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Measurements of low pT direct photons in PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo 04/11/2008WWND South Padre.

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Presentation on theme: "Measurements of low pT direct photons in PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo 04/11/2008WWND South Padre."— Presentation transcript:

1 Measurements of low pT direct photons in PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo 04/11/2008WWND 2008 @ South Padre Island 1/16

2  Quark Gluon Plasma De-confined phase of quarks and gluons Experimental approach at RHIC  √s NN = 200GeV Au+Au collisions  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 pT) – Initial hard scattering, Pre- equilibrium  Thermal photons (Low pT) – Carry the thermodynamic information from QGP and hadron gas Introduction 04/11/2008WWND 2008 @ South Padre Island hard: thermal: Decay photons (  0 → ,  → , …) 2/16

3 Photon Measurement in PHENIX  Hard photon Strong suppression of high pT hadrons helps to improve the S/N ratio Successfully measured for both p+p and Au+Au collisions  Measured pT range – up to 14GeV/c Good agreement with pQCD  Thermal photon Thermal radiation from QGP  Primary contributor in low pT range up to 3-6GeV/c Direct evidence of thermal equilibration Thermal photon measurement is very challenging because it is very hard due to a large background from hadron decays. 04/11/2008WWND 2008 @ South Padre Island Au+Au S.Turbide et al PRC 69 014903 3/16

4 p+p direct photon Low pT Photons Long-awaited results for both p+p and Au+Au  Experimental determination is very important since applicability of pQCD is doubtable in low pT region. In ‘real’ photon measurement  Measured yield with a large systematic error Difficulty on measuring low pT “real” direct photons 1.Finite energy resolution of the EMCal 2.Large hadron background Advantages on measuring ‘virtual’ photons 1.High momentum resolution of the Drift Chamber 2.Reliable estimation of the hadron decay components using Kroll-Wada formula Alternative method to measure low pT direct photons → Measure e + e - pairs from ‘virtual’ direct photons 04/11/2008WWND 2008 @ South Padre Island q  g q e+e+ e-e- 4/16

5 Virtual Photon Measurement  Case of Hadrons Obviously S = 0 at M ee > M hadron  Case of  * – If pT 2 >>M ee 2  Possible to separate hadron decay components from real signal in the proper mass window.  Any source of real  can emit  * with very low mass.  Convert direct  * fraction to real direct photon yield Kroll-Wada formula S : Process dependent factor 04/11/2008WWND 2008 @ South Padre Island 5/16

6 Signal Extraction arXiv: 0706.3034arXiv: 0802.0050 Au+Aup+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 jets 04/11/2008WWND 2008 @ South Padre Island 6/16

7 Hadronic Cocktail Calculation 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 …  Hadronic cocktail was well tuned to individually measured yields of mesons in PHENIX for both p+p and Au+Au collisions. 04/11/2008WWND 2008 @ South Padre Island  Remaining pairs after background subtraction Real signal + Hadron decay components – Estimate hadron components using hadronic cocktail 7/16

8 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 low mass region Integrated yield in150MeV < m ee < 750MeV – Real/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model) 8/14

9 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 pT Sliced Mass Spectra  Shape differences between p+p and Au+Au are larger at lower pT. ○ Au+Au ● p+p 04/11/2008WWND 2008 @ South Padre Island Normalized by the yield in m ee < 100MeV 9/16

10 WWND 2008 @ South Padre Island04/11/2008 pT Dependence Au+Au  pT<1GeV/c Large enhancement → thermal qqbar and  annihilations? p+pAu+Au  pT<1GeV/c Large enhancement → thermal qqbar and  annihilations?  pT>1GeV/c && m ee <300MeV Enhancement is also seen. Small expected contributions from qqbar and  annihilations Satisfy pT 2 >>m ee 2 → Virtual photon analysis can be performed! 10/16

11 Cocktail Comparison p+pAu+Au (MB) PHENIX Preliminary 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 04/11/2008WWND 2008 @ South Padre Island 11/16

12 Determination of  * fraction, r r : direct  * /inclusive  * Direct  * /inclusive  * is determined by fitting the following function for each pT bin.  Fit in 80-300MeV gives – Assuming direct  * 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. M ee (GeV/c 2 ) Reminder : f direct is given by Kroll-Wada formula with S = 1. 04/11/2008WWND 2008 @ South Padre Island 12/16

13 direct  * /inclusive  * μ = 0.5p T μ = 1.0p T μ = 2.0p T 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 04/11/2008WWND 2008 @ South Padre Island Base line 13/16

14 Direct Photon Spectra p+p  First measurement in 1-4GeV/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 comes from thermal photons? 04/11/2008WWND 2008 @ South Padre Island 14/16

15 Theory Comparison 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 04/11/2008WWND 2008 @ South Padre Island S.Turbide, R.Rap, C.Gale, Phys.Rev.C 69 (2004) T 0 max =370 MeV  0 =0.33 fm/c 15/16

16 Summary & Outlook  Direct photon measurements with virtual photon method in p+p and Au+Au collisions have been done at RHIC-PHENIX.  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.  This is the first time that direct photon production in p+p collisions has been measured in 1<pT<4GeV/c. Direct photon yield in p+p collisions is consistent with NLO pQCD. 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.  p+p analysis with more statistics is now ongoing. Result with higher quality will be provided. pT region will be extended upward.  Same analysis will be done in d+Au collisions. 04/11/2008WWND 2008 @ South Padre Island 16/16

17 backup 04/11/2008WWND 2008 @ South Padre Island 17/16

18 PHENIX Detector Minimum Bias data sample (triggered by BBC) & Electron triggered data sample (p+p) BBC – z-vertex DC, PC1 – Tracking RICH & EMCal – Electron ID – Electron Trigger 04/11/2008WWND 2008 @ South Padre Island 18/16

19 Material Conversion Pair Cut 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 04/11/2008WWND 2008 @ South Padre Island 19/16

20 Additional Correlated Background 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% 04/11/2008WWND 2008 @ South Padre Island 20/16

21 Combinatorial Background (Au+Au) 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% 04/11/2008WWND 2008 @ South Padre Island 21/16

22 Centrality Dependence WWND 2008 @ South Padre Island04/11/2008  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 22/16

23 Theory Comparison 2 Freeze-out Cocktail + “random” charm +  spectral function Low mass M>0.4GeV/c 2 : some calculations OK M<0.4GeV/c 2 : not reproduced 04/11/2008WWND 2008 @ South Padre Island 23/16

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