Unruh-DeWitt detector's response to a non-relativistic particle

Unruh-DeWitt (UDW) detector that couples locally to a quantum field is an important tool for operationally studying field properties. Here, we study the response of an inertial UDW detector to a non-relativistic particle state of a massive scalar field and we find that the transition probability of the detector splits into the vacuum contribution and the matter contribution. We show that the matter part oscillates with the interaction time duration and the oscillation period depends on the difference between the mass of the particle and the energy gap of the detector; a strong resonance pattern for the transition probability is found when such difference equals to zero. We compare the matter part contribution with the non-relativistic probability density of the particle and find that they provide qualitatively similar description of the particle.