The study of the various inelastic processes induced by photons, electrons, pions, and (anti) neutrinos from nucleons is very important as it provides information about the excitation mechanism of nucleons. This enables us to investigate the structure of nucleons as a composite of quarks and the role of gluons in the quark–quark forces using quantum chromodynamics (QCD). The inelastic processes induced by (anti)neutrinos are a unique source to determine the axial vector aspects of the nucleon structure and relate it to the pion physics. Moreover, in recent years, inelastic reactions induced by (anti)neutrinos leading to the production of mesons like pions, kaons, and ηs have become more relevant in the search for proton decay and for neutrino oscillations.
Within the standard model, proton stability is associated with baryon number conservation, as the proton being the lightest baryon, cannot decay into any other baryon when it is in the free state. It is believed that the baryon number conservation law is not a fundamental law; and there are models of grand unified theory (GUT) , which predict the lifetime of the proton to be of the order of 1031 years. To estimate a proton's lifetime, some calculations have also been performed using supersymmetry (SUSY) GUTs like SUSY SU(5) and SUSY SO(10). Generally, SUSY GUT models favor kaons (K0 or K+) in the final state if a proton decays. However, some of these models also predict decay modes where an eta meson is produced in the final state . In the minimal SU(5) GUT, the predicted proton lifetime and decay to e+ π0 is 1031±1 years, which has been ruled out by IMB , Kamiokande [506, 507], and Super-Kamiokande . Although these experiments have not observed any event for the decay p → e+ π0, they have provided the limit for the proton's lifetime to be 1033±1 years. The best limit on the proton's lifetime comes from Super-Kamiokande  for the channel p → νK+ > 5.9 × 1033 years. Soudan-2  looked for eta in the final state in the proton years. Soudan-2  looked for eta in the final state in the proton decay searches and found the best limit for the channels p → μ+ η and p → e+ η to be 8.9 × 1031 years and 8.1 × 1031 years, respectively.