It is a new fundamental particle which oscillates back and forth in between closed nucleons with a velocity which is comparable to the velocity of light. This particle is detected by various radiation experiments. These particles can be positively, negatively charged or can be electrically neutral. Mesons are about 275 times heavier than electrons.
Due to presence of any type of charge, these particles interact with protons and neutrons. Due to this interaction, proton may change into neutron and a neutron may change into proton over a small period of time.
Proton + negative meson = Neutron
Neutron + positive meson = Proton
This exchange of meson particle in between the closed nucleons results into the force of attraction between protons and neutrons.
