Takao Sakaguchi BNL 2/9/2012 1T. Sakaguchi, RBRC lunch meeting
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-
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
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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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 00 Invariant Mass(p T =4GeV, peripheral) 0 efficiency
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
Then, we make this. Taking 0 ratio cancels out systematic errors on energy scale measurement 2/9/2012 8T. Sakaguchi, RBRC lunch meeting
9 (fm/c) log t hadron decays sQGP hard scatt jet Brems. jet-thermal parton-medium interaction hadron gas EE Rate Hadron Gas sQGP Jet-Thermal Jet Brems. Hard Scatt See e.g., Turbide, Gale, Jeon and Moore, PRC 72, (2005)
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 S/B ratio
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, (2005) R. Fries et al., PRC72, (2005) Turbide et al., PRC77, (2008) Liu et al., arXiv: , etc..
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2/9/2012T. Sakaguchi, RBRC lunch meeting 13 Adding virtuality in photon measurement (fm/c) log t hadron decays hadron gas sQGP hard scatt Mass (GeV/c 2 ) * 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 )
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:
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+
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
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: Au+Au Won Nishina memorial prize!
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, (2010) Theory calculations: d’Enterria, Peressounko, EPJ46, 451 Huovinen, Ruuskanen, Rasanen, PLB535, 109 Srivastava, Sinha, PRC 64, Turbide, Rapp, Gale, PRC69, Liu et al., PRC79, Alam et al., PRC63, (R) 18
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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, (2006), etc.. 2/9/2012 T. Sakaguchi, RBRC lunch meeting 20 Turbide et al., PRC77, (2008) Thermal photons Bremsstrahlung (energy loss) jet jet photon conversion v 2 > 0 v 2 < 0 For prompt photons: v 2 ~0
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 GeV minimum bias preliminary 21 R comes from virtual photon measurement
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 v20 v2 GeV minimum bias inclusive photon v 2 preliminary 22
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 GeV minimum bias preliminary 23 PHENIX, arXiv:
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: v1 thermal diluted by prompt Chatterjee, Srivastava PRC79, (2009) Hydro after 0
thermal + prim. van Hees, Gale, Rapp, PRC84, (2011) 2/9/ 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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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
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
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
Forward direct photons shed light on time evolution scenario ◦ Real photons, *->ee, *-> 2/9/2012 T. Sakaguchi, RBRC lunch meeting 30 T. Renk, PRC71, (2005)
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
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
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, (2003)
By changing rapidity, we can cover the missing region of √s NN, with high statistics. NPA772(2006)167 2/9/ T. Sakaguchi, RBRC lunch meeting My eye fit My rough stat calc.
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, (2003).
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, (2006)
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)
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
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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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, (2008)
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