Spontaneous excitation of a circularly accelerated atom coupled with vacuum Dirac field fluctuations

• Spontaneous excitation of a circularly accelerated atom is studied.
• The atom interacts with the Dirac field through nonlinear coupling.
• A cross term involving vacuum fluctuations and radiation reaction contributes.
• The atom in circular motion does not perceive pure thermal radiation.
• The contribution of the cross term changes as the atomic trajectory varies.

We study the spontaneous excitation of a circularly accelerated atom coupled with vacuum Dirac field fluctuations by separately calculating the contribution to the excitation rate of vacuum fluctuations and a cross term which involves both vacuum fluctuations and radiation reaction, and demonstrate that although the spontaneous excitation for the atom in its ground state would occur in vacuum, such atoms in circular motion do not perceive a pure thermal radiation as their counterparts in linear acceleration do since the transition rates of the atom do not contain the Planckian factor characterizing a thermal bath. We also find that the contribution of the cross term that plays the same role as that of radiation reaction in the scalar and electromagnetic fields cases differs for atoms in circular motion from those in linear acceleration. This suggests that the conclusion drawn for atoms coupled with the scalar and electromagnetic fields that the contribution of radiation reaction to the mean rate of change of atomic energy does not vary as the trajectory of the atom changes from linear acceleration to circular motion is not a general trait that applies to the Dirac field where the role of radiation reaction is played by the cross term.