BY A PEDESTRIAN Related publications direct photon in Au+Au  PRL94, (2005) direct photon in p+p  PRL98, (2007) e+e- in p+p and Au+Au  arXiv: (PRC) Direct (virtual) photon in Au+Au  arXiv: (PRL) Latest results on direct photon from Phenix 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting

New findings on Hot Quark Soup PHENIX STAR 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting

Perfect liquid hot enough to be quark soup Hydro fits  T ini ~ 300 – 600 MeV (k B T ~ eV K -1 ) T ~ 4 – K 170 MeV Perfect liquid 300 MeV600 MeV free Gas ? 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting (Arxiv: )

How to measure the temperature ? Looking at photons  Hot matter emits thermal radiation  Temperature can be measured from the emission spectrum Hard parton scattering Hard parton scattering  High pT photons (> 6 GeV) QGP photons QGP photons  Low pT photons (1 – 3 GeV) Hadron gas photons Hadron gas photons  Very low pT photons (<2 GeV) Measuring the photons  In p+p : get the baseline  In Au+Au : get the temperature time hard parton scattering Au Hadron Gas freeze-out quark-gluon plasma Space Time expansion   pK     05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting

Sources of photons in A+A In A+A collisions  High pT photons (pT> 6 GeV): non thermal  Initial parton-parton scattering: as in p+p  not affected by Hot and Dense Matter  test the theoretical description of A+A collisions with pQCD  Low pT photons (pT < 3 GeV) : thermal  Come from the thermalized medium  Carry information about the initial temperature of the Quark Gluon Plasma  Thermal photons are created in the QGP as well as in the hadron gas over the entire lifetime of these phases  test hydro models  Low and intermediate pT photons (up to 6 GeV)  Interaction of the quarks and gluons from the hard scattering processes with the QGP q hard + g QGP  q +   get a large fraction of the momentum of q hard 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting

Phenix detector e+e+ ee   2 central arms: electrons, photons, hadrons  charmonium J/ ,  ’  e + e -  vector meson , ,   e + e -  high p T  o  +,  -  direct photons  open charm  hadron physics 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting |y|<0.35

Direct photons in p+p : the baseline PRL98, (2007) In p+p collisions  Direct photons  Compton scattering q + g  q +   qq annihilation q + q  g +   Bremsstrahlung (intial state)  Fragmentation photons  Brem. From final state partons  Final state hadron decay (background)   0, , K 0,…   +  Measured p+p yield compatible with NLO pQCD calculations p+p 200 GeV 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting

Thermal photons thermal direct photons are measured as « excess » above hadron decay photons From Monte Carlo : take a parametrization of measured  0 as input and propagate the particles through detectors background photons = remaining photons (from  0 ) after all cuts background photons  0 from MC Direct photons candidates : obtained after rejecting photons pairs falling within 110<M  <170 MeV/c² (  0 ) and 500<M  <620 Mev/c² (  ) Direct photons candidates (from data)  0 from data 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting Difficult to measure below p T <3 GeV/c (the yield of thermal photons is only 1/10 of that of hadron decay photons)

Alternative: « quasi real » virtual photons 9 05/02/2010 F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting e+e+ e-e- Source of real photon should also be able to emit virtual photon LMR I (Low Mass Region I) (M ee  0) p T ee >> M ee  « quasi real » photons (When M << E  ) Advantages :  0 decay photons are removed  reduce the bkg by ~80% S ignal /B ackground improved by a factor 5 (See arXiv : (appendix B) for more details) J/    DD ’’

r = fraction of direct photons = direct/inclusive Measuring lepton (electron) pairs Remove (dalitz)  0  e + e -   M ee > 100 MeV/c² Remove other sources of bkg  M ee < 300 MeV/c² Look at 100<M ee <300 MeV Several p T bins  0 cut-off f tot =(1-r) f c (M ee ) + r f dir (M ee ) f dir (M ee ) = expected shape from direct photon spectrum normalized to the data for mee<30 MeV/c² f c (M ee ) = cocktail mass distribution normalized to mee<30 MeV/c² 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting Cocktail  (Note : identical shape for f dir and f c for m ee <30 MeV/c²)

Fraction of direct photons Several pT bins: 1.0 < p T < 1.5 GeV/c 1.5 < p T < 2.0 GeV/c 2.0 < p T < 2.5 GeV/c 2.5 < p T < 3.0 GeV/c 3.0 < p T < 4.0 GeV/c 4.0 < p T < 5.0 GeV/c p+p consistent with NLO pQCD Au+Au : large excess above NLO pQCD 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting NLO pQCD calculations ×T AA T AA = glauber nuclear overlap function

Direct photon spectrum : the final plot Direct (real) photons in p+p Phys. Rev. Lett. 98, (2007) Direct (real) photons in Au+Au Phys. Rev. Lett. 94, (2005) NLO pQCD calculations Modified Power Law Fit on p+p MPL Fit on p+p × T AA Au+Au Min bias central mid Exponential + MPLF on p+p × T AA p+p 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting Normalized cocktail e+e- yield For m ee < 30 MeV/c² Photon yield from normalized cocktail Measured e+e- yield for m ee < 30 MeV/c² Fit = exponential + (MPLF×T AA ) = + (MPLF×T AA ) T = 221 ±23±18 MeV Starting from (identical shape for f dir and f c for m ee <30 MeV/c²) (arXiv: )

Comparison with hydro models 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting From hydro models 300 MeV< T ini < 600 MeV 0.17 fm <  0 < 0.6 T C from Lattice QCD ~ 170 MeV T AuAu (fit) ~ 220 MeV 170 MeV Perfect liquid 300 MeV600 MeV

Conclusion 05/02/ F. Fleuret - LLR Rencontre ions lourds/Heavy Ion Meeting