Magnetic transitions in ultraperipheral collisions

Abstract

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 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 never investigated. In previous work, we proposed a test of the quality of the classical approximation. We calculated an observable quantity using the classical magnetic field and also using photons as input. We focused 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 → ∆ → N'+π. In ultraperipheral relativistic heavy-ion collisions this conversion can be induced by the classical magnetic field of one of 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 calculated the cross-sections in these two independent ways and found that they differ from each other by ~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.