91Ó°ÊÓ

Quarks are divided into six flavors’: up, down, charm, weird, top, and bottom. The masses of up and down quarks are the smallest of all quarks.

Except in the case of weak force interactions, flavor is a preserved quantity in a reaction. The W^_ boson, the positively charged W⁺boson, and the neutral Z⁰ boson are the three types of force carriers in the weak force. A weak interaction is mediated by one of the above-mentioned bosons in response to the electric charge change associated with the change in flavor, depending on the flavor change. Let's have a look at \({{\rm{\beta }}^{\rm{ - }}}\)decay as an example. The up-quark u in a proton, which carries charge \({\rm{q(u) = + }}\frac{{\rm{2}}}{{\rm{3}}}\), changes flavor into a down quark d, which carries charge \({\rm{q(d) = - }}\frac{{\rm{1}}}{{\rm{3}}}\), in \({{\rm{\beta }}^{\rm{ - }}}\)decay. We have a charge change of -1 overall, and since total charge must be preserved in an interaction vertex, we deduce that the reaction must be mediated by a W^_ boson with a charge of \({\rm{q}}\left( {{{\rm{W}}^{\rm{ - }}}} \right){\rm{ = - 1}}\). Taking into consideration the W^_ most likely decay route, which is

\({W^ - } \to {e^ - } + {\bar \nu _e}\)

we get a resulting interaction which is consistent with our knowledge about \({{\rm{\beta }}^{\rm{ - }}}\)decays.