  We study the decoherence and thermalization of an Unruh-DeWitt detector
linearly coupled to the free massless scalar field in flat spacetime of
arbitrary dimensions ($d\geq 2$). The initial state of the detector is chosen
to be a pure state consisting of a linear superposition of ground and excited
states, and we calculate the time evolution of reduced density matrix of the
detector. Using perturbation method, we analytically derive the transition rate
of the detector (the rate of change of the diagonal elements in the density
matrix) and the decoherence rate (the rate of change of the off-diagonal
elements in the density matrix). We find that the results are not the same in
odd and even dimensional spacetimes, but the unitarity of the qubit is
preserved in both cases. The real part of the decoherence rate is related to
the transition rate, while the imaginary part may contain different forms of
divergence terms in different dimensions due to the temporal order product
operator and the singularities of the Wightman function for quantum field
theory. We derive the recurrence formula to obtain the divergence terms in each
dimension and analyze the renormalization problem.
