# Quark Matter 2014 - XXIV International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

19-24 May 2014
Europe/Zurich timezone

## Soft direct photon $v_{2}$ and $n_{q}$ scaling

20 May 2014, 16:30
2h

### spectrum

Board: G-08
Poster Electromagnetic Probes

### Speaker

Sarah Campbell (Iowa State University)

### Description

In heavy ion collisions at RHIC and the LHC, direct photons are produced in excess of their respective $T_{AA}$-scaled p+p spectra at low $p_T$ from 1-4 GeV/c. These results suggest early-stage thermal partonic emission from the hot dense medium. Additionally, these soft photons are produced with a large azimuthal anisotropy, an elliptic flow ($v_2$) as large as that of pions. Hydrodynamic models struggle to quantitatively reproduce the large soft photon $v_2$ because the pressure gradients that generate these anisotropies need time to develop, suggesting later-stage hadronic emission. The similarly-sized pion $v_2$ in this $p_T$ range is understood as resulting from the recombination of quarks near the phase transition as evidenced by quark number ($n_q$) scaling. In this poster, I present a coalescence-like model to describe the soft photon flow as the result of increased photon production from $q-\bar{q}$ interactions as the system approaches confinement. The published PHENIX soft photon and identified particle measurements are used to test this approach with both a $\chi^2$ analysis and comparison to model curves.

### Primary author

Sarah Campbell (Iowa State University)

 Poster