# Beta decay?

1. Jul 25, 2015

### fxdung

Energy of beta ray and neutrino is equal Q=M(mass) of nucleous before-M of nucleous after,so it about 1Mev.But the mass of W boson is 80 MeV,so the least energy of electron and neutrino must be 80 MeV.
Why there is the difference?Why does it seem that energy were not conservation?

2. Jul 25, 2015

### bcrowell

Staff Emeritus
The W is a virtual particle. It's "off shell," so it doesn't have to have an energy and momentum that are consistent with its mass.

3. Jul 25, 2015

### Staff: Mentor

Energy and momentum are both conserved at each vertex of a Feynman diagram, so one can calculate the energy and momentum of the virtual W from the energies and momenta of the outgoing (anti)neutrino and electron. However, because the W is virtual, $\sqrt{E^2 - (pc)^2}$ does not equal the mass of a real W. For a real W, $\sqrt{E^2 - (pc)^2} = m_W c^2$.

Particle physicists use the jargon that real (virtual) particles are "on (off) the mass shell," referring to the quantity $\sqrt{E^2 - (pc)^2}$. Or as Ben said, just "on (off) shell" for short.

Last edited: Jul 25, 2015
4. Jul 25, 2015

### fxdung

I can understand your teaching! Thanks very much!

5. Jul 26, 2015

### Orodruin

Staff Emeritus
Also, the mass of the W is 80 GeV.