arXiv 2011.007262020-11hep-ph

相对论性重离子碰撞中的磁场：经典近似的检验

Magnetic field in relativistic heavy ion collisions: testing the classical approximation

I. Danhoni, F. S. Navarra

备注：: 6 pages, 5 figures. arXiv admin note: text overlap with arXiv:1910.00711
发表信息：: Phys. Rev. C 103, 024902 (2021)
DOI：: 10.1103/PhysRevC.103.024902

摘要：

人们相信，在非中心相对论性重离子碰撞中，会形成一个非常强的磁场。有几个研究这个领域的影响，其中$\vec{B}$计算与经典电动力学的表达式。当每个场模中的场量子数足够高时，量子场可以近似为经典场。如果场源足够强，则可能发生这种情况。在重离子物理学中，经典处理的有效性没有被研究过。在这项工作中，我们提出了一个测试的经典近似的质量。我们使用经典的磁场和光子作为输入来计算可观测量。如果两种方法的结果一致，这将表明经典近似是有效的。更准确地说，我们关注的是一个核子转化为δ共振的过程，然后δ共振衰变为另一个核子和π介子，即，$ N \to Δ\to N' π$。在超周边相对论性重离子碰撞中，一个离子的经典磁场作用于另一个离子可以引起这种转换。或者，我们可以用一个被目标核子吸收的等效光子流来代替经典磁场。用这两种方法计算了碰撞截面，发现在所考虑的碰撞能范围内，它们相差$\simeq 10$ %.这表明这两种形式是等价的，并且磁场的经典近似是合理的。

英文摘要：

It is believed that in non-central relativistic heavy ion collisions a very strong magnetic field is formed. There are several studies of the effects of this field, where $\vec{B}$ is calculated with the expressions of classical electrodynamics. A quantum field may be approximated by a classical one when the number of field quanta in each field mode is sufficiently high. This may happen if the field sources are intense enough. In heavy ion physics the validity of the classical treatment was not investigated. In this work we propose a test of the quality of the classical approximation. We calculate an observable quantity using the classical magnetic field and also using photons as input. If the results of both approaches coincide, this will be an indication that the classical approximation is valid. More precisely, we focus on the process in which a nucleon is converted into a delta resonance, which then decays into another nucleon and a pion, i.e., $ N \to Δ\to N' π$. In ultra-peripheral relativistic heavy ion collisions this conversion can be induced by the classical magnetic field of one the ions acting on the other ion. Alternatively, we can replace the classical magnetic field by a flux of equivalent photons, which are absorbed by the target nucleons. We calculate the cross sections in these two independent ways and find that they differ from each other by $\simeq 10$ \% in the considered collision energy range. This suggests that the two formalisms are equivalent and that the classical approximation for the magnetic field is reasonable.

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