Determination of the top quark mass through the measurement of thefrac{{Gamma ({rm Z} to bbar b)}}{{Gamma _{hadr} - Gamma (Z to bbar b)}} ratio at high luminosity LEP

We investigate a possible role of invariant (the integral of motion) in nonstationary field theory by employing an explicit example of a scalar field interacting with a time-dependent external electromagnetic field. We show how in a unique manner the invariant operator leads to the particle concept in the Schrödinger picture. This approach gives an intermediate connection between the operator and functional-Schrödinger-equation formalisms. An application to the problem of particle creation is also discussed briefly.