1. Takao Sakaguchi BNL 2/9/2012 1T. Sakaguchi, RBRC lunch meeting

2.  Production Process ◦ Compton and annihilation (LO, direct) ◦ Fragmentation (NLO) ◦ Escape the system unscathed  Carry dynamical information of the state  Temperature, Degrees of freedom ◦ Immune from hadronization (fragmentation) process at leading order ◦ Initial state nuclear effect  Cronin effect (k T broardening) 2/9/2012 T. Sakaguchi, RBRC lunch meeting 2 Photon Production: Yield   s   e+e+ e-e-

3. 2/9/2012 T. Sakaguchi, RBRC lunch meeting3 Before RHIC  In 1986, search for direct photon started in heavy ion collisions at CERN ◦ Upper limits published in 1996 from WA80(S+Au at 200GeV/u ◦ Followed by WA93  Third generation experiment, WA98, showed the first significant result ◦ Pb+Pb  s NN =17.3GeV, PRL85, 3595(2000). p+Pb data shows initial nuclear effect Baumann, QM2008

4.  Au+Au = p+p x T AB holds – pQCD factorization works  NLO pQCD works.  Non-pert. QCD may work in Au+Au system 2/9/2012 T. Sakaguchi, RBRC lunch meeting 4 Blue line: N coll scaled p+p cross-section

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6.  Real Photon measurement ◦ EMCal(PbSc, PbGl): Energy measurement and identification of photons ◦ Tracking(DC, PC): Veto to charged particles  Dilepton measurement ◦ RICH: Identify electrons ◦ EMCal(PbSc, PbGl): Identify electrons ◦ Tracking(DC, PC): Momentum measurement of electrons 2/9/2012 T. Sakaguchi, RBRC lunch meeting 6 00  Invariant Mass(p T =4GeV, peripheral)  0 efficiency

7.  Statistically subtract photon contributions from  0 /  /  ’/  ◦ Measure or estimate yield of these hadrons ◦ Measure: Reconstruct hadrons via 2  invariant mass in EMCal ◦ Mass = (2E 1 E 2 (1-cos  )) 1/2  Or, tag photons that are likely from these hadrons event-by- event ◦ Possible if density of produced particles is low (p+p or d+Au)  Subtract remaining background contributions: ◦ Photons that are not from collision vertex ◦ Hadrons that are misidentified as photons  Correct for detection efficiency of photons  Signal is very small. ◦ ~ 5% S/B in 1-3GeV/c ◦ Extremely difficult 2/9/2012 T. Sakaguchi, RBRC lunch meeting 7 Direct  hadron decay  Inclusive photon

8.  Then, we make this. Taking  0 ratio cancels out systematic errors on energy scale measurement 2/9/2012 8T. Sakaguchi, RBRC lunch meeting

9. 9 (fm/c) log t 1 10 10 7 hadron decays sQGP hard scatt jet Brems. jet-thermal parton-medium interaction hadron gas EE Rate Hadron Gas sQGP Jet-Thermal Jet Brems. Hard Scatt See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005)

10.  Thermal radiation from QGP (1<pT<3GeV) ◦ S/B is ~5-10% ◦ Spectrum is exponential. One can extract temperature, dof, etc..  Hadron-gas interaction (pT<1GeV/c):  (  )   (  ),  K*  K  2/9/2012 T. Sakaguchi, RBRC lunch meeting 10 f B : Bose dist.  em : photon self energy photons dileptons Interesting, but S/B is small 5432154321 S/B ratio

11. 2/9/2012 T. Sakaguchi, RBRC lunch meeting11 New production mechanism introduced Jet in-medium bremsstrahlung Jet-photon conversion Both are “thermal  hard” Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung) Compton scattering of hard scattered and thermal partons (Jet-photon conversion) Turbide et al., PRC72, 014906 (2005) R. Fries et al., PRC72, 041902 (2005) Turbide et al., PRC77, 024909 (2008) Liu et al., arXiv:0712.3619, etc..

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13. 2/9/2012T. Sakaguchi, RBRC lunch meeting 13 Adding virtuality in photon measurement (fm/c) log t 1 10 10 7 hadron decays hadron gas sQGP hard scatt Mass (GeV/c 2 ) 0.5 1  *  e+e- virtuality jet Brems. jet-thermal parton-medium interaction By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c 2 )

14. 2/9/2012 T. Sakaguchi, RBRC lunch meeting 14  Focus on the mass region where  0 contribution dies out  For M<<p T and M<300MeV/c 2 ◦ qq ->  * contribution is small ◦ Mainly from internal conversion of photons  Can be converted to real photon yield using Kroll-Wada formula ◦ Known as the formula for Dalitz decay spectra Low p T photons with very small mass Compton q  g q e+e+ e-e- Internal conv. One parameter fit: (1-r)f c + r f d f c : cocktail calc., f d : direct photon calc. PRL104,132301(2010), arXiv:0804.4168

15.  Reconstruct Mass and pT of e+e- ◦ Same as real photons ◦ Identify conversion photons in beam pipe using and reject them  Subtract combinatorial background  Apply efficiency correction  Subtract additional correlated background: ◦ Back-to-back jet contribution ◦ well understood from MC  Compare with known hadronic sources 2/9/2012 T. Sakaguchi, RBRC lunch meeting 15 π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+

16.   fraction = Yield direct / Yield inclusive  Largest excess above pQCD is seen at Au+Au. ◦ Moderately in Cu+Cu also. 2/9/2012 T. Sakaguchi, RBRC lunch meeting No excess in d+Au (no medium) Excess also in Cu+Cu 16

17. d+Au Min. Bias  Inclusive photon ×  dir /  inc  Fitted the spectra with p+p fit + exponential function ◦ T ave = 221  19 stat  19 syst MeV (Minimum Bias)  Nuclear effect measured in d+Au does not explain the photons in Au+Au 2/9/2012 T. Sakaguchi, RBRC lunch meeting 17 PRL104,132301(2010), arXiv:0804.4168 Au+Au Won Nishina memorial prize!

18.  Initial temperature T i ◦ 300 ~ 600 MeV (different assumptions) ◦ Depends on thermalization time   2/9/2012 T. Sakaguchi, RBRC lunch meeting T c ~170MeV from lattice QCD PHENIX, Phys. Rev. C 81, 034911 (2010) Theory calculations: d’Enterria, Peressounko, EPJ46, 451 Huovinen, Ruuskanen, Rasanen, PLB535, 109 Srivastava, Sinha, PRC 64, 034902 Turbide, Rapp, Gale, PRC69, 014903 Liu et al., PRC79, 014905 Alam et al., PRC63, 021901(R) 18

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20.  Depending the process of photon production, angular distributions of direct photons may vary  Jet-photon conversion, in-medium bremsstrahlung (v 2 <0) ◦ Turbide, et al., PRL96, 032303(2006), etc.. 2/9/2012 T. Sakaguchi, RBRC lunch meeting 20 Turbide et al., PRC77, 024909 (2008) Thermal photons Bremsstrahlung (energy loss) jet jet photon conversion v 2 > 0 v 2 < 0 For prompt photons: v 2 ~0

21. 2/9/2012 T. Sakaguchi, RBRC lunch meeting Calculation of direct photon v 2 = inclusive photon v 2 - background photon v 2 (    etc) inclusive photon v 2 Au+Au@200 GeV minimum bias preliminary 21  R comes from virtual photon measurement

22.   0 v 2 ◦ similar to inclusive photon v 2  Two interpretations ◦ There are no direct photons ◦ Direct photon v 2 is similar to inclusive photon v 2 2/9/2012 T. Sakaguchi, RBRC lunch meeting 0 v20 v2 Au+Au@200 GeV minimum bias inclusive photon v 2 preliminary 22

23.  Very large flow in low pT  v 2 goes to 0 at high p T ◦ Hard scattered photons dominate 2/9/2012 T. Sakaguchi, RBRC lunch meeting Au+Au@200 GeV minimum bias preliminary 23 PHENIX, arXiv:1105.4126

24.  Later thermalization gives larger v 2 (QGP photons)  Large photon flow is not explained by models for QGP 2/9/2012 T. Sakaguchi, RBRC lunch meeting 24 Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1 thermal diluted by prompt Chatterjee, Srivastava PRC79, 021901 (2009) Hydro after  0

25. thermal + prim.  van Hees, Gale, Rapp, PRC84, 054906 (2011) 2/9/2012 25 T. Sakaguchi, RBRC lunch meeting  Large flow can not be produced in partonic phase, but could be in hadron gas phase  This model changed ingredients of photon spectra drastically! ◦ We realized the importance of the data…

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27.  We might have found that the QGP is formed ◦ High enough temperature to induce phase transition ◦ Need even precise measurement with larger statistics  How does the system thermalize? ◦ In ~0.3fm/c ? How? ◦ A hypothesis says at 0.3fm/c, the system is not thermalized  What happens in the pre-equilibrium state? ◦ Longitudinal expansion. Landau? Bjorken? ◦ What it the initial state condition? Glasma?  Penetrating probe might shed light on the pre- equilibrium states and thermalization mechanism 2/9/2012 T. Sakaguchi, RBRC lunch meeting 27

28.  Since the thermalization time is very fast, let’s base on Landau picture (extreme case)  Less thermal photons flying to higher rapidity (   ) may be produced than those to mid- rapidity (   ) ◦ with refer to the QGP formation time. ◦ dz ~ 2R/100, dx ~ 2R  One could see more photons produced in pre-equilibrium states ◦ Rapidity dependence photon measurement may play a role as a system clock 2/9/2012 T. Sakaguchi, RBRC lunch meeting 28   dz ~ 2R/100 dx ~ 2R

29. 2/9/2012 T. Sakaguchi, RBRC lunch meeting 29 central collision of equal nuclei at differ mostly by initial conditions proper time space-time rapidity

30.  Forward direct photons shed light on time evolution scenario ◦ Real photons,  *->ee,  *->  2/9/2012 T. Sakaguchi, RBRC lunch meeting 30 T. Renk, PRC71, 064905(2005)

31.  Strong gluon field (Glasma) preceded by CGC + fluctuation  Strong color-electric and magnetic field in a flux tube ◦ extended in z-direction  May play an important role on rapid thermalization  Is there any way to detect Glasma state? ◦ Photons from early stages, i.e., high rapidity? 2/9/2012T. Sakaguchi, RBRC lunch meeting 31

32. Singular point in phase diagram that separates 1 st order phase transition (at small T) from smooth cross-over (at small  b ) 2/9/2012 T. Sakaguchi, RBRC lunch meeting 32 Finding the QCD Critical Point Quark-number scaling of V 2 saturation of flow vs collision energy  /s minimum from flow at critical point Critical point may be observed via: fluctuations in & multiplicity K/π, π/p, pbar/p chemical equilibrium R AA vs  s, …. VTX provides large azimuthal acceptance & identification of beam on beam-pipe backgrounds

33.  Higher the rapidity goes, higher the baryon density we may be able to reach  BRAHMS plot. Another way to access to the critical point? 2/9/2012T. Sakaguchi, RBRC lunch meeting 33 BRAHMS, PRL90, 102301 (2003)

34.  By changing rapidity, we can cover the missing region of √s NN, with high statistics. NPA772(2006)167 2/9/2012 34T. Sakaguchi, RBRC lunch meeting My eye fit My rough stat calc.

35.  Charged hadron results and some pion/proton ratio results  Might be an idea to extend our measurement to  0 /direct photons/dileptons 2/9/2012T. Sakaguchi, RBRC lunch meeting 35 BRAHMS, PLB 684(2010)22. BRAHMS, PRL91, 072305(2003).

36.  Genuine process that involves “quark” ◦ Quark energy loss can be measured ◦ Need a lot of help from model calculations 2/9/2012 T. Sakaguchi, RBRC lunch meeting 36 Hot matter created in HIC S. Turbide, C. Gale, D. Srivastava, R. Fries, PRC74, 014903 (2006)

37.  Take Axel’s strawman’s design (in TPD workshop) ◦ Cover’s rapidity range of y = 3-4 2/9/2012T. Sakaguchi, RBRC lunch meeting 37 ~7m Charge VETO pad chamber ~7m EMCal & (Hcal)

38.  Muon Piston Calorimeter extension (MPC-EX) (3.1<|  |<3.8) ◦ Shower max detector in front of existing MPC. Now sits at ~1m from IP ◦ Measure direct photons/  0 in forward rapidity region in p+p, p+A  Study of how high in centrality in A+A we can go is on-going ◦ In the future, placing in a very far position (from Interaction Point) would be an option 2/9/2012 T. Sakaguchi, RBRC lunch meeting 38

39.  Interesting physics are explored by direct photon measurements in HI collisions ◦ Hard photons, Thermal photons, elliptic flow of photons  Rapidity may be a new degree of freedom on photon measurement  I would like to see many predictions on direct photons and dileptons at high rapidity! ◦ I’d be happy to be involved in the theory effort, also. 2/9/2012 T. Sakaguchi, RBRC lunch meeting 39

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41.  A calculation tells that even in low pT region(pT~2GeV/c), jet- photon conversion significantly contributes to total  What do we expect naively? ◦ Jet-Photon conversions  Ncoll  Npart  (s 1/2 ) 8  f(xT), “8” is xT-scaling power ◦ Thermal Photons  Npart  (equilibrium duration)  f( (s 1/2 ) 1/4 ) ◦ Bet: LHC sees huge Jet-photon conversion contribution over thermal?  Together with v 2 measurement, the “thermal region” would be a new probe of medium response to partons 2/9/2012 T. Sakaguchi, RBRC lunch meeting 41 ~15GeV?~6GeV? Jet-photon conversion Thermal pQCD LHC Turbide et al., PRC77, 024909(2008)

42.  Excess in d+Au? ◦ No exponential excess  High-p T direct photon results from PHENIX and STAR ◦ d+Au  Agree with T AB scaled pQCD  consistent with PHENIX and STAR ◦ p+p  Agree with pQCD and PHENIX  Low-p T direct photon ◦ No publication data at STAR 2/9/2012 T. Sakaguchi, RBRC lunch meeting STAR, Phys.Rev.C81,064904(2010) 42