This
quantum number is the charge of a
global/
gauge U(1) symmetry in some
Grand Unified Theory models, called . Unlike baryon number alone or lepton number alone, this hypothetical symmetry would not be broken by
chiral anomalies or
gravitational anomalies, as long as this symmetry is global, which is why this symmetry is often invoked. If exists as a symmetry, then for the
seesaw mechanism to work has to be
spontaneously broken to give the
neutrinos a nonzero mass. The
anomalies that would break baryon number conservation and lepton number conservation individually cancel in such a way that is always
conserved. One hypothetical example is
proton decay where a
proton () would decay into a
pion () and
positron (). The
weak hypercharge is related to via X + 2\,Y_\text{W} = 5\,( B - L ), where
X charge (not to be confused with the
X boson) is the conserved
quantum number associated with the global
U(1) symmetry
Grand Unified Theory. ==See also==