versions. The Masses of weak gauge bosons are generated by the Higgs mechanism, and those of quarks and charged leptons are given via the Yukawa interaction after the symmetry breaking.

In the SM a scalar iso-doublet field, the Higgs field, is introduced to trigger the spontaneous symmetry breaking, whose energy density determines the structure of vacuum. It is a remarkable feature of the SM that all the massive elementary particles but neutrinos obtain masses which are proportional to the vacuum expectation value of the Higgs field. The scale for the mass of each particle is determined by the magnitude of its coupling with the Higgs boson. In the SM, therefore, hierarchical structure among quark masses observed at experiments is a consequence of differences in the strength of the Yukawa interaction for each quark. As a result the question of the quark mass hierarchy remains a problem, which must be solved in the framework of new physics beyond the SM.

The top quark is exceptionally heavy as compared to the other quarks. Its mass has been measured to be at the scale of electroweak symmetry breaking, so that its Yukawa coupling has turned out to be of order one within the 