Laplace method for the simplest diagrams of elastic scattering of scalar particles

Abstract

We propose an algorithm for the application of the Laplace method for the calculation of the simplest Feynman diagram with a single loop in the scalar {\phi}^3 theory. The calculation of the contribution of such a diagram to the scattering amplitude requires the calculation of a fourfold integral over the four-momenta components circulating in a loop. The essence of the Laplace method for the calculation of multiple integrals lies in the fact that if the module of an integrand has a point of sufficiently sharp maximum inside the integration domain, then the integral can be replaced by a Gaussian integral by representing the integrand in the form of an exponent from the logarithm and expanding this logarithm into Taylor series in the vicinity of a maximum point up to the second degree terms. We show that there are two-dimensional and non-intersecting surfaces inside the four-dimensional region of integration, on which the maximum of the module of integrand is reached. This leads to a problem that the integrand is non-analytically dependent on the parameters responsible for bypassing the poles. Also the derivatives of logarithm of the scattering amplitude are non-analytically dependent on these parameters. However, in the paper we show that these non-analyticities compensate each other. As a result of such a procedure, three of the four integrations can be done analytically, and the calculation of the contribution of the diagram to the scattering amplitude is reduced to a numerical calculation of the single integral in finite bounds from an expression that does not contain non-analyticities. The described calculation method is used to construct a model dependence of elastic scattering differential cross section d{\sigma}_elastic/dt on the square of the transmitted four-momentum t (Mandelstam variable).