stat) ± 142 (sys) ± 200 (model)  b Simultaneous fit of charm and bottom: –  cc =518 ± 47 (stat) ± 135 (sys) ± 190 (model)  b –  bb = 3.9 ± 2.4 (stat) +3/-2 (sys)  b submitted to Phys. Lett.B arXiv:

Charm and bottom cross sections CHARM BOTTOM Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end) Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end) First measurements of bottom cross section at RHIC energies!!!

Au+Au Cocktail Comparison Data absolutely normalized Cocktail filtered in PHENIX acceptance Charm from –PYTHIA –Single electron non photonic spectrum w/o angular correlations  cc = N coll x 567±57±193  b submitted to Phys. Rev. Lett arXiv: Low-Mass Continuum: enhancement 150 <m ee <750 MeV: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Intermediate-Mass Continuum: Single-e  p T suppression & non-zero v 2 : charm thermalized? PYTHIA single-e p T spectra softer than p+p but coincide with Au+Au Angular correlations unknown Room for thermal contribution?

pp – AuAu comparison pp and AuAu normalized to  0 region p+p: follows the cocktail Au+Au: large Enhancement in Agreement in intermediate mass and J/  just for ‘coincidence’ (J/  happens to scale as  0 due to scaling with Ncoll + suppression) p+p NORMALIZED TO m ee <100 MeV submitted to Phys. Lett.B arXiv: submitted to Phys. Rev. Lett arXiv:

Characterize the enhancement Mass [GeV/c 2 ] p T [GeV/c] centrality

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

p T dependency p+p: follows the cocktail Au+Au: enhancement concentrated at low p T 0<p T <0.7 GeV/c 0.7<p T <1.5 GeV/c 1.5<p T <8 GeV/c All p T p+p Au+Au arXiv: arXiv:

p T dependency II p+p: follows the cocktail for all the mass bins Au+Au: significantly deviate at low p T p+p Au+Au

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

NA60 p T analysis 2. Slice in mass 1. Subtract the cocktail 3. Project in p T 4. Extract T eff hadrons (  ) –T eff depends on mass –T eff smaller for , decouples early –T eff large for , decouples late low mass excess –clear flow effect visible –follows trend set by hadrons –possible late emission intermediate mass excess –no mass dependence –indication for early emission 5. Interpret 6. But…? How about the steepening at low p T ?

p T dependency 0<p T <0.7 GeV/c 0.7<p T <1.5 GeV/c 1.5<p T <8 GeV/c 0<p T <8.0 GeV/c p+p Au+Au arXiv: arXiv: Au+Au Large enhancement at low p T Enhancement is also observed in p T above 1GeV/c in the low-mass below 300MeV

p+pAu+Au (MB) Gluon Compton q  g q e+e+ e-e- Dileptons at low mass and high p T m<2  only Dalitz contributions p+p: no enhancement at low p T, only small towards higher p T Au+Au: large enhancement at low p T Any source of real  produces virtual  with very low mass Assuming internal conversion of direct photon  extract the fraction of direct photon PHENIX Preliminary r : direct  * /inclusive  * Direct  * /Inclusive  * determined by fitting each p T bin 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV

Direct photon spectra NLO pQCD μ = 0.5p T μ = 1.0p T μ = 2.0p T p+p Consistent with NLO pQCD – better agreement with small µ Au+Au Clear enhancement above NLO pQCD p+p Consistent with NLO pQCD Au+Au Above binary scaled NLO pQCD Excess from thermal photons? The virtual direct photon fraction is converted to the direct photon yield.

Theory Comparison I D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) T 0 ave =360 MeV (T 0 max =590 MeV)  0 =0.15 fm/c S.Turbide, R.Rap, C.Gale, Phys.Rev.C 69 (2004) T 0 ave =370 MeV  0 =0.33 fm/c

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

Summary First measurements of dielectron continuum at RHIC p+p Low mass Excellent agreement with cocktail Intermediate mass Extract charm and bottom  c = 544 ± 39 (stat) ± 142 (sys) ± 200 (model)  b  b = 3.9 ± 2.4 (stat) +3/-2 (sys)  b Au+Au Low mass Enhancement above the cocktail: in 150<M ee <750 MeV 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Centrality dependency: increase faster than N part p T dependency: enhancement concentrated at low p T T~120 MeV No flow Intermediate mass Agreement with PYTHIA: coincidence? First measurements of direct photons for p T ~1GeV at RHIC p+p Excellent agreement with NLO pQCD Au+Au above NLO pQCD: thermal radiation?

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

Outlook PHENIX: –New results coming soon d+Au: cold nuclear matter Cu+Cu: low N part Di-muon continuum (forward rapidity) –Upgrades: HBD: reduction of combinatorial background  LMR SVTX: tagging of charm  IMR –Low-Energy Scans: search for the critical point STAR: upgrades (HFT and TOF)

Backup

A Hadron Blind Detector (HBD) for PHENIX signal electron Cherenkov blobs partner positron needed for rejection e+e+ e-e-  pair opening angle ~ 1 m Dalitz & Conversion rejection via opening angle –Identify electrons in field free region –Veto signal electrons with partner HBD concept: –windowless CF4 Cherenkov detector –50 cm radiator length –CsI reflective photocathode –Triple GEM with pad readout –Reverse bias (to get rid of ionization electrons in the radiator gas) Status –installed and taking data in RUN7 –x3 more statistics

Background studies HBD simulation –Change acceptance filter in simulation –Assume HBD rejects hadrons 1:10 –Assume single electron rejection 1:10 Expect S/B 1:10 at m~ 500 MeV Not included –Realistic hadron p T distribution –Mass resolution – Additional conversion background total reject hadrons reject electron charm

Projections N. events 1x x10 9

RHIC at low energy Npart = (MinBias Au+Au) Nch/Npart and at mid-rapidity at sqrt(s_NN)=17.2 from SPS pT distributions and particle ratios according to CERES paramterization in EXODUS pt*(a1*exp(-1.0*mt/T1)+a2*exp(- 1.0*mt/T2)+a3*exp(-1.0*mt/T3)) other hadrons: –T = *mass.; –pt*exp(-1.0*mt/T); PHENIX acceptance filter for +- magnetic field configuration The expected ZDC rates at beam energy of ~8.5GeV is 100Hz. * 75% to convert into a BBC rate * 50% for the lifetime  event rate of 37.5Hz Expected whole vertex minbias event rate [Hz] Which scaling do we use?

Projections 50M events (=2 weeks runtime = 1.2*10 6 s) 500M events (+ electron cooling)

Relativistic Heavy-Ion Collider BNL STAR PHOBOS PHENIX Specifications: 3.83 km circumference 2 independent rings: 120 bunches/ring 106 ns crossing time A + A √ s NN = 200 GeV Luminosity: 2·10 26 cm -2 s -1 (~1.4 kHz) p+p √ s max = 500 GeV p+A √ s max = 200 GeV 4 experiments: BRAHMS, PHENIX, PHOBOS, STAR Runs (2000 – 2008): 200, 130, 62.4 GeV 200, 62.4 GeV 200 GeV 200, 62.4, 22.4 GeV BRAHMS

The matter is so dense that it melts(?) J/  (and regenerates it ?) Introduction: 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?

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

The unfiltered calculations black: our standard cocktail red : hadronic spectrum using the VACUUM rho spectral function green: hadronic spectrum using the IN-MEDIUM rho spectral function blue : hadronic spectrum using a rho spectral function with DROPPING MASS magenta : QGP spectrum using the HTL-improved pQCD rate

CuCu dN/dm – Min Bias π η η’ ω ρ φ cc J/ψ ψ’

Unphysical correlations II Yield as a function of cut: at some point (when the cut removes all the ghosts) the yield should saturate The yield has a minimum then increases then saturates The saturation value is different for converter and non converter runs (different electron multiplicity) The minimum value and the saturation value are the same for converter and non converter runs Cut value Corrected yield in MeV cut optimized for pions Cut optimized for electrons Converter Non converter UNLIKE mass pTpT This is not reproducible in mixed events  cut differently in real and mixed events  need a larger cut Cut applied as event cut Real events: discarded and never reused Mixed events: regenerated to avoid topology dependence

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 44 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

Dielectrons in p+p advantages and challenges Advantages: improved signal/background Challenges: 1. triggered data  complications for the mixed-event technique EMC RICH Used triggered dataset to increase statistics i.e. event triggered by track that hit RICH and EMC (pT threshold: 0.4GeV) Trigger bias on combinatorial background Remove random benefit (only accept pairs in which at least one electron has fired the trigger) Generate mixed events from MinBias dataset, with same requirement on the pair

Continuum in p+p: more challenges 2. Need to correct for trigger efficiency –Hadron cocktail –Depends on triggered dead area –Determine trigger efficiency from MinBias (triggered electron/all electrons) –Simulate trigger efficiency EMC sector by sector –Project into mass vs. p T electrons from MB events + ERT triggered electrons from MB events + ERT triggered electrons from ERT triggered events

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

Acceptance Comparison of PHENIX and NA60 acceptance (p T vs. y CM ) Experiments measure in different regions of phase space NA60

Efficiency Correction Single electron pT efficiency m ee (GeV/c 2 ) Correction for RICH ghost cut Correction for eID cut mass pTpT Mee 0.8 MeV Pair eff = single eff 2 Mee 0.4—0.8 pair eff decreases as consequence of single eff drop Mee < 0.4 MeV: pair eff increases as consequence of mT cutoff (from single pT cutoff which truncates the single distribution leading to larger pT

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

Torsten Dahms - Stony Brook University51 Centrality Dependence π 0 production scales with Npart Low Mass: If in-medium enhancement from ππ or qq annihilation  yield should increase faster than proportional to Npart Intermediate Mass: charm follows binary scaling  yield should increase proportional to Ncoll LOW MASS INTERMEDIATE MASS submitted to Phys. Rev. Lett arXiv: Enhancement rise does not depend on 1D vs 2D correction Enhancement present even before correction

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.

Centrality dependence of background

Thermal photons? Direct measurement of photons (EMCAL) in this energy region impaired by: –Neutral hadron contamination –Energy resolution in π 0 reconstruction  Try to use dielectrons! No significant excess at low p T hard: thermal: Decay photons (  0 → ,  → , …)

Compton q  g q q  g q e+e+ e-e- phase space factorform factor invariant mass of virtual photon invariant mass of Dalitz pair phase space factorform factor invariant mass of Dalitz pair invariant mass of virtual photon The idea Start from Dalitz decay Calculate inv. mass distribution of Dalitz pairs‘ Now direct photons Any source of real  produces virtual  with very low mass Rate and mass distribution given by same formula –No phase space factor for m ee << p T photon

q  g q q  g q e+e+ e-e- Low mass dielectron pairs Invariant mass distribution of Dalitz pairs fully calculable with QED Different contributions are well measured Any source of real  (on-shell) produces virtual  with very low mass (~ mass shell) for m ee << p T photon : Rate and mass distribution same shape (no parent  no phase space factor  no cutoff) ÷ ÷ ÷ MeV R data Experimental technique Measure yield in different mass windows (different rate of different contributions) Calculate ratios of various M inv bins to lowest one: R data If no direct photons: ratios correspond to Dalitz decays If excess: direct photons

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  )