The spin particles can have no mass terms because there is no right/left helicity pair with the same SU -LRB- NUMBER@1 -RRB- and SU -LRB- NUMBER@2 -RRB- representation and the same weak hypercharge .

This rule says that no two fermions can be in the same `` place '' at the same time , because no two fermions in an atom can have the same quantum numbers at the same time .
