  We study the statistics of thermal energy transfer in the nonequilibrium
(two-bath) spin-boson model. This quantum many-body impurity system serves as a
canonical model for quantum energy transport. Our method makes use of the
Majorana fermion representation for the spin operators, in combination with the
Keldysh nonequilibrium Green's function approach. We derive an analytical
expression for the cumulant generating function of the model in the steady
state limit, and show that it satisfies the Gallavotti-Cohen fluctuation
symmetry. We obtain analytical expressions for the heat current and its noise,
valid beyond the sequential and the co-tunnelling regimes. Our results satisfy
the quantum mechanical bound for heat current in interacting nanojunctions.
Results are compared with other approximate theories, as well as with a
non-interacting model, a fully harmonic thermal junction.
