# Question about conservation of lepton and baryon

1. Dec 8, 2009

### kelvin490

I would like to ask what is mean by conservation of lepton and baryon? Does this imply conservation of mass? In addition, does this theory contradicts mass-energy conversion E=mc2?

2. Dec 9, 2009

### Bob S

Both lepton-number conservation and baryon-number conservation are empirical laws. This means that in any process (decay, collisions, etc.) the number of leptons (3 kinds) AND the number of baryons cannot change (similar to charge conservation). So when a neutron decays, it has to decay into another baryon (proton) AND a pair of leptons (including energy conservation requirements). So the question might be "why doesn't the proton decay into mesons and leptons?" Many searches have been made for proton decay into mesons and leptons. My Particle Data Group book
http://pdg.lbl.gov/2009/tables/contents_tables.html
shows that the proton decay lifetime is presently over 1032 years.
Bob S

3. Dec 9, 2009

### bcrowell

Staff Emeritus
No. For example, in the reaction $e^+ + e^- \rightarrow 2\gamma$, lepton number is conserved (0=0), but mass is not.

No. In the same reaction, mass is converted to energy.