Number conservation of bosons and fermions Why is the number of bosons not conserved while the number of fermions is conserved? Does it have something to do with the Pauli exclusion principle?
 A: If what you mean by conservation of "number of fermions" is that the number of fermions in the initial state must be equal to the number of fermions in the final state, well, it's wrong. Consider for example this nuclear reaction (which exists):
$$p + p \to \pi^+ + d$$
where $p\equiv$ proton (spin 1/2), $\pi^+$ pion (spin 0) and $d\equiv$ deuterium (spin 1), you see that there are 2 fermions in the initial state and none in the final. So no conservation of both the number of fermions and the number of bosons!
What is conserved by all reactions (so far…) are:


*

*Total angular momentum (and not only the spin)

*Energy/Momentum

*Total lepton number

*Baryon number 

*electric charge


(plus the symmetry CPT but it's a bit different the context of this question).
Now, one can define the fermion number as $B+L$ where $B$ is the baryon number and $L$ the total lepton number. Its conservation is a consequence of the conservation of both $B$ and $L$ individually. But beware that fermion number doesn't mean "number of fermions".
A: I was just only making a comment, let me expand.The global charges of Fermion (f), baryon B)  and Lepton (L)  number are conserved in the standard model. But Global charges are not related to any gauge symmetry so there no strong reason for their conservation. In theories beyond the standard model they are predicted to be NOT conserved, though in some models ,  B-L charge Is conserved. Global charge conservation numbers must be violated to explain why our universe is mostly matter and not equal amounts of matter and anti matter which would give us a universe of mostly photons. Some models  gauge B-L charge, these are left-right models, but these seem to be ruled out now by CERN. Beyond the standard models there is proton decay in  SO(10)/SUSY SU(5) unification models and double beta neutrino-less decays which violate these global charges but none of this have been observed 
A: With regard to Fermion number it gets strange in grand unification , because superforce  bosons seem to carry the global charge of fermion number 
