Nonperturbative Calculation of Shear Viscosity from the Basis of the Keldysh Field in Thermal Ф⁴ Theory

The nonperturbative result of the shear viscosity in thermal
Ф⁴ theory is given by solving the Bethe-Salpeter (B-S) integral equation in the closed time formalism in real time. By introducing a two-legs-truncated Green function it is shown that the B-S equation is decoupled in the basis of Keldysh field.