The Weak Magnetic Photon Emission from Quark-gluon Plasma

来自夸克-胶子等离子体的弱磁辐射

There must be electromagnetic fields created during high-energy heavy-ion collisions.Although the electromagnetic field may become weak with the evolution of the quark-gluon plasma(QGP),compared to the energy scales of the strong interaction,they are potentially important to some electromagnetic probes.In this work,we propose the coupled effect of the weak magnetic field and the longitudinal dynamics of the background medium for the first time.We demonstrate that the induced photon spectrum can be highly azimuthally anisotropic when the quarkgluon plasma is in the presence of a weak external magnetic field.On the other hand,the weak magnetic photon emission from quark-gluon plasma only leads to a small correction to the photon production rate.After hydrodynamic evolution with a tilted fireball configuration,the experimentally measured direct photon elliptic flow is well reproduced.Meanwhile,the used time-averaged magnetic field in the hydrodynamic stage is found no larger than a few percent of the pion mass square.

在相对论重离子对撞中,必然有电磁场产生。尽管伴随夸克胶子等离子体演化的电磁场可能很弱,但它们有可能对电磁探针有重要意义。本工作首次提出了弱磁场和背景介质的纵向动力学的耦合效应。阐明了当夸克-胶子等离子体存在弱外部磁场时,由磁场诱导的光子是高度各向异性的。另一方面,来自夸克-胶子等离子体的弱磁光子发射对光子产量只有很小的修正。在具有倾斜构型的火球经流体动力学演化之后,可以很好地重现实验测量的直接光子椭圆流。同时,在流体动力学阶段使用的时间平均磁场不大于介子质量平方的百分之几。