Spontaneous Excitation Of A Circularly Accelerated Atom Coupled With Spin-3/2 Fields Fluctuations

In studying the interaction between an atom and the radiation fields, the physical mechanism of Spontaneous emission has been discussed and it may be attributed to vacuum fluctuations, radiation reaction or a combination of them.The ambiguity in physical interpretation was solved, when Dalibard, Dupont-Roc and Cohen-Tannoudji(DDC) chose a symmetric ordering between the operators of the atom and the fields and which renders vacuum fluctuations and radiation reaction possess an independent physical meaning. This formalism was widely used in the studies of the evolution of the atom coupled with the fields subsequently.We study the spontaneous excitation of a uniformly accelerated atom and an atom in circular motion coupled with scalar, electromagnetic and Dirac field fluctuations in vacuum and found the excitation would spontaneously occur for the atom in its ground state as if in a thermal bath, which is consistent with Unruh effect. However, the coupling between the atom and the vacuum scalar and electromagnetic field are both linear, thus the average rate of change of atomic energy can be distinctively separated into the contributions of vacuum fluctuations and radiation reaction. When we generalized the DDC formalism to the case of an atom coupled with vacuum Dirac field fluctuations where the coupling between the atom and field is nonlinear, the evolution of the atom is governed in the leading order by vacuum fluctuations and the cross term of vacuum fluctuations and radiation reaction. The cross term plays the same role as that of radiation reaction in the scalar and electromagnetic field cases, which always leads to a loss of atomic energy irrespective of whether the atom is initially in the ground orexcited state, but the difference is that the contribution of the cross term varies as the trajectory of the atom changes.In this paper, We study the spontaneous excitation of a two-level atom in circular motion coupled nonlinearly to vacuum massless Rarita-Schwinger fields in the ultrarelativistic limit and demonstrate that the spontaneous excitation occurs for ground-state atoms in circular motion in vacuum. A noteworthy feature is that terms of odd powers in acceleration appear in the excitation rate whereas in the linear acceleration case there are only terms of even powers present. Furthermore,the excitation rate is not of a pure thermal form since the Planckian factor which appears in the linear case is now replaced by a non-Planckian exponential factor.On the other hand, what makes the present case unique in comparison to the atom’s coupling to other fields that are previously studied is the appearance of the terms proportional to the seventh and ninth powers of acceleration in the mean rate of change of atomic energy which are absent in the scalar, electromagnetic and Dirac field cases.