标题： 强磁化高温QCD等离子体中光子发射的椭圆偏振特性 显示英文标题

标题： Ellipticity of photon emission from strongly magnetized hot QCD plasma

摘要： 利用光夸克Landau能级跃迁表象来表示光子极化张量的虚部，我们研究了强磁化夸克-胶子等离子体中直接光子发射的角度依赖性。由于磁场的作用，耦合常数零阶的三个过程（记为$\alpha_s$）主导了主要阶次的光子发射率：(i) 夸克裂变 ($q\rightarrow q+\gamma $)，(ii) 反夸克裂变 ($\bar{q} \rightarrow \bar{q}+\gamma $)，以及(iii) 夸克-反夸克湮灭 ($q + \bar{q}\rightarrow \gamma$)。在适中的高温范围内，即 $T\gtrsim m_\pi$，以及强磁场条件下，即 $|eB|\gtrsim m_\pi^2$，直接光子产生主要由这两个裂变过程主导。我们证明了夸克态的朗道能级量子化在光子发射的能量和角度依赖性方面起着重要作用。除此之外，它还导致光子椭圆度系数 $v_2$的非平凡动量依赖性，该系数在小横向动量时取负值，在大横向动量时取正值。 这两个区域之间的交叉发生在 $k_T\simeq \sqrt{|eB|}$附近。 应用于重离子碰撞时，这意味着可以直接解释光子的较大值$v_2$可能部分是由夸克-胶子等离子体中的磁场引起的。 显示英文摘要

摘要： 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.