weak decays beta decay ofneutron

problem energy and momentum not conserved e n p

W. Pauli: 1930  Neutrino mass ~ m(e)

weak decays beta decay ofneutron

e n p

direct interaction of four fermions  Enrico Fermi, ~1936 p n electron antineutrino

weak interactions current-current interaction weak currents ~ lefthanded:

lefthanded fermion S= - 1/2 righthanded antifermion S = + 1/2

maximal breaking of parity

weak interaction of the quarks beta – decay d  u

weak decays of strange particles ( u d s )( u u d ) s  u

Cabibbo angle:  flavor mixing

19

W(+) W(-) Z photon

rotation symmetry spontaneous symmetry breaking in classical mechanics

spontaneous symmetry breaking baki

 direction singled out  rotation symmetry broken

Peter Higgs

Kibble Guralnik Hagen Englert Brout

potential V example 1: real scalar field

=> 2 ground states

example 2: complex scalar field

exact symmetry: broken symmetry:

scalars: doublet

degrees of freedom before symmetry breaking:

After symmetry breaking:

scalars: triplet

„Higgs“ scalar

photon

first term lefthanded  parity violation

neutral current interaction parity violation observed in atomic physics

neutrino massless (only lefthanded component)

The masses of the weak bosons are predicted, once the weak angle is measured. The electron mass cannot be predicted ( the coupling constant g is unknown ).

electroweak theory: mass generation by ??? SSB ??? QCD: mass generation by confinement problem: mass generation

hypothetical Higgs boson

L3 I ATLAS CMS LHCb Alice

December 2011: ATLAS and CMS report signal at 125 GeV decay into Z + Z W + W, photon + photon

Real World 3 lepton-quark families mass

Real World 6 leptons – 6 quarks 2 x 3 doublets

flavormixing