Search for Thermal Photons in PHENIX

1. Search for Thermal Photonsin PHENIX - Torsten Dahms - Stony Brook University 23rd Winter Workshop On Nuclear Dynamics February 13, 2007

2. Outline • Motivation: signature of QGP • Conventional direct photon measurements • New attempts at low pT: • Tagging of EMCal photons • External conversions in detector material • Low mass internal conversions • Summary Torsten Dahms - Stony Brook University

3. The “Little Bang” in the lab g e- e+ Final state probes Freeze-out Soft probes Hadronization QGP Thermalization Penetrating probes Hard Scattering Au Au time g  Expansion  space • electro-magnetic radiation: γ, e+e-, +- • rare, emitted “any time”; reach detector unperturbed by strong final state interaction Torsten Dahms - Stony Brook University

4. Thermal photons? Decay photons(p0→g+g, h→g+g, …) hard: thermal: No significant excess at low pT Torsten Dahms - Stony Brook University

5. Direct photons in Au+Au Conventional method: • Measure inclusive photonsγincl = γdecay + γdirect • Calculate double ratio:(γincl/π0)measured / (γdecay/ π0)background = γincl/ γdecay=1+ γdirect/ γdecay • If double ratio > 1 direct photons • Run4: more statistics, but still no conclusive measurement Torsten Dahms - Stony Brook University

6. The PHENIX experiment • Charged particle tracking: • DC, PC1, PC2, PC3 • Electron ID: • Cherenkov light RICH • shower EMCal • Photon ID: • shower EMCal • Lead scintillator calorimeter (PbSc) • Lead glass calorimeter (PbGl) • charged particle veto p g e+ e- Torsten Dahms - Stony Brook University

7. New Idea eg = g efficiency ag = g acceptance • Measure inclusive photon spectrum, but a very clean sample • Tag all photons, which combined with a photon from a second (less clean) sample can be identified as pion decay product DATA eg’= loose g efficiency f = conditional probability of having a loose photon (g’) in the acceptance, once you already have a clean g the acceptance SIMULATION DOUBLE RATIO everything cancels out except for eg’~90%  minimal systematics Torsten Dahms - Stony Brook University

8. Clean Photon Sample no pair cut with pair cut Dalitz Conversions • Method I: • Only use EMCal clusters which fulfill very strict PID cuts • Method II: • Identify conversion photons in beam pipe • Additional advantage: • very good momentum resolution of charged tracks at low pT • No detector artifacts r~mass f PHENIX Beam Pipe Torsten Dahms - Stony Brook University

9. p0 signal extraction Real events Mixed event Beam pipe conversions • Clean EMCal sample has better S/B ratio Clean EMCal γ sample • BG subtraction within pT bins • Normalized outside the π0 peak Torsten Dahms - Stony Brook University

10. Results in Au+Au • Agreement of all three results within their errors • There seems to be an excess above the decay photons at low pT Torsten Dahms - Stony Brook University

11. Another Idea e+ Compton e- g* q g g g q p0 p0 e+ g* g e- Compton g q g q • Start from Dalitz decay • Calculate inv. mass distribution of Dalitz pairs‘ invariant mass of Dalitz pair invariant mass of Dalitz pair invariant mass of virtual photon invariant mass of virtual photon form factor form factor phase space factor phase space factor • Now direct photons • Any source of real g produces virtual gwith very low mass • Rate and mass distribution given by same formula • No phase space factor formee<< pT photon Torsten Dahms - Stony Brook University

12. 0-30 90-140 140-200 200-300 MeV Rdata ÷ ÷ ÷ In practice • Material conversion pairs removed by analysis cut • Combinatorial background removed by mixed events • Calculate ratios of various mee bins to lowest one: Rdata • If no direct photons: ratios correspond to Dalitz decays • If excess:direct photons • Fit of virtual photon shape to data in principle also possible(done for d+Au) Torsten Dahms - Stony Brook University From conventional measurement

13. Comparison • Agreement of all three methods within their errors • Also internal conversions favor an excess above decay photons Torsten Dahms - Stony Brook University

14. The spectrum • Compare to NLO pQCD • L.E. Gordon and W. Vogelsang • Phys. Rev. D48, 3136 (1993) • Above (questionable) pQCD • Compare to thermal model • D. d’Enterria, D. Peressounko • nucl-th/0503054 • Data above thermal at high pT • Data consistent with thermal+pQCD • Needs confirmation from p+p measurement 2+1 hydro T0ave=360 MeV (T0max=570 MeV) t0=0.15 fm/c Torsten Dahms - Stony Brook University

15. Internal Conversions: d+Au • Internal conversion method provides smaller systematic errors • But not as small as in the case of Au+Au collisions in Run-4(Large background of external conversion on MVD detector in Run-3) Torsten Dahms - Stony Brook University

16. Internal Conversion: d+Au • Internal conversion extends range of significant points to pt > 2 GeV • Data agrees with pQCD predictions in full pt range  No indication for nuclear effects Torsten Dahms - Stony Brook University

17. Comparison: d+Au & Au+Au • There is room for thermal photons • Systematic errors are still too big to decide whether excess seen in Au+Au can be assign to thermal source Direct photon spectrum measured in d+Au collisions and scaled with <Ncoll> agrees pretty well with spectrum measured in Au+Au at high pT Torsten Dahms - Stony Brook University

18. Summary • Photons are penetrating hard and soft probes for relativistic heavy ion collisions • Conventional Calorimeter measurement • Systematic uncertainties at low pT too large to make definite statement about thermal photon contribution • New methods show excess above decay photons • Consistent with each other • Internal conversions • Promising new technique to measure direct photons • Thermal photon scenario consistent for pT<3GeV/c • Because of the large systematic errors comparison of binary scaled d+Au spectrum with Au+Au does not allow to make a statement on the origin of the excess above pQCD observed in Au+Au • Same analysis of p+p is needed as confirmation Torsten Dahms - Stony Brook University

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20. Backup

21. The PHENIX experiment West Arm East Arm e+ e- e- e- Beam Pipe γ γ Collision Vertex γ e+ e+ • electrons: • momentum reconstruction (1% resolution) • particle ID: RICH (loose cuts because clean signature of conversion peak) • same or opposite arms: different pT acceptance • photons: EMCal (loose cuts high efficiency ~ 98%) track reconstruction assumes vertex in the interaction point  conversion at radius r≠0: e+e- pairs ‘acquire’ an opening angle  they acquire an invariant mass m =  B dl ~ r > 0 if r=4 cm (beam pipe) m =20 MeV Torsten Dahms - Stony Brook University

22. Invariant e+e- mass spectrum ofRun 4 Au+Au: Dalitz decays beampipe conversions air conversions & combinatorial background • Conversion pairs are created off-vertex • Track reconstruction produces apparent opening angle • Leads to apparent mass ~20MeV/c2 Torsten Dahms - Stony Brook University

23. Pair properties MC Simulation MC Simulation all pairs dalitz decay beam pipe conversions all pairs dalitz decay beam pipe conversions z z e- Conversion pair Dalitz decay B B y y e- x x e+ e+ • Dalitz decays have a real opening angle due to the π0 mass • Conversion pairs have small intrinsic opening angle • magnetic field produces opening of the pair in azimuth direction • orientation perpendicular to the magnetic field Torsten Dahms - Stony Brook University

24. Simulations: Nγhadr(pT) and Nγπ0 tag(pT) • Inclusive photon spectrum • π0, η → γe+e- • π0 parameterization from measured data • η from mT scaling, yield normalized at high pT (0.45 from measurement) • Use Dalitz decay (π0→ γ γ ~ π0 → γ γ* → γe+e- for pT > 0.8 GeV/c) • All e+e- (from π0, η) in the acceptance  pT spectrum of e+e- • If γ from π0 is also in acceptancepTspectrum of e+e- from π0 all e+e- pairs e+e- pairs from π0 1/Nevt dN/dpT [c/GeV] Torsten Dahms - Stony Brook University

25. Cocktail ingredients (pp): p0 • most important: get the p0 right (>80 %), assumption: p0 = (p+ + p-)/2 • parameterize PHENIX pion data: • most relevant: the h meson (Dalitz & conversion) • also considered: r, w, h’, f • use mT scaling for the spectral shape, i.e. • normalization from meson/p0 at high pT as measured (e.g. h/p0 = 0.45±0.10) Torsten Dahms - Stony Brook University

26. Gale QM05 Torsten Dahms - Stony Brook University