Motivation for Higgs Mechanism Gauge symmetry agrangian is invariant under local phase transformation QED local gauge invariance -massless photon field A, U(1) QCD: local gauge invariance =8 massless vector gluon fields G. SU(3) o Weak interaction: massive/Z instead of massless(1983 o A solution: spontaneous breaking of a local gauge symmetry(introduce mass without breaking gauge invariance)(1960s) Or ignore the experiment factor that massive W/z quarks mediators have been discovered LEPTONSMotivation for Higgs Mechanism 7  Gauge Symmetry Lagrangian is invariant under local phase transformation  QED : local gauge invariance → massless photon field Aμ  QCD: local gauge invariance → 8 massless vector gluon fields  Weak interaction: massive W/Z instead of massless (1983).  A solution: spontaneous breaking of a local gauge symmetry (introduce mass without breaking gauge invariance) (1960s).  Or ignore the experiment factor that massive W/Z mediators have been discovered. a G U(1) SU(3)