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New degree of freedom in thermal photon measurement

New degree of freedom in thermal photon measurement. Takao Sakaguchi Brookhaven National Laboratory. Serious Preface. Discovery and quantification are both important and should go along Mandatory condition as far as I think, in order to expand a field (and get budget)

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New degree of freedom in thermal photon measurement

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  1. New degree of freedom in thermal photon measurement Takao Sakaguchi Brookhaven National Laboratory

  2. Serious Preface • Discovery and quantification are both important and should go along • Mandatory condition as far as I think, in order to expand a field (and get budget) • Challenging and promising measurements are therefore important and should go along • We have promising future measurements, but not (many) challenging measurements • Let’s think about challenging measurement. • Make the case successful, we need both good theoretical predictions and experimental feasibility study T. Sakaguchi, Thermal photons and dileptons

  3. Electromagnetic probes (was challenging) Photon Production: Yield  s • 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 (kTbroardening) e+ g* e- g T. Sakaguchi, Thermal photons and dileptons

  4. Blue line: Ncoll scaled p+p cross-section First gdir in Au+Au (hard scattering) T. Sakaguchi, Thermal photons and dileptons • Au+Au = p+px TAB holds – pQCD factorization works • NLO pQCD works. Non-pert. QCD may work in Au+Au system

  5. Possible sources of photons See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005) Rate hard scatt HadronGas jet Brems. parton-medium interaction jet-thermal sQGP sQGP hadron gas hadron decays Jet-Thermal Jet Brems. log t 1 107 10 (fm/c) Eg Hard Scatt T. Sakaguchi, Thermal photons and dileptons

  6. photons dileptons fB: Bose dist. em: photon self energy Difficult objects! Photons from QGP~big challenge~ • 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 Interesting, but S/B is small S/B ratio 5 4 3 2 1 T. Sakaguchi, Physics Colloquium

  7. Adding virtuality in photon measurement hard scatt jet Brems. parton-medium interaction jet-thermal sQGP hadron gas hadron decays g*  e+e- virtuality log t 1 107 0.5 10 (fm/c) 1 By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2) Mass (GeV/c2) T. Sakaguchi, Thermal photons and dileptons

  8. Focus on the mass region where p0 contribution dies out For M<<pT and M<300MeV/c2 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 pT photons with very small mass e+ Compton e- g* q g q PRL104,132301(2010), arXiv:0804.4168 One parameter fit: (1-r)fc + rfd fc: cocktail calc., fd: direct photon calc. Internal conv. T. Sakaguchi, Thermal photons and dileptons

  9. Inclusive photon × gdir/ginc Fitted the spectra with p+p fit + exponential function Tave = 221  19stat  19syst MeV (Minimum Bias) Nuclear effect measured in d+Au does not explain the photons in Au+Au Low pTphotons in Au+Au (thermal?) Won Nishina memorial prize! PRL104,132301(2010), arXiv:0804.4168 Au+Au d+AuMin. Bias T. Sakaguchi, Thermal photons and dileptons

  10. Adding collision geometry dependence Depending the process of photon production, path length dependence of direct photon yield varies v2 of the direct photons will become a source detector Later thermalization gives larger v2 jet fragment photon annihilation compton scattering jet v2 > 0 Bremsstrahlung (energy loss) v2 < 0 For prompt photons: v2~0 T. Sakaguchi, Thermal photons and dileptons

  11. Later thermalization gives larger v2 (QGP photons) Large photon flow is not explained bymodels for QGP Results on path-length dependence Hydro after t0 Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1 thermal diluted by prompt Chatterjee, SrivastavaPRC79, 021901 (2009) T. Sakaguchi, Thermal photons and dileptons

  12. Jet-photon conversion LHC Thermal pQCD ~6GeV? ~15GeV? LHC is a good place for thermal photons/dileptons? • 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 (s1/2)8 f(xT), “8” is xT-scaling power • Thermal Photons Npart (equilibrium duration) f( (s1/2)1/4 ) • Bet: LHC sees huge Jet-photon conversion contribution over thermal? • Together with v2 measurement, the “thermal region” would be a new probe of medium response to partons Turbide et al., arXiv:0712.0732 T. Sakaguchi, Thermal photons and dileptons

  13. New degree of freedom? T. Sakaguchi, Thermal photons and dileptons

  14. One step forward with electromagnetic probes.. • 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 T. Sakaguchi, Thermal photons and dileptons

  15. Rapidity as a clock of system evolution g2 dx ~ 2R g1 dz ~ 2R/100 • Since the thermalization time is very fast, let’s base on Landau picture (extreme case) • Less thermal photons flying to higher rapidity (g1) may be produced than those to mid-rapidity (g2) • 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 T. Sakaguchi, Thermal photons and dileptons

  16. Landau and Bjorkenexpansion models central collision of equal nuclei at differ mostly by initial conditions space-time rapidity proper time T. Sakaguchi, Thermal photons and dileptons

  17. T. Renk, PRC71, 064905(2005) Rapidity dependence ~system expansion~ • Forward direct photons shed light on time evolution scenario • Real photons, g*->ee, g*->mm T. Sakaguchi, Thermal photons and dileptons

  18. Rapidity dependence ~probing initial condition~ Glasma Color Glass Condensate T. Sakaguchi, Thermal photons and dileptons

  19. CGC -> Glasma -> QGP, how? • 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? T. Sakaguchi, Thermal photons and dileptons

  20. Finding the QCD Critical Point Singular point in phase diagram that separates 1st order phase transition (at small T) from smooth cross-over (at small b) • Quark-number scaling of V2 • saturation of flow vs collision energy • /s minimum from flow at critical point • Critical point may be observed via: • fluctuations in <pT> & multiplicity • K/π, π/p, pbar/p chemical equilibrium • RAA vs s, …. • VTX provides large azimuthal acceptance & identification of beam on beam-pipe backgrounds T. Sakaguchi, Thermal photons and dileptons

  21. High Rapidity as a high baryon system • Higher the rapidity goes, higher the baryon density we may be able to reach • BRAHMS plot. Another way to access to the critical point? BRAHMS, PRL90, 102301 (2003) T. Sakaguchi, Thermal photons and dileptons

  22. Review ~BRAHMS results BRAHMS, PLB 684(2010)22. BRAHMS, PRL91, 072305(2003). • Charged hadron results and some pion/proton ratio results • Might be an idea to extend our measurement to p0/direct photons/dileptons T. Sakaguchi, Thermal photons and dileptons

  23. Drell-Yan as an energy loss probe Hot matter created in HIC S. Turbide, C. Gale, D. Srivastava, R. Fries, PRC74, 014903 (2006) • Genuine process that involves “quark” • Quark energy loss can be measured • Need a lot of help from model calculations T. Sakaguchi, Thermal photons and dileptons

  24. How about measurement? ~Detector Plan~ EMCal & (Hcal) Charge VETO pad chamber ~7m ~7m • Take Axel’sstrawman’s design • Cover’s rapidity range of y = 3-4 T. Sakaguchi, Thermal photons and dileptons

  25. How about measurement?~A technology choice: MPC-EX~ • Muon Piston Calorimeter extension (MPC-EX) (3.1<|h|<3.8) • Shower max detector in front of existing MPC. Now sits at ~3m from IP • Measure direct photons/p0 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 T. Sakaguchi, Thermal photons and dileptons

  26. Summary • Rapidity may be a new degree of freedom on photon/dilepton measurement • Higher rapidity may shed light to the pre-equilibrium state as well as time evolution of the system • 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. T. Sakaguchi, Thermal photons and dileptons

  27. Backup T. Sakaguchi, Thermal photons and dileptons

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