Heisenberg operators of a Dirac particle interacting with the quantum radiation field

We consider a quantum system of a Dirac particle interacting with the quantum radiation field, where the Dirac particle is in a 4×4-Hermitian matrix-valued potential V. Under the assumption that the total Hamiltonian HV is essentially self-adjoint (we denote its closure by ), we investigate properties of the Heisenberg operator (j=1,2,3) of the j-th position operator of the Dirac particle at time t∈R and its strong derivative dxj(t)/dt (the j-th velocity operator), where xj is the multiplication operator by the j-th coordinate variable xj (the j-th position operator at time t=0). We prove that D(xj), the domain of the position operator xj, is invariant under the action of the unitary operator for all t∈R and establish a mathematically rigorous formula for xj(t). Moreover, we derive asymptotic expansions of Heisenberg operators in the coupling constant q∈R (the electric charge of the Dirac particle).