The Standard Model, a theory of elementary interactions, describes very well electromagnetic, weak and strong interactions of elementary particles. Despite the success, there are some shortcomings like the description of neutrino mass and the origin of dark matter. It is unclear, why there are exactly three fermion families and why the electric charge is quantized. It is also unpleasant to have a theory with so many free parameters. There are attempts to describe all the three forces uniformly in the Grand Unified Theories. They could solve some of the Standard-Model problems. Some of them allow proton to be unstable. This is because they predict interactions that violate baryon number conservation and therefore force the proton to decay. The force carriers of these interactions are leptoquarks. They are hypothetical bosons that transform quarks into leptons and vice versa. I this thesis the leptoquark $S_1$ is considered. This leptoquark has electric charge $1/3$, no spin and is a singlet of the electroweak interaction. He could explain why there are discrepancies between neutron lifetime measurements of different methods. At the same time, leptoquark $S_1$ makes proton unstable. In the beginning, an overview of the Standard Model is made, followed by a description of Grand Unified Theories and the $SU(5)$ model, the simplest representative of them. Next, a classification of leptoquarks is made. Then measurements of the neutron and proton lifetimes are described. What follows, are the derivations of decay widths $\Gamma(p\to e^+\pi^0)$, $\Gamma(p\to e^+\gamma)$ and $\Gamma(n\to \chi\gamma)$. The results are used together with neutron and proton lifetime and atomic parity violation measurements to check the ratio between the $S_1$ coupling constants and its mass $m_{S_1}$. In the end, we estimate the value of $m_{S_1}$ and propose a new bound on $\tau(p\to e^+\gamma)$.
|
