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Centrality Dependent Soft Direct Photon Yield and v n Measurements by PHENIX Richard Petti (BNL) for the PHENIX Collaboration QCD Chirality Workshop 2015 1-21-2015 1

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Photon Production in Heavy Ion Collisions http://u.osu.edu/vishnu/author/shen-201-2/ photons are produced throughout the entire lifetime of the fireball due to a low cross-section with QCD matter, they escape virtually unmodified, allowing a glimpse of the matter at all times definition: Direct photons are all photons NOT from a hadron decay 2

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Direct Photon Production in Heavy Ion Collisions The term direct photon encompasses production from many possible physics sources the initial collision (hard) parton scattering pre-equilibrium production (soft?) from a glasma from high B fields thermal radiation from QGP (soft) high energy parton interaction with QGP matter (hard) thermal radiation from hadron gas (soft) meson-meson scattering in the hadron gas (soft) others? Extremely difficult (impossible?) to separate these experimentally Our handle is theoretical calculations of all sources folded with the space-time evolution of the fireball can it reproduce the data? 3

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Important questions need to be answered for a complete understanding of the evolution of HICs What processes contribute to the direct photon yield and v 2 that we measure? is it truly all thermal or is there something else? if thermal sources dominate, when is most of the radiation emitted in time? hydro calculations indicate that v 2 of thermal photons from QGP is small and grows as system hadronizes initial expectation was that the bulk of thermal production was early in time when the fireball is the hottest these two ideas seem to be inconsistent with the data Is the v 2 dominated from hydrodynamics and flow or is there some contribution from an asymmetric production decoupled from flow? B fields glasma 4

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What do we know from experiment? RHIC results on production yields (200 GeV Au+Au) Min. Bias (0-93% Centrality) arXiv:1405.3940 0-80% Centrality C. Yang et al. Nuclear Physics A 00 (2014) 14 5

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What do we know from experiment? PHENIX results on production yields (system dependence) Phys.Rev. C87 (2013) 054907 6

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What do we know from experiment? PHENIX results on v n (200 GeV Au+Au) P.R.L. 109, 122302(2012) v2v2 v2v2 v3v3 7

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What do we know from experiment? ALICE results on production yields and v 2 (2.76 TeV Pb+Pb) Nuclear Physics A 904-905 (2013) 573c-576c arXiv:1212.3995 8

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A few details about the measurement of soft photon production at PHENIX Large background from hadron decays makes analysis difficult 3 techniques at PHENIX external photon conversions measure real photons greatly reduce hadron contamination gives the lowest p T reach photons deposit energy into emcal best at high momentum internal photon conversions measure virtual photons reduce background from 0 Dalitz decays 9

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External Conversion Method at PHENIX PHENIX does all tracking (before Run 10) outside of the magnetic field requires one to assume each track originates from the primary vertex for reconstruction Employ an alternate track model to identify di-electron pairs from external conversions focus on conversions in the HBD detector parameterize the mis-reconstruction of electrons from the HBD readout plane (at 60cm) allows us to both correct for the mis- reconstruction, as well as to exploit this to identify mis-reconstruction effect on a pair -> artificial opening angle -> apparent mass calculate two masses for every e + e - pair one for standard tracking assuming 0cm vertex one for alternate tracking assuming 60cm vertex 10

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Converted photon identification projection to 1D Dalitz pairs HBD conversion pairs Dalitz pairs HBD conversion pairs 11

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Measuring direct photons from inclusive photons experimentally we measure all photons (inclusive) we calculate the fraction of direct photons in the inclusive sample by measuring R = N incl /N decay to minimize systematic uncertainties, we measure the ratio via a double ratio this ratio is used to calculate the direct photon yield and v n, along with the inclusive and hadron decay photon observables 12

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Measuring the 0 tagged inclusive photons tag inclusive photon as coming from pions by reconstructing pions 13

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Real photon measurement compared to virtual photon measurement arXiv:1405.3940 real photon measurement via external conversions (2007 + 2010) consistent with virtual photon measurement! 14

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Centrality dependence of the yield arXiv:1405.3940 calculate direct photon yield from R and the hadron decay photon yield from the cocktail direct = (R 1) hadron estimate hard scattering component by fitting p+p yield, with, and scaling by N coll (the green line below) we subtract the hard component to isolate the excess yield fit a limited range of the excess with to extract the inverse slope, which are consistent within uncertainties subtract scaled p+p to isolate excess 15

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Shape of the excess yield with N part Excess of photon yield increases with power-law function, =1.480.08(stat.)0.04(sys.) 3/2 The centrality dependence is not an artifact of the very low p T points: same slope as we increase lower limit of integration (upper limit is always 2GeV/c). The shape of direct photon p T spectra doesnt depend on centrality. arXiv:1405.3940 16

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Centrality dependence of v n direct photon v 2 calculated from two methods of measurement are consistent within uncertainties hint that the direct photon v 2 is non-zero at zero p T 17

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Direct photon and pion v n comparison direct photon and pion v n are very similar in magnitude non-zero v 2 above p T = 5GeV results from di-jet correlations, not flow P.R.L. 109, 122302(2012) v2v2 v3v3 Min. Bias 18

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Data vs Theory - Yields Linnyk et al.: PHSD transport model; Linnyk, Cassing, Bratkovskaya, P.R.C 89, 034908(2014) vHees et al.: Fireball model; van Hees, Gale, Rapp; P.R.C 84, 054906(2011) Shen et al.: Ohio hydro for two different initial conditions; Shen, Heinz, Paquet, Gale; P.R.C 84, 064903(2014) The yield itself is still not perfectly described. arXiv:1405.3940 19

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Non-zero v 2 a signature of high B fields? recent paper from B. Mueller et. al indicates that a non-zero v 2 can be realized from magnetic field effects use the holographic duality between a strongly couple N=4 SYM and AdS 5 x S 5 include massless and massive quarks B z = 1( T) 2 B z = 0.1( T) 2 B z = 0.2( T) 2 in plane polarization out of plane polarization averaged emission BERNDT MLLER, SHANG-YU WU, AND DI-LUN YANG PHYSICAL REVIEW D 89, 026013 (2014) p T so p T 1-4GeV /(T) 1.5-6 at T = 200 MeV 20

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Moving Forward need concrete evidence of the existence of strong magnetic fields effects can have the same qualitative signatures of other known effects difficult to separate need to answer the question, is the large v 2 at low p T dominated by enhanced late time production coupled with hydrodynamic flow or from some other production asymmetry due to magnetic fields? models will need to accommodate all sources, taking into account magnetic field effects with the QGP and hadron gas and the space- time evolution of the fireball More experimental results in the works working on direct photon yield in 62GeV and 39GeV working on extending direct photon yield baseline in p+p to lower momentum 21

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Summary Experiments have now accumulated a whole suite of direct photon measurements in different systems and energies over a wide p T range yield v n PHENIX has unique measurements focusing on the very low energy production (even below 1GeV) by measuring real photons that externally convert to di- electron pairs in detector material Current models do not completely describe observations Is a non-zero v 2 a signature of large B fields? 22

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Backups 23

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Some Simulations of Conversions Full GEANT simulation of photon conversions Assume all particles come from a radius of 60cm (ATM) 24

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Hadron decay photon cocktail 25

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charged pion v 2 seems to go to zero at zero p T (contrary to direct photon v 2 ) Phys.Rev.Lett. 91 (2003) 182301 26

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Shape of the excess yield with N part Excess of photon yield increases with power-law function, =1.480.08(stat.)0.04(sys.) 3/2 The centrality dependence is not an artifact of the very low p T points: same slope as we increase lower limit of integration (upper limit is always 2GeV/c). The shape of direct photon p T spectra doesnt depend on centrality. arXiv:1405.3940 P.R.C 89, 044910(2014) Theory Data 27

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