Nonthermal signature of the Unruh effect in field mixing

Mixing transformations for a uniformly accelerated observer (Rindler observer) are analyzed within the quantum field theory framework as a basis for investigating gravitational effects on flavor oscillations. In particular, the case of two charged boson fields with different masses is discussed. In spite of such a minimal setting, the standard Unruh radiation is found to lose its characteristic thermal interpretation due to the interplay between the Bogoliubov transformation hiding in field mixing and the one arising from the Rindler spacetime structure. The modified spectrum detected by the Rindler observer is explicitly calculated in the limit of the small mass difference.

Figure 1

The proper coordinate system of a uniformly accelerated observer in the Minkowski spacetime. The hyperbola represents the world line of an observer with proper acceleration $a$.

Figure 2

Pictorial interpretation of the modified spectrum of mixed particles in $|0_M⟩$ for a Rindler observer. Different colors of dots correspond to different particle/antiparticle flavors. Contrary to the case of absence of mixing, the vacuum is populated by particle-antiparticle pairs, both of the same (blue-blue, red-red) and different (blue-red) types.