Neutron Beta Decays and Search of Deviations from Standard Model

Despite the success of the Standard Model (SM), it is believed to be incomplete and part of some larger physical framework. The neutron is a system, which is ideal for testing the limits of the SM and probing for hints of new physics. The neutron beta decay and its investigation belong to high-precision elementary particle physics, which is able to indicate a failure of the SM without directly specifying the new physics compared to high-energy scale experiments. Neutron decay can determine the Cabibbo-Kobayashi-Maskawa (CKM) matrix element V_ud through increasingly precise measurements of its main observables, the neutron lifetime and decay correlation coefficients. The neutron decay is also sensitive to possible right-handed currents, scalar and tensor terms in the weak lepton- nucleon interaction, time reversal violation and, by using CPT invariance, sensitive to CP violation. In addition the neutron lifetime is an important ingredient for the model of the formation of the universe. Higher precision in experiments on the neutron decay have to be accompanied by a theoretical analysis including higher corrections, i.e. contributions of "smaller effects". Currently, a new set of experiments is planned within the DFG/FWF program 1491 "Precision experiments in particle- and astrophysics with cold and ultracold neutrons" coordinated by Prof. Abele at the Institute of Atomic and Subatomic Physics in Vienna and by the group of Prof. Serebrov at the Petersburg Nuclear Physics Institute (PNPI) in Gatchina, which should place new standards for observables of the neutron beta decay. For the preparation of the experimental devices new theoretical data are needed, for example for the calibration of the geometry of angular distributions of charged particles in the final state of the neutron decay for the improvement of the accuracy of measurements. Due to this close interconnection between theory and experiment this "Joint Project" between the experimental groups of the priority program and Prof. Serebrov as well as the theoretical group headed by Dr. Pitschmann is initiated. The main aim of investigations of the theoretical part is the investigation of the radiative neutron beta decay. The final state of this decay channel contains in addition to the proton, electron and antineutrino also a soft photon. The importance of this analysis is related to the fact that in all experiments on the measurements of the lifetime of the neutron and correlation coefficients one cannot separate the contribution of the usual continuum state beta decay from the radiative beta decay. Using results for the branching ratio of the radiative decay obtained within Heavy-Baryon Chiral Perturbation Theory and the experimental value of the lifetime of the neutron gives a lower bound lifetime of the continuum state decay which agrees only within three standard deviations with the current theoretical value. Thus, a better knowledge of the rate of the radiative beta decay is of crucial importance, since it opens a window to new physics, which can be really determined from the experimental data on the lifetime of the neutron and the correlation coefficients of the electron energy spectrum. This "Joint Project" is an unique opportunity to compare theoretical and experimental data, obtained with the same level of accuracy, allowing for the first time a real search for possible deviations from the Standard Model.

Despite the success of the Standard Model of elementary particle physics, it is believed to be incomplete and part of some larger physical framework at higher energies. The neutron is a system, which is ideal for testing the limits of the Standard Model and probing for hints of new physics. The neutron beta decay and its investigation belong to high-precision elementary particle physics, which is able to indicate a failure of the Standard Model without directly specifying the new physics compared to high-energy scale experiments. The corresponding experimental observables in neutron decay are the lifetime of the unstable neutron, which is an important ingredient for the model of the formation of the universe, as well as the angular distributions of its decay products, the so-called correlation coefficients. The Standard Model provides to all observables precise predictions. Any deviations from these predictions are due to a deviation of the underlying theory, the Standard Model. Experiments on the neutron decay carried out to higher precision have to be accompanied by a theoretical analysis including higher corrections, i.e. contributions of "smaller effects", for their interpretation. A new set of experiments has been carried out within the DFG/FWF program 1491 "Precision experiments in particle- and astrophysics with cold and ultracold neutrons" coordinated by Prof. Abele at the Institute of Atomic and Subatomic Physics in Vienna and by the group of Prof. Serebrov at the Petersburg Nuclear Physics Institute (PNPI) in Gatchina, which have placed new standards for observables of the neutron beta decay. For the preparation of the experimental devices new theoretical data were needed. Due to this close interconnection between theory and experiment this "Joint Project" between the experimental groups of the priority program and Prof. Serebrov as well as the theoretical group headed by Dr. Pitschmann has been initiated. The main aim of investigations of the theoretical part has been the investigation of the radiative neutron beta decay. Thereby, the final state of this decay contains in addition to the proton, electron and antineutrino also a soft photon. For the description of possible deviations due to new physics, phenomenological coupling constants have been employed, which allow to investigate such deviations without recourse to specific hypothetical models describing new physics at high energies. In this "Joint Project" theoretical and experimental data have been obtained with the same level of accuracy, which allowed for the first time a real search for possible deviations from the Standard Model.