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Di-electron Continuum at PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo Rencontres de Moriond - QCD and High Energy Interactions.

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Presentation on theme: "Di-electron Continuum at PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo Rencontres de Moriond - QCD and High Energy Interactions."— Presentation transcript:

1 Di-electron Continuum at PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 1/14

2  Quark Gluon Plasma – De-confined phase of quarks and gluons – Predicted by lattice QCD Temperature – higher than 150-200MeV Energy density – higher than 1GeV/fm 3 – Relevance to the history of the early universe – Properties of QGP are of keen interest Strongly interacting system Perfect fluid  Experimental approach – Heavy ion collisions at RHIC √s NN = 200GeV Au+Au collisions Introduction Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 2/14

3 hard: thermal: Decay photons (  0 → ,  → , …) Photons & Di-leptons Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 Compton q  g q e+e+ e-e-  Photons and di-leptons are emitted from thermally equilibrated medium. – Penetrating the strongly interacting matter – Carrying the thermodynamic information directly  Topics on di-electron continuum in low mass region (Below 1GeV) 1.Measurement of electron-pair yield in low- mass low-pT region in p+p and Au+Au Enhancement possibly due to thermal q-qbar and  -  annihilations 2.Measurement of direct photon yield in p+p and Au+Au with virtual photon method Thermal photons are primary contributor in 1-5GeV/c. R. Rapp nucl-th/0204003 3/14

4 Rencontres de Moriond - QCD and High Energy Interactions - Signal Extraction 03/14/2008 arXiv: 0706.3034 arXiv: 0802.0050 Au+Au p+p  Real signal di-electron continuum  Background sources 1. Combinatorial background 2. Material conversion pairs 3. 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 After subtraction  Remaining pairs Real signal + Hadron decay components – Estimate hadron components using hadronic cocktail – Tuned to individually measured yields of mesons at PHENIX 4/14

5 Cocktail Comparison Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 arXiv: 0802.0050arXiv: 0706.3034 Au+Aup+p  p+p Excellent agreement with cocktail  Au+Au Large enhancement in 150MeV < m ee < 750MeV Integrated yield – Real/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model) 5/14

6 Centrality Dependence Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008  Integrated yield divided by N part /2 a.150MeV<m ee <750MeV Strong centrality dependence → Increases faster than N part a.m ee <100MeV (  0 region) Agreement with cocktail arXiv: 0706.3034 ● Au+Au ● p+p ▒ Cocktail 6/14

7 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 Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 Shape Difference 7/14  Shape differences between p+p and Au+Au are larger at lower pT. ○ Au+Au ● p+p

8 Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 pT Dependence p+p  Consistent with cocktail for all mass bins Au+Au  Large enhancement at low pT as presented p+p  Consistent with cocktail for all mass bins Au+Au  Large enhancement at low pT  Enhancement is also observed in pT above 1GeV/c in the low-mass below 300MeV → Second topic of this presentation 8/14 p+pAu+Au

9 Virtual Photon Measurement  Hadron mass distribution Obviously S = 0 at M ee > M hadron   * mass distribution – If pT 2 >>M ee 2  Possible to separate hadron decay components from real signal in the proper mass window. Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008  Any source of real  can emit  * with very low mass.  Convert direct  * fraction to real direct photon yield Kroll-Wada formula 9/14 S : Process dependent factor Invariant mass of  *

10 Cocktail Comparison Rencontres de Moriond - QCD and High Energy Interactions - p+pAu+Au (MB) PHENIX Preliminary 03/14/2008 p+p  Good agreement between real and cocktail  Small excess at higher pT Au+Au  Good agreement in M ee < 50MeV  Enhancement is clearly seen above 100MeV. 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV 10/14

11 Determination of  * fraction, r Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 r : direct  * /inclusive  * Direct  * /inclusive  * is determined by fitting the following function for each pT bin.  Fit in 80-300MeV gives – Assuming direct  * mass shape  2 /NDF=11.6/10 – Assuming  shape instead of direct  * shape  2 /NDF=21.1/10 → Assumption of direct  * is favorable. 11/14 M ee (GeV/c 2 ) Reminder : f direct is given by Kroll-Wada formula with S = 1.

12 direct  * /inclusive  * Rencontres de Moriond - QCD and High Energy Interactions - μ = 0.5p T μ = 1.0p T μ = 2.0p T 03/14/2008 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 12/14 p+pAu+Au

13 Direct Photon Spectra Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 p+p  First measurement in 1-3GeV/c  Consistent with NLO pQCD → Serves as a crucial reference The virtual direct photon fraction is converted to the direct photon yield. Au+Au  Above binary scaled NLO pQCD → Excess from thermal photons? 13/14

14 Summary & Outlook Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008  Di-electron analyses in p+p and Au+Au collisions have been done at RHIC- PHENIX.  Large enhancement at low mass is observed in Au+Au collisions while the result in p+p collisions is consistent with a hadronic cocktail. Strong centrality dependence Large enhancement at low pT  The fractions of direct  * to inclusive  * above pT of 1GeV/c are obtained by making a shape comparison between real pairs and a hadronic cocktail.  Direct photon yield in p+p collisions is consistent with NLO pQCD. First measurement of direct photons for 1<pT<3GeV/c in p+p collisions. The result in p+p serves as a crucial reference to Au+Au result.  Excess of direct photon yield above binary scaled NLO pQCD in Au+Au collisions is observed.  The paper on direct photon measurement with virtual photon method will be submitted soon.  pT region of direct photon yield in p+p collisions will be extended upward with more statistics.  Same analysis will be done in d+Au collisions. 14/14

15 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - backup 15/14

16 PHENIX Detector 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - Minimum Bias data sample (triggered by BBC) & ERT triggered data sample Analyzed events = 1.6B events BBC – z-vertex DC, PC1 – momentum RICH & EMCal – Electron ID – Energy – Track Matching – Electron Trigger 16/14

17 Material Conversion Pair Cut 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay The pairs from material conversion should be removed. These pairs can be recognized by its orientation relative to the magnetic field. r ~ mass PHENIX Beam Pipe  MVD support structure 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 17/14

18 Additional Correlated Background 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+ π0π0 γ e+e+ e-e- e-e- e+e+ Jet cross pair Dalitz + conversion cross pair Correlated + combinatorial background is very good agreement with the real like sign mass spectrum. Systematic error due to background subtraction ≈ 2% 18/14

19 Combinatorial Background (Au+Au) 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - Normalization factor is determined by like sign pairs.  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% 19/14

20 Hadronic Cocktail Calculation (p+p) Rencontres de Moriond - QCD and High Energy Interactions -03/14/2008 arXiv: 0802.0050  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 –  Other mesons are measured individually.  Fit with mT scaling of  0 parameterization –  Hadronic cocktail is well tuned to PHENIX data. 20/14

21 Theory Comparison 1 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - Freeze-out Cocktail + “random” charm +  spectral function Low mass M>0.4GeV/c 2 : some calculations OK M<0.4GeV/c 2 : not reproduced 21/14

22 Theory Comparison 2 03/14/2008Rencontres de Moriond - QCD and High Energy Interactions - 22/14 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

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