Exact real-time correlation functions of the XXZ antiferromagnetic chain in the massive regime

We apply a thermal form factor expansion method to evaluate the real-time longitudinal spin-spin correlation functions of XXZ chain in the antiferromagnetic massive parameter regime. In the zero temperature limit we obtain a series of multiple integrals with fully explicit integrands composed of functions of the q-gamma family and of q-hypergeometric functions. This has to be contrasted with previous results where the integrands (the form factor densities) were given in terms of sums over multiple integrals or (in the better case) in terms of Fredholm determinants. The new series allows for an accurate numerical calculation of the real-time correlation functions at short and large distances and opens interesting perspectives for future directions.