Renormalization-group description of nonequilibrium critical short-time relaxation processes: a three-loop approximation

The influence of nonequilibrium initial values of the order parameter on its evolution at a critical point is described using a renormalization group approach of the field theory. The dynamic critical exponent $\theta'$ of the short time evolution of a system with an $n$-component order parameter is calculated within a dynamical dissipative model using the method of $\varepsilon$-expansion in a three-loop approximation. Numerical values of $\theta'$ for three-dimensional systems are determined using the Pad\'{e}-Borel method for the summation of asymptotic series.