# Mass conservation in meson theory

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

$$n\rightarrow n + \pi^{0}$$
$$p\rightarrow p + \pi^{0}$$
$$n\rightarrow p + \pi^{-}$$
$$p\rightarrow n + \pi^{+}$$

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?

## Answers and Replies

Norman
Mass conservation? Total energy must be conserved, but mass is not. The incident particle (or original particle) must have some energy available (say through kinetic energy) to emit the pion.
Make sense?
Cheers,
Ryan

Reshma
Hi Ryan, thank you for replying. So the energy is conserved here. However, pions do have mass. How is this mass estimated?

marlon
Reshma said:
Hi Ryan, thank you for replying. So the energy is conserved here. However, pions do have mass. How is this mass estimated?

How about this one ?

marlon

Mentor
In the pion-exchange model, aren't the pions virtual instead of real? If they're virtual, they can temporarily violate energy conservation during the exchange process; but the total energy of the two nucleons before the exchange equals their total energy after the exchange.