27 September 2015 to 3 October 2015
Kobe, Fashion Mart, Japan
Japan timezone

Photon production from gluon mediated quark-anti-quark annihilation prior to hadronization

29 Sept 2015, 16:30
2h
Exhibition space 3 & 4

Exhibition space 3 & 4

Board: 0704
Poster Electromagnetic Probes Poster Session

Speaker

Sarah Campbell (Columbia University)

Description

Heavy ion collisions produce direct photons at low transverse momentum, $p_{T}$ from 1-3 GeV/c, in excess of the $p$+$p$ spectra scaled by the nuclear overlap factor, $T_{AA}$. These low $p_{T}$ photons have a large azimuthal anisotropy, $v_{2}$, suggesting late-stage production. Theoretical models, including hydrodynamic models, struggle to quantitatively reproduce the large low $p_{T}$ direct photon excess and $v_{2}$. This talk investigates increased photon production as the system becomes color neutral from soft-gluon mediated $q$-$\bar{q}$ interactions in a process analogous to coalescence. This production mechanism will generate photons that follow constituent quark number, $n_{q}$, scaling of $v_{2}$ with an $n_{q}$ value of two for direct photons. A $\chi^{2}$ comparisons of the published direct photon and identified particle $v_{2}$ measurements and a coalescence-like Monte Carlo is used to test this description. The $\chi^{2}$ comparison finds that $n_{q}$-scaling applied to the direct photon $v_{2}$ data prefers the value $n_{q}=1.8$ and agrees with $n_{q}=2$ within errors in most cases. The coalescence-like Monte Carlo simulation calculates the direct photon $v_{2}$ while describing the shape of the direct photon $p_{T}$ spectra in an internally consistent manner. The simulation, while systematically low when compared to the data, is in agreement with the 0-20% and 20-40% Au$+$Au PHENIX measurement at $p_{T}$ less than 3 GeV/c. This model predicts that higher order flow harmonics, $v_{n}$, in direct photons will follow the modified $n_{q}$-scaling laws seen in identified hadron $v_{n}$ with an $n_{q}$ value of approximately two. Comparisons to preliminary direct photon results, including higher-order anisotropies and the status of simulations at LHC energies, will also be shown.
On behalf of collaboration: NONE

Primary author

Sarah Campbell (Columbia University)

Presentation materials

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