I am told there are four fundamental forces, and each of these forces has a boson that acts as its carrier. Reading this http://www.fnal.gov/pub/science/inquiring/questions/higgs_boson.html I find ...
W and Z bosons are observed/discovered. But as force carrying bosons they should be virtual particles, unobservable? And also they require to have mass, but if they are virtual they may be off-shell, ...