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Direct Photon measurement at RHIC-PHENIX

Direct Photon measurement at RHIC-PHENIX. Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration. Outline. What can we learn from photons? Photon contribution from various sources pQCD, thermal, etc. Detector and Analysis Result

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Direct Photon measurement at RHIC-PHENIX

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  1. Direct Photon measurement at RHIC-PHENIX Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration

  2. Outline • What can we learn from photons? • Photon contribution from various sources • pQCD, thermal, etc. • Detector and Analysis • Result • Comparison with theoretical calculations • Summary Takao Sakaguchi, BNL

  3. Direct Photon measurement • Photons come from all the stage from initial hard scattering to final hadronic state. • Transparent in the strongly interacting medium • Carry thermodynamical information of the state from that they are emitted • Temperature • Degree of freedom • QGP phase transition can be detected through this direct probe • Formation time • Time profile of phase transition A Compilation: hep-ph/0410282, Aurenche Takao Sakaguchi, BNL

  4. Direct Fragment Photon Production • Leading Order photon production • Annihilation process • Compton scattering process • Next to Leading Order photon production • Bremsstrahlung • Next-to-Next-to… • Jet Fragment photon • Another Sources? • Pre-equilibrium photon • Jet-Photon conversion in presence of hot dense medium • Scattering of hard scattered parton with thermal partons • Diagram shown in Drell-Yan process (similar to real photon) hep-ph/0201311 Takao Sakaguchi, BNL

  5. Photon production (pQCD) • Produced in initial hard scattering of partons in nucleons • pT>~3-4GeV to very high pT • Rumor.. • pQCD calculation describes yields very well. • How well? • Typically, 20-30% theoretical uncertainty due to the various choice of scale • Comparison to experiment: Factor of two difference remains. • Also dependent of beam energy • Shooting the factor in experiments • PHENIX contributes one • Shown in red points • Photon is not affected by final state interaction • Directly comparable with calculation. • Distinct initial state from final state effect Apanasevich, et al., PRD63(2003)014009 PHENIX p-p data in red points! Takao Sakaguchi, BNL

  6. Initial State Effect (kT-smearing) • Prompt photon production in -Be, p-Be (Tevatron E706) • Any data don’t agree with simple NLO pQCD calculation. • Just with primordial kT • Including additional kT describes data very well. • Cronin effect! • Suggesting Cronin effect is not due to recombination • d-Au high pT hadron spectra showed kT broadening behavior as well • d-Au photon data should show initial state effect only • Direct measurement of kT is possible • Kill or save recombination model E706 Collaboration, PRD70(2004)092009 • Center-of-mass energy, and xT region: • RHIC: sNN=200GeV, xT ~0.04-0.25 • E706: sNN=32-40GeV, xT~0.2-0.5 Takao Sakaguchi, BNL

  7. Thermal & Hadron Gas~Photon is Difficult!~ • Thermal radiation from QGP state. • LO, NLO, LPM included • Resummed 22 process • exchange partons • The state of art calculation from different sources shows that radiation from QGP state is dominant in pT=1-3GeV • pT>3GeV, initial pQCD • pT<1GeV, Hadron Gas interaction • ()(), K*K • Here is where we want to shoot! • Estimated excess  in 1.5<pT<3GeV/c: ~10% • Systematic error should be less than this. • Needs precise estimate of background photons from known hadronic sources • Needs precise knowledge of detector response • High photon efficiency, and charged and neutral particle rejection PRC69(2004)014903 Takao Sakaguchi, BNL

  8. Single photons in Heavy Ion collisions ~Before RHIC~ • WA80, WA98 are the dedicated experiments for direct photon search in relativistic heavy Ion collisions. • WA98 data can be either explained by kT-smearing or higher initial temeprature • Any data did not see pT>4GeV, where pQCD photons dominate • No information on kT does not allow us to resolve the issue. • Recent data points at ~100MeV available from WA98. • By analysis of correlation strength in interferometry, • WA98, PRL93(2004)022301 WA98 data and theoretical interpretation PRC69(2004)014903 Takao Sakaguchi, BNL

  9. And RHIC… • STAR measurement of 0 and photon in Au-Au at sNN=130GeV • 0.5<pT<2.5 • PRC70(2004)044902 • Using photon conversion method • Systematic Errors • 0 normalization error: 49% • 19% additional error on 0-11% centrality data • pT spectra fit errors: 5-11% • Basically, no excess is seen. Takao Sakaguchi, BNL

  10. PHENIX Detector • Use of both Lead scintillater and lead glass electro-magnetic calorimeter (EMCal) • Radiation Length: PbSc: 18X0, PbGl: 16X0 • Coverage: |h|<0.38, f = p • PbSc: Energy Resolution: 8.1/E(GeV)  2.1 % Position Resolution: 5.7/E(GeV)  1.6 mm • PbGl: Energy Resolution : 5.9/E(GeV)  0.8 % Position Resolution: 8.4/E(GeV)  0.2 mm • RICH and Tracking: Energy calibration of the calorimeter • Using electrons • Pad chamber and Tracking: Estimate of Charged particle contamination PbGl PbSc • Events used • 30M Minimum bias trigger events • 55M Minbias-equivalent LVL-2 trigger events Takao Sakaguchi, BNL

  11. 0’s and ’s • 0’s and ’s spectra are measured in Run2 Au-Au Run. • /0 ratio is obtained: 0.45+/-0.1 • ~95% of background sources are already determined • Other is less than ~5%.(-> by some estimate) 0 data PRL 91,072301 (2003) + high pT triggered data prel. Takao Sakaguchi, BNL

  12. From top to bottom 1, 0++’+ 2, 0+ 3, 0 pT [GeV/c] Estimate of Background photons • Fit the measured 0 transverse momentum spectra with a function. • Move each data points up and down by systematic errors • Fit the new spectra again • Iteration of Generating  from 0, and estimating /0 ratio error due to fit. • Assume Flat Rapidity • , ’ ：　Incorporate pT of the fit function by sqrt(pT2+M2-M2) • Normalization scale obtained by /0 ratio measured (/0=0.450.1) • Generate photons, and count the number of photons that entered calorimeter. • Systematic Error on estimated background /0 is 3-4% • Note that it is the error to Ratio, not to  or 0 individually. Takao Sakaguchi, BNL

  13. Real photon measurement Efficiency to various particles • “Photon ID likelihood function consists of “shower dispersion, ratio of energy in several towers” • f(Ecent,Ecore,2, etc.,) • Apply threshold to the function • Efficient Photon ID and hadron rejection • Photon ID efficiency, hadronic cluster background, and detector response as a function of centrality estimated by event embedding method • Unfold the real photon spectra from measured cluster energy distribution • Systematic Errors: • Non  correction error: ~3% •  Yield correction (off-vertex, conversion): ~10% • Energy Scale of  :12-15% • 0 Yield extraction: 7-10% • 0 Yield correction (off-vertex, conversion): ~12% • Total Systematic Error after combining PbGl and PbSc: • Spectra: ~15-20% • /0 double ratio: ~12-16% 1 Electron, photon 0.1 ,K,p 0.01 0 2 4 6 8 10 12 Deposited energy [GeV] Takao Sakaguchi, BNL

  14. (/0)meas/(/0)estimated = (meas/estimated) Systematic Error on Data: 12-16% of the ratio Gray box shows all errors including systematic uncertainty Showing the contribution over the background photons from known hadronic sources Suppression of 0 and  contributions enhanced ratios Red lines show thickness-scaled NLO pQCD calculation + known background contribution Yellow shades show uncertainty from thickness function and theoretical calculation Well described by the calculation Results (/0 double ratio), Au-Au) Error Bar: Statistical error Takao Sakaguchi, BNL

  15. Results (Direct photon spectra) • Direct photon spectra over centralities • Systematic Error: ~15-20% • Clearly seen that we measured photons over the order of 1027! • See the scale please.. • Again, Thickness-scaled NLO QCD calculation describes all the spectra very well • From Central to Peripheral • No exception within current errors • Yellow bands show uncertainty on NLO pQCD calculation and thickness function Takao Sakaguchi, BNL

  16. Results for p-p Bands represent systematic errors. (Subtraction) Errors on the backgrounds result in enlarged errors on the signal, especially at low-pT region. • NLO-pQCD calculation • CTEQ6M PDF. • Gluon Compton scattering + fragmentation photon • Set Renormalization scale and factorization scale pT/2,pT,2pT • Systematic Error: • 20(high pT)-45(low pT)% The theory calculation shows a good agreement with our result. Takao Sakaguchi, BNL

  17. Results (RAA) • Photon RAA is consistent with unity over all the centrality compared to 0 results. • Clear evidence of that the yield follows thickness-scaled hard scattering • p-p reference from NLO pQCD Calculation • 0 RAA decreases to ~0.2 at Npart=320 • Dotted line shows uncertainty of thickness function • Error bars show total error (systematics + statistical) except thickness function error • Yellow shows uncertainty on pQCD calculation Direct g p0 Takao Sakaguchi, BNL

  18. Comparison with calculations • Any of pQCD calculations describe data well • Adding kT broadening makes factor of ~2 difference • Around same factor as E706 • Calculation suggests that slopes of the spectra at RHIC and E706 are same • Jet Photon included calculation (Fries et al., PRL 90, 132301 (2003)) is also shown • Fits very well above 4GeV! • Assuming existence of hot dense medium • Prompt partons scatter with thermal partons • The line approaches to simple pQCD calculation in high pT Takao Sakaguchi, BNL

  19. Jet Photon overwhelms QGP? • Break-up of Fries prediction • Jet Photons overwhelms all the other contributions below 7GeV/c • Jet production rate calculated by LO pQCD with K factor compensation of 2.5 • pQCD photon calculation from LO with no K factor • Fitting too good! • In Peripheral, the calculation should fit the data as well • RAA and spectra themselves tell you what happens • Calculation is assuming existence of hot dense medium, which is not the case in peripheral! Takao Sakaguchi, BNL

  20. State-of-Art; Zooming into low pT • Most realistic calculation • Including all the contributions • We may be able to see QGP contribution in 1-3GeV/c in Run4! PRC69(2004)014903 Thanks to Ralf Rapp for providing theoretical calculations! Takao Sakaguchi, BNL

  21. Next Target? ? Takao Sakaguchi, BNL

  22. Summary • First high pT direct photon observed in heavy ion collisions. • Yield scaled by thickness function • Thickess-scaled NLO pQCD calculation describing initial hard scattering process fits well. • Any Model fits to data in the current errors. • Jet Photon model is quite unlikely given the fact that the spectra scales to thickness function • Needs Improvement of systematic errors to scope lower pT region • Run4 data will extract more precise information in the intermediate pT region Takao Sakaguchi, BNL

  23. Backup Slides Takao Sakaguchi, BNL

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