Abstract
Large hadronic elliptic flow v2 is considered as an evidence for the existence of a strongly-coupled QGP fluid in high-energy heavy-ion collisions. On the other hand, direct photon v2 has recently been found to be much larger than that of hydrodynamic estimations, which is recognized as “photon v2 puzzle”. In this study, I discuss the implication of late production of quarks in an initially gluon-rich medium because photons are coupled to quarks. Numerical analyses imply that thermal photon v2 can be visibly enhanced. This indicates that interplay of equilibration processes and collective expansion would be important.
Highlights
Azimuthal anisotropy of particle spectra is a unique observable in heavy-ion collisions
Given the success of hydrodynamic models so far, excess of direct photon v2 compared with the model predictions was a surprising discovery [4, 5]
Direct photons are defined as the sum of thermal photons, which are emitted softly from the medium, and prompt photons, which are created in the initial hard processes (Fig. 1)
Summary
Azimuthal anisotropy of particle spectra is a unique observable in heavy-ion collisions. Direct photons are defined as the sum of thermal photons, which are emitted softly from the medium, and prompt photons, which are created in the initial hard processes (Fig. 1). Some of the possible approaches to the problem would be categorized as follows: (i) modified thermal photon emission mechanism, (ii) modified prompt photon emission mechanism, (iii) other sources of photon emission, (iv) unknown medium-photon interaction, (v) modified bulk evolution and (vi) improvement of experimental data. I focus on (i) and (v) and discuss the effects of quark chemical equilibration in the bulk medium on thermal photon emission rate in the early stages of hydrodynamic evolution because color glass theory indicates that the medium can be initially gluon-rich [6, 7]
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