The understanding that protons do not decay into an electron and a positron with the emission of two neutrinos comes from experimental observations and theoretical considerations. The decay of a proton into an electron and a positron with the simultaneous emission of two neutrinos would violate several fundamental principles, such as the conservation of lepton number and energy-momentum conservation.
Experimental observations have shown that protons are stable particles with an extremely long lifetime. No evidence of proton decay with such a decay mode has been observed in extensive experiments conducted over many decades.
Additionally, theoretical considerations play a crucial role in establishing the limitations on possible decay modes. The Standard Model of particle physics, which is the currently accepted framework for understanding elementary particles and their interactions, incorporates the principles of conservation of lepton number and energy-momentum conservation. These principles have been well-tested and confirmed by numerous experiments.
The theoretical framework of the Standard Model, coupled with the absence of any experimental evidence for proton decay with the emission of two neutrinos, strongly suggests that such a decay mode is highly unlikely or prohibited. The conservation laws and experimental constraints place stringent bounds on the allowed decay modes of particles, including the proton.
It is important to note that the hypothetical proton decay into an electron, a positron, and two neutrinos is not predicted or supported by our current understanding of particle physics. However, it is always possible that new experimental discoveries or theoretical advancements may lead to revisions or extensions of our current knowledge in the future.