weak interactions
weak decays beta decay of neutron
e n neutrino p direct interaction of four fermions Enrico Fermi, ~1936
Enrico Fermil
1964 ===> gauge theory electroweak
1957 weak bosons W J. Schwinger
( Glashow, Salam, Weinberg ) - 1964 - 1968 gauge theory of the electroweak interactions ( Glashow, Salam, Weinberg )
Glashow Salam Weinberg 1964-1968
weak and electromagnetic interactions partial unification
gauge group SU(2)xU(1)
S U (2) x U (1) weak interaction electromagnetic interaction
SU(2) x U(1) 3 + 1 gauge bosons photon Z W(+) W(-)
SU(2)xU(1) weak interactions electromagnetism neutral current
SU(2)xU(1) neutral current CERN 1972
photon: superposition of SU(2) and U(1) – gauge boson Why photon massless? Why W – boson massive?
by spontaneous symmetry breaking mass generation by spontaneous symmetry breaking ( “ Higgs “ – mechanism)
Minimal electroweak theory Gauge group: SU(2) x U(1)
SU(2) U(1) fermions
symmetry breaking: SU(2) and U(1) are broken, but the electric charge is unbroken
neutral current interaction parity violation
neutral current interaction parity violation observed in atomic physics
weak interactions
weak decays beta decay of neutron
e n neutrino p direct interaction of four fermions Enrico Fermi, ~1936
Minimal electroweak theory Gauge group: SU(2) x U(1)
symmetry breaking: SU(2) and U(1) are broken, but the electric charge is unbroken
M(W) = 80.4 GeV M(Z)= 91.2 GeV CERN, 1984 =>
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: QCD: mass generation by SSB mass generation problem: mass generation electroweak theory: mass generation by SSB QCD: mass generation by confinement
hypothetical Higgs boson
limit on mass of Higgs boson from LEP: 114 GeV
L3 I ATLAS CMS LHCb Alice LEP - LHC
muon H===> Z Z LHC Higgs particle
W W M(W)~100 MeV - other possibility to generate the masses for the weak bosons: W =>pion=>W pion W W M(W)~100 MeV
W =>technipion=>W - technicolor W =>technipion=>W technipion W W M(W)~100 GeV
composite weak bosons, leptons and quarks other possibility: composite weak bosons, leptons and quarks