Spontaneous Excitation Of An Accelerated Atom Interacting With A Massive Scalar Field

Spontaneous emission is one of the most significant features of atoms. In vac-uum, static atoms in the excited states will spontaneously transit to the ground states, losing energy and emitting photons, whereas for those in the ground state, the spontaneous transition to the excited states is impossible. It is believed that the atomic spontaneous emission is induced by the vacuum fluctuations and ra-diation reaction, but there is uncertainty. Atomic spontaneous emission can be attributed to a result of vacuum fluctuations, or that of radiation reaction, or even a combination of them. Later, Dalibard, Dupont-Roc and Cohen-Tannoudji (DDC) found that this uncertainty comes from the ambiguity in operator-ordering in the interaction Hamiltonian. Taking the symmetry ordering of the atomic and field operators, one can eliminate this uncertainty, so that the vacuum fluctuations and radiation reaction have a physical meaning separately.Using DDC theories, Audretsch and Mueller had studied the spontaneous excitation of an accelerated atom interacting with a massless scalar field, and got a conclusion:the behavior of a uniformly accelerated atom is equivalent to that of a static atom in thermal reservoir of Unruh temperature Tu=a/2π. Are spontaneous excitation of a uniformly accelerated atom and that of a static atom in a thermal reservoir due to the same mechanism? Does this equivalence still hold if we replace the massless scalar field with the massive scalar field, or add a certain boundary to the field. To answer this question, and further understand the Unruh effect and provide a theoretical basis, we will study the spontaneous excitation of an uniformly accelerated atom interacting with a massive scalar field.An introduction will firstly be made in this paper, in which we would briefly introduce the DDC methods and the Unruh effect. Then we will apply the DDC method, considering a two-level atom coupled with a massive scalar field, for the two cases of uniform motion in a vacuum and at rest in a thermal reservoir, and we calculate the average energy change rate of the atom and the Einstein coefficients of spontaneous emission and spontaneous excitation, respectively. Our main work will be focused on the case of uniformly accelerated motion in a vacuum, in which atomic average energy change rate and the atomic Einstein coefficients of the radiation and the excitation are calculated. We will compare the results with that of the case of the massless scalar field and the case of static atoms in a thermal reservoir at the Unruh temperature, and discuss the correction by the mass of the field. Finally, some conclusions would be drawn for the nature of atomic radiation and excitation owing to the introduction of the mass of field. Especially, we find that when the atom coupled with a massive scalar field, the selection rules is valid for energy level transition of a static atom in the Unruh temperature thermal reservoir, but for a uniformly accelerated atom, it would no longer work. This shows that the equivalence between a uniformly accelerated atom in the vacuum with a static atom in the thermal reservoir of Unruh temperature is not valid. Finally, we discuss the approximate results and their physical significance in the following cases:the low mass limit, the high mass limit, the low acceleration limit and the high acceleration limit.