Introduction

The study of the various inelastic processes induced by photons, electrons,pions, and (anti) neutrinos from nucleons is very important as it providesinformation about the excitation mechanism of nucleons. This enables us toinvestigate the structure of nucleons as a composite of quarks and the roleof gluons in the quark–quark forces using quantum chromodynamics(QCD). The inelastic processes induced by (anti)neutrinos are a uniquesource to determine the axial vector aspects of the nucleon structure andrelate it to the pion physics. Moreover, in recent years, inelasticreactions induced by (anti)neutrinos leading to the production of mesonslike pions, kaons, and ηs have become more relevantin the search for proton decay and for neutrino oscillations.

Within the standard model, proton stability is associated with baryon numberconservation, as the proton being the lightest baryon, cannot decay into anyother baryon when it is in the free state. It is believed that the baryonnumber conservation law is not a fundamental law; and there are models ofgrand unified theory (GUT) [503], which predict the lifetime of the protonto be of the order of 1031 years. To estimate a proton'slifetime, some calculations have also been performed using supersymmetry(SUSY) GUTs like SUSY SU(5) and SUSY SO(10). Generally, SUSY GUT modelsfavor kaons (K0 orK+) in the final state if a protondecays. However, some of these models also predict decay modes where an etameson is produced in the final state [504]. In the minimal SU(5) GUT, thepredicted proton lifetime and decay to e+π0 is 1031±1 years,which has been ruled out by IMB [505], Kamiokande [506, 507], andSuper-Kamiokande [508]. Although these experiments have not observed anyevent for the decay p → e+π0, they have provided the limit for theproton's lifetime to be 1033±1 years. The bestlimit on the proton's lifetime comes from Super-Kamiokande [509] forthe channel p → νK+> 5.9 × 1033years. Soudan-2 [510] looked for eta in the final state in the proton years.Soudan-2 [510] looked for eta in the final state in the proton decaysearches and found the best limit for the channels p →μ+η and p →e+η to be 8.9 ×1031 years and 8.1 × 1031years, respectively.