E Kistenev BNL PHENIX Direct photons in PHENIX

E. Kistenev, BNL, PHENIX Direct photons in PHENIX at RHIC p g e- e+ 2 central arms: electrons, photons, hadrons Talk outline: D C QM 2011, Annecy, France 10/24/2021 - Direct g yield - FF measurements - Flow 1

Direct (prompt) photons n n 30% of energy released when two particles collide are photons; Most are tertiary, they are products of electromagnetic decays of secondary hadrons and leptons; Some are direct – produced in partonic hard scattering, emitted by fragmenting partons or by media during freese out; Those due to hard scattering are also called prompt, their production in NN interactions is well studied and commonly used as a proof of validity for p. QCD treatment 2

Direct photons – real and virtual Nature to the resque Typically direct photons are small “excess” above hadron decay photons in the total inclusive yield Statistical approach: measure inclusive photons and subtract hadronic (decay) component Huge g/p 0 ratio at high p. T reflects high-p. T hadron (p 0) suppression in Au. Au central collisions Excess at low p. T is less then 15% so precision measurements of direct photon yield in thermalreagion are notoriously difficult Real photon yield can be measured from virtual photon yield, which is observed as low mass e+e- pairs Direct g h 3

Virtual photons (internally converted) Relation between the g* yield and real photon yield is known (Kroll-Wada formular in case of hadrons (p 0, h), equality in case of direct photons) where One parameter fit: (1 -r)fc + r fd here fc: cocktail calc. , fd: direct photon calc. 4

Direct photons in Au. Au 200 Ge. V Curves: collision scaled pp direct g yield Photons in calorimeters 2001 -2010 Hard prompt photons are isolated collision scaling works in Au. Au 5

p. QCD photons RAA 2004 Isospin effect (Arleo: wave function differences between p and n)? Initial state effect? Initial state effect: “energy deficit” induced by multiple interactions in CNM. B. Kopeliovich, J. Nemchik,

p. QCD photons and partonic FF 6. 89 ± 0. 64 Rate 9. 49 ± 1. 37 Hadron Gas Thermal QGP Thermal “Pre-Equilibrium” Thermal? Jet Re-interaction ? LO 7

Emerging thermal g’s Virtual photon measurement helped to extend p. T range down to ~1 Ge. V/c and establish thermal dominance in the direct g yield below p. T~5 Ge. V/c first reliable sighting of thermal 8

Thermal enhancement p p g q Exp fit to Au+Au data / scaled pp data: r g q g Tave = 221 19 stat 19 syst Me. V experimental lower bound on T Min. Bias 9

Thermal photons and thermalization time Thermal photon dominate below p. T~ 5 Ge. V/c; PHENIX: Tthermal = 221 19 stat 19 syst Me. V Theory is uncertain about equilibration time (t 0) and temperature (T 0) when hydro reigns, recent estimates vary. Ge. V from 0. 15 fm/c – 600 Me. V to 0. 5 fm/c – Au+Au@200 Ge. V 300 Me. V; ar. Xiv: 1105. 4126 minimum bias p 0 v 2 If photons are radiated inside an expanding matter having v 2, their momenta add or subtract for radiation along or opposite to. Direct the photons motion. Neglecting pion mass, thermal photons must have the same or greater v 2 as inclusive photons pions , if it comes from this mechanism. (thermal are ~10%) BKopeliovich (pr. comm. ) late thermalization or preequilibrium emission ? 10

Direct photon flow: from intuition to theory Hydro after t 0 ar. Xiv: 1105. 4126 Curves: Holopainen, Räsänen, Eskola ar. Xiv: 1104. 5371 v 1 2011 thermal diluted by prompt Chatterjee, Srivastava PRC 79, 021901 (2009) Pattern is right, scale can now be tuned to match experiment 11

Summary: • Direct g p. T spectra are consistent with LO (prompt) & thermal photons being major contributors with little or no visible NLO contribution; • PHENIX prompt photons yield measurements confirm collision scaling; • Prompt g’s do not flow; • Prompt g’s provide a reasonable estimate for the energy of recoil jet; • Medium softens partonic FF; • Thermal g’s show the flow similar to that of hadrons. 12

BACKUP 13

Partonic FF shape modification p p p x. E = -p. Th/p. Tg cos(Df)~z. T x = -ln x. E - MLLA variable Universal scaling in pp Enhancement at very low z Suppression at large z. T - p. QCD photons provide a reasonable estimate for the energy of recoil jet (q or g); - Measured medium FF is softer compared to vacuum fragmentation; - Further studies with jets tagged by strangess (gluon / quark jet separation) follow 14