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## Main Question or Discussion Point

In the Meson theory of nuclear forces, exchange of pi meson is given by:

[tex]n\rightarrow n + \pi^{0}[/tex]

[tex]p\rightarrow p + \pi^{0}[/tex]

[tex]n\rightarrow p + \pi^{-}[/tex]

[tex]p\rightarrow n + \pi^{+}[/tex]

Here the charge is conserved. But I don't understand how mass conservation takes place as in some of the cases a lighter mass gives rise to a heavier mass as in case of protons giving rise to neutons. So how is mass conservation obeyed here and mass of pion is estimated?

[tex]n\rightarrow n + \pi^{0}[/tex]

[tex]p\rightarrow p + \pi^{0}[/tex]

[tex]n\rightarrow p + \pi^{-}[/tex]

[tex]p\rightarrow n + \pi^{+}[/tex]

Here the charge is conserved. But I don't understand how mass conservation takes place as in some of the cases a lighter mass gives rise to a heavier mass as in case of protons giving rise to neutons. So how is mass conservation obeyed here and mass of pion is estimated?