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Dileptons @RHIC a clock and a thermometer of relativistic heavy ion collisions Alberica Toia for the PHENIX Collaboration Stony Brook University / CERN International School of Nuclear Physics “Heavy ion collisions from the Coulomb barrier to the Quark-Gluon Plasma” Erice, September 16-24 2008
ALICE hadronization [A TeV] g g m e Jet p f Time p cc p L K time freeze-out // // // // // // expansion 10 158 [A GeV] formation and thermalization of quark-gluon matter? 17 200 √sNN [GeV] hard parton scattering Space Au Au Dileptons: radiation from the medium HADES CBM NA60 PHENIX DLS (KEK E235) CERES • direct probes • large emission rate in hot and dense matter • light vector mesons: signals of chiral transition? • Measurement at RHIC: • different energy • different initial conditions • different hydro evolution • hard processes have larger cross sections • collider geometry
Dilepton Signal Extraction arXiv: 0706.3034 Au+Au • Single electron • Track reconstruction • Electron Identification (RICH + EMCal) • 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 • Real signal • di-electron continuum arXiv: 0802.0050 p+p
Hadronic Cocktail Calculation • Remaining pairs after background subtraction • Real signal + Hadron decay components • Parameterized PHENIX p0 data with assumption of p0 = (p++p-)/2 Other mesons are fit with mT scaling of π0 parameterization pT→√(pT2+mmeson2-mπ2) fit the normalization constant • All mesons mT scale!!! • Hadronic cocktail was well tuned to individually measured yield of mesons in PHENIX for both p+p and Au+Au collisions. • Mass distributions from hadron decays are simulated by Monte Carlo. • p0, h, h’, w, f, r, J/y, y’ • Effects on real data are implemented. • PHENIX acceptance, detector effect, efficiencies … arXiv: 0802.0050
Cocktail Comparison p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034 • p+p • Excellent agreement with cocktail • Au+Au • Large enhancement in low mass region • Integrated yield in 150MeV < mee < 750MeV • data/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model)
Centrality Dependency p0 region: • Agreement with cocktail Low Mass: • yield increases faster than proportional to Npart enhancement from binary annihilation (ππ or qq) ? Intermediate Mass: • yield increase proportional to Ncoll charm follows binary scaling submitted to Phys. Rev. Lett arXiv:0706.3034 LOW MASS INTERMEDIATE MASS
Source of the excess? • Freeze-out Cocktail + “random” charm + r spectral function Low mass • M>0.4GeV/c2: some calculations OK • M<0.4GeV/c2: data still above theory calculations Intermediate mass • Random charm + thermal partonic may work
pT-Sliced Mass Spectra 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c Normalized by the yield in mee < 100MeV • Au+Au • p+p PHENIX Preliminary • The low mass enhancement decreases with higher pT • Different shape at low and high pT • low mass enhancement does not contribute to m<300 MeV/c2 and pT>1 GeV/c?
Low mass & High pT region p+p Au+Au (MB) 1 < pT < 2 GeV/c 2 < pT < 3 GeV/c 3 < pT < 4 GeV/c 4 < pT < 5 GeV/c • p+p • Good agreement between real and cocktail • Small excess at higher pT • Au+Au • Good agreement in Mee < 50MeV/c2 • Enhancement is clearly seen above 100MeV/c2.
e+ Gluon Compton g* e- q g q Rising at low mass (~1/m): due to Dalitz decays of baryonic/mesonic resonances a1 pg a1 pg* pe+e- N* Ng N* Ng* Ne+e- where the underlying process is: Quarks and gluons can be “free” (in QGP) or “hidden” in hadrons Theoretical predictions: ppre+e- threshold at 2mp~300MeV Cocktail Hadron Gas QGP (qqg*e+e-) qqbarg*e+e- “threshold-like” mq < 150 MeV? Much smaller than cocktail Mass (GeV/c2) Sources of e+e- pairs Dalitz: p0ge+e- hge+e- wp0e+e- fhe+e- Drell-Yan: qqe+e- Direct: re+e- we+e- fe+e- J/ye+e- y’e+e- Heavy flavor: cce+e- +X bbe+e- +X cocktail
Direct Photons Measurement e+ q e- g* • Any source of real g can emit g* with very low mass. • If the Q2 (=m2) 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 g q Kroll-Wada formula S : Process dependent factor • Case of Hadrons • Obviously S = 0 at Mee > Mhadron • Case of g* • If pT2>>Mee2 • Possible to separate hadron decay components from real signal in the proper mass window.
Determination ofg* fraction, r Direct g*/inclusive g* is determined by fitting the following function for each pT bin. Reminder : fdirect is given by Kroll-Wada formula with S = 1. r : direct g*/inclusive g* • Fit in 80-300MeV/c2 gives • Assuming direct g* mass shape • c2/NDF=11.6/10 • Assuming h shape instead of direct g* shape • c2/NDF=21.1/10 • Twice as much as measured h yield • Assumption of direct g* is favorable.
directg*/inclusive g* p+p Au+Au μ = 0.5pT μ = 1.0pT μ = 2.0pT Base lineCurves : 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
Direct Photon Spectra exp + TAA scaled pp 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? Fit to pp NLO pQCD (W. Vogelsang) exponential scaled pp
1st 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, 450-660 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.) D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) From data: Tini > 220 MeV > TC From models: Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c
Dilepton Spectra p+p Au+Au • p+p • Agreement with cocktail • Au+Au • pT>1GeV/c: small excess internal conversion of direct photons • pT<1GeV/c: large excess other source???
Summary p+p LOW MASS: Excellent agreement with hadronic decay cocktail INTERMEDIATE MASS: Extract charm and bottom cross sections σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb σbb= 3.9 ± 2.4 (stat) +3/-2 (syst) μb DIRECT PHOTONS p+p in agreement with pQCD Au+Au LOW MASS: Enhancement above the cocktail expectations:3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Centrality dependency: increase faster than Npart Enhancement concentrated at low pT INTERMEDIATE MASS: Coincidence agreement with PYTHIA Room for thermal radiation? DIRECT PHOTONS: Dielectron mass shape for pT > 1 GeV and mee < 300MeV consistent with internal conversions of virtual photons Au+Au above pQCD excess with inv. slope: Teff = 221 ± 23 (stat) ± 18 (syst) well described by hydrodynamical models with initial coonditions of Tinit=300–600 MeV atτ0 = 0.15–0.6 fm/c • First dielectron continuum measurement at RHIC • Despite of low signal/BG • Thanks to high statistics • Very good understanding of background normalization • HBD upgrade will reduce background great improvement of systematic and statistical uncertainty (LMR) • Silicon Vertex detector will distinguish charm from prompt contribution (IMR) 17
sQGP @ RHIC strongly interacting Quark-Gluon Plasma (sQGP) in HI collisions at RHIC The matter is so opaque that even a 20 GeV p0 is stopped The matter is so dense that even heavy quarks are stopped What does it emit? What is the temperature? The matter is so strongly coupled that even heavy quarks flow PHENIX preliminary The matter is so dense that it modifies the shape of jets The matter is so dense that it melts(?) J/y (and regenerates it ?)
Not a new idea… J.H.Cobb et al., PL 78B, 519 (1978) • The idea of measuring direct photon via low mass lepton pair is not new one. It is as old as the concept of direct photon. • This method is first tried at CERN ISR in search for direct photon in p+p at 55GeV. They look for e+e- pairs for 200<m<500 MeV, and they set one of the most stringent limit on direct photon production at low pT • Later, UA1 measured low mass muon pairs and deduced the direct photon cross section. Credit to try it in PHENIX goes to Y.Akiba
Previous measurements NA60 CERES CERES measured an excess of dielectron pairs, confirmed by NA60, rising faster than linear with centrality attributed to in-medium modification of the r spectral function from pp annihilation. NA60 CERES The enhancement is concentrated at low pT
Understanding the pT dependency • Comparison with cocktail • Single exponential fit: • Low-pT: 0<mT<1 GeV • High-pT: 1<mT<2 GeV • 2-components fits • 2exponentials • mT-scaling of p0 + exponential • Low pT: • inverse slope of ~ 120MeV • accounts for most of the yield
Extract 2 components 2 EXPONENTIALS HAGEDORN + EXPONENTIAL • We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) • We fit Hagedorn to Mee<100MeV (p0-dominated) • Then we fit (a*mT-scaling + exponential) to the other mass bins • Because of their different curvature, mT-scaling and the exponential account for more or less of the yield in the low-pT region.
YIELDS Low-pT yield 2expo fit mT-scaling +expo fit Total yield (DATA) Yields and Slopes SLOPES • Intermediate pT: inverse slope increase with mass, consistent with radial flow • Low pT: • inverse slope of ~ 120MeV • accounts for most of the yield
e+ Gluon Compton g* e- q g q q q R.Rapp + H.vanHees K.Dusling + I.Zahed E.Bratkovskaja + W.Cassing Theory Comparison II • Freeze-out Cocktail + “random” charm + r spectral function Low mass • M>0.4GeV/c2: some calculations OK • M<0.4GeV/c2: not reproduced Intermediate mass • Random charm + thermal partonic may work Low-pT slope not reproduced PARTONIC HADRONIC p-p annihilation q-q annihilation
Theory Comparison II Calculations from R.Rapp & H.vanHees K.Dusling & I.Zahed E.Bratovskaja & W.Cassing (in 4p)
Questions SPS RHIC 1. Enhancement at M<2Mp If pions are massless can pp annihilation produce ee with M<300MeV? 2. Enhancement at low pT, with T~120 MeV and now flow Is the same low-pT 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?
g p DC e+ e- PC1 magnetic field & tracking detectors PC3 PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) designed to measure rare probes:+ high rate capability & granularity + good mass resolution and particle ID - limited acceptance Au-Au & p-p spin • 2 central arms: electrons, photons, hadrons • charmonium J/, ’ -> e+e- • vector mesonr, w, -> e+e- • high pTpo, p+, p- • direct photons • open charm • hadron physics • 2 muon arms: muons • “onium” J/, ’, -> m+m- • vector meson -> m+m- • open charm • combined central and muon arms: charm production DD -> em • global detectors forward energy and multiplicity • event characterization
z Dalitz decay Conversion pair z e- B B y y x e+ e- x e+ Photon conversion rejection • ge+e- at r≠0 have m≠0(artifact of PHENIX tracking: • no tracking before the field) • effect low mass region • have to be removed Conversion removed with orientation angle of the pair in the magnetic field Photon conversion r ~ mee Inclusive Removed by phiV cut After phiV cut Beampipe MVD support structures
Photon conversion cut No cut M<30 MeV & fV<0.25 & M<600 MeV & fV<0.04 M<600 MeV & fV<0.06 M<600 MeV & fV<0.08 M<600 MeV & fV<0.10 M<600 MeV & fV<0.12 M<600 MeV & fV<0.14 M<600 MeV & fV<0.20 M<600 MeV & fV<0.40
γ e- e+ Conversion pair Dalitz decay e+ π0 z z e+ e- e- π0 B B π0 γ e- y y γ e- e+ e+ x x Physical background Semi-correlated Background Background is charge-independent Calculate the shape with MC Normalize to the like-sign spectra Good description of the data • p0g g* e+e- e+e- • “jets” X arXiv: 0802.0050 Photon conversion ge+e- at r≠0 have m≠0(artifact of PHENIX tracking) Conversion removed with orientation angle of the pair in the magnetic field
Combinatorial Background • PHENIX 2 arm spectrometer acceptance: • dNlike/dm ≠ dNunlike/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-- = 1.004Systematic error (conservative): 0.2% LIKE SIGN SPECTRA TOTAL SYSTEMATIC ERROR = 0.25% Use same event topology (centrality, vertex, reaction plane) Remove every unphysical correlation
Comparison of BG subtraction Methods Monte Carlo method Like sign method(with some variations) give consistent results over the full invariant mass range to determine syst. uncertainty: spread of two extreme cases (blue & orange): 5-10% 33
Acceptance q0 • Define acceptance filter (from real data) • Correct only for efficiency IN the acceptance • “Correct” theory predictions IN the acceptance charge/pT z vertex pT f0 • Single electron pT > 200 MeV • Pair mT > 400 MeV Not an analysis cut, but a constrain from the magnetic field mass
Ne Electron yield converter 0.8% 0.4% 1.7% With converter Photonic W/O converter Dalitz : 0.8% X0 equivalent radiation length Non-photonic 0 Material amounts: 0 Cross check Converter Method We know precise radiation length (X0) of each detector material The photonic electron yield can be measured by increase of additional material (photon converter was installed) The non-photonic electron yielddoes not increase Photonic single electron: x 2.3 Inclusive single electron :x 1.6 Combinatorial pairs :x 2.5 Photon Converter (Brass: 1.7% X0)