Standard Model has two accidental global symmetries $U(1)_{B}$ baryon number and $U(1)_{e} \times U(1)_{\mu} \times U(1)_{\tau}$ lepton number, which holds for each family. We define baryon number $B = 1$ for baryons $(n,p)$ and $B = -1$ for antibaryons $(\overline{n}, \overline{p})$. For lepton number we define $L_{e,\mu,\tau} = 1$ for leptons and $\overline{L}_{e,\mu,\tau} = -1$ for antileptons. Both symmetries are conserved in Standard Model for which oscillations between baryons and proton decay are forbidden in Standard Model. Grand Unified Theory (GUT) introduces new hypothetical particles Leptoquarks (LQ) for which we get theory Beyond Standard Model (BSM). LQ are new interaction mediators that allow us to violate the baryon number for one or two units. Violation for one unit gives us proton decay and violation for two units gives us neutron antineutron oscillations. There is also an experimental search for double beta dacay, which would violate a lepton number for $\Delta L = 2$ two units and give us proof that the neutrino is a Majorana fermion. In this thesis we consider oscillations between baryons with emphasis on the neutron. We calculate decay amplitude for the transition of a neutron into Majorana fermion $n \rightarrow \chi$ on tree-level and one-loop level. At the tree level, LQ $S_{1}$ is interaction mediator, and $\overline{S}_{1}$ is the interaction mediator for one loop. Experimental observations of the neutron antineutron oscillation would greatly impact the asymmetry between matter and antimatter. Many experiments are trying to measure oscillation time between neutron and antineutron, which would prove the existence of neutron antineutron oscillation.
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