Ellipticity of photon emission from strongly magnetized hot QCD plasma

By making use of an explicit representation for the imaginary part of the photon polarization tensor in terms of transitions between the Landau levels of light quarks, we study the angular dependence of direct photon emission from a strongly magnetized quark-gluon plasma. Because of the magnetic field, the leading order photon rate comes from the three processes of the zeroth order in the coupling constant $\alpha_s$: (i) the quark splitting ($q\rightarrow q+\gamma$), (ii) the antiquark splitting ($\bar{q} \rightarrow \bar{q}+\gamma$), and (iii) the quark-antiquark annihilation ($q + \bar{q}\rightarrow \gamma$). In a wide range of moderately high temperatures, $T\gtrsim m_\pi$, and strong magnetic fields, $|eB|\gtrsim m_\pi^2$, the direct photon production is dominated by the two splitting processes. We show that the Landau-level quantization of quark states plays an important role in the energy and angular dependence of the photon emission. Among other things, it leads to a nontrivial momentum dependence of the photon ellipticity coefficient $v_2$, which takes negative values at small transverse momenta and positive values at large transverse momenta. The crossover between the two regimes occurs around $k_T\simeq \sqrt{|eB|}$. In application to heavy-ion collisions, this suggests that a large value of $v_2$ for the direct photons could be explained in part by the magnetic field in the quark-gluon plasma.