Nonperturbative and Instanton Phenomena in Particle Physics

In this dissertation we study some nonperturbative phenomena in high energy particle physics. First, we show an example of a semiclassical transition in phi^4 theory with positive coupling constant. This process can be described by the classical O(4)-invariant solution, considered on a contour in the complex time plane. The transition is technically analogous to the one-instanton transition in the electroweak model. It is suppressed by the factor exp(-2S _0), where S_0 is the Lipatov instanton action. This process describes a semiclassical transition between two coherent states with a much smaller number of particles in the initial state than in the final state. Therefore, it could be relevant to the problem of calculation of amplitudes for multiparticle production in phi^4-type models in the context of baryon number violating processes. Then, using an ansatz for the multi-instanton contribution borrowed from the study of high energy baryon number violating processes, we investigate whether instanton corrections could spoil the universal relation between the nonperturbative contributions to the lepton energy spectrum in semileptonic Bto X_{u }l v decays and to the photon energy spectrum in radiative Bto X_{s }+gamma decays. We find that the universality may well fail unless the spectrum is smeared over a region which is considerably larger than had previously been thought necessary. This result affects the possibility of performing a reliable measurement of V_{ub} using inclusive decays. In the last part we calculate the shape of the photon spectrum for Bto X_{s }+gamma decay in the presence of a background chromomagnetic field for the case of SU(2) gauge group. The effect of the external field resembles the Zeeman effect --the parton model peak is split into two slightly asymmetric peaks, located around the kinematic endpoint. We investigate the analytic properties of the spectrum and calculate the total decay rate and moments of the spectrum. We also analyze the cases of SU(2) "electric" and SU(3) "magnetic" background fields. The decay in the background field may serve as a model for the investigation of gluon condensate effects and their influence on the shape of the spectrum.