Lecture 13 – the weak force When is it weak and when is it strong ? Weak interactions of quarks Cabibbo angle CKM matrix Handedness in charged current interactions. FK7003

When is a force weak or strong ? FK7003

gX X,|Q| gX FK7003

When the weak force is weak FK7003

When the weak force become stronger ne e- e- g,Z0 W- p p FK7003

Electromagnetic and weak force strength FK7003

The weak force in detail Charged currents Basic processes for quarks and leptons Lepton-quark symmetry Cabibbo angle CKM matrix V-A theory FK7003

Basic charged current processes involving leptons Weak coupling at a vertex: gW FK7003

Quark interactions via the weak force. FK7003

Cabibbo mixing FK7003

Lepton quark symmetry c s’ c s c d gcd FK7003

Determination of the Cabibbo angle s K- u gud gus FK7003

Cabibbo–Kobayashi–Maskawa Matrix gab FK7003

More on the CKM Matrix The CKM matrix is generalisation of Cabibbo’s mixing for the light quarks by Kobayashi and Maskawa When generalising CP violation was introduced/discovered as a property of weak decays. Kobayashi and Maskawa won the 2008 Nobel prize for this work. Many particle physicists (probably Cabibbo included) were surprised at Cabibbo’s exclusion. FK7003

Decay of W FK7003

Spin dependence of charged current interactions FK7003

V-A interactions s s s s s FK7003

Top quark non-discovery 1984 CERN UA1 experiment pp (630 GeV cm energy) Something they would rather forget FK7003

Top quark discovery quark pdf q(x) Anti-quark pdf q(x) FK7003

Top quark lifetime FK7003

Selected top quark decays at the Tevatron FK7003

A top quark pair production ”event” Event observed at the D0 experiment. FK7003

Top quark discovery top background FK7003

Summary The weak force was studied The weak force is of comparable strength with the electromagnetic force for interactions with |Q|>MW Charged current interactions take place in quark doublets of mixed physical states. Cabibbo theory and the CKM matrix formalise mixing. At relativistic energies, the W couples preferentially to left- handed fermions and right-handed antifermions (V-A theory). Next lecture – neutral currents, electroweak unification and the Higgs. FK7003