Universality of weak interactions?

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Presentation transcript:

Universality of weak interactions? Do all leptons and quarks carry the same unit of weak charge? Yes, for leptons and no for quarks for quarks, the couplings to the weak gauge bosons depend on the quark flavors, due to “quark-mixing”  CKM mechanism Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 Weak decays of quarks Consider the (semileptonic) weak decay Assuming universality of weak decays of quarks, we expect both decays would happen in similar rate, but... Intro. to elementary particle physics Y. Kwon 11/24/2003

Weak decays of quarks (2) Intro. to elementary particle physics Y. Kwon 11/24/2003

Weak decays of quarks (3) It was also noticed that the value of the Fermi constant GF deduced from nuclear b-decay was slightly less than that obtained from muon decay. So, what are we going to do? Discard the universality of weak interaction? Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 Cabibbo theory Try to keep the universality, but modify the quark doublet structure… Assume that the charged current (W) couples the “rotated” quark states where d’, s’ (weak interaction eigenstates) are linear combinations of mass eigenstates d, s Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 Cabibbo theory (2) What we have done is to change our mind about the charged current: “Cabibbo-favored” vs. “-suppressed” effective weak coupling for DS=0 (duW) is cos qc effective weak coupling for DS=1 (suW) is sin qc Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 Cabibbo theory (3) (Ex) What is the relationship between the weak couplings for muon decay (Gm=GF) and nuclear b-decay (Gb) ? Intro. to elementary particle physics Y. Kwon 11/24/2003

Suppression of flavor-changing neutral currents a very stringent suppression of flavor-changing neutral current reactions (Ex) Draw the decay diagrams for the above two reactions! Is it easy to understand this stringent suppression? Intro. to elementary particle physics Y. Kwon 11/24/2003

Flavor-changing neutral currents (FCNC) Neutral-current reactions for (u,d’) quarks In this picture, FCNC is perfectly allowed by theory ??? Intro. to elementary particle physics Y. Kwon 11/24/2003

GIM mechanism for FCNC suppression In 1970, Glashow, Iliopoulos & Maiani (GIM) proposed the introduction of a new quark of Q=+2/3, with label c for ‘charm’. With this new quark, a second quark doublet is also introduced. Then we have additional terms for the neutral current reactions. Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 GIM mechanism (2) FCNC has disappeared! Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 GIM mechanism (3) At the price of a new quark ‘charm’ and another quark doublet, the (experimentally) unwanted FCNC has been removed! Later, in 1974, the bound state of charm-anti-charm was discovered: J/ Indeed, just before this discovery, it had been possible to estimate the mass of the new quark!!  by considering mixing Intro. to elementary particle physics Y. Kwon 11/24/2003

Extension of Cabibbo theory to 3 quark generations Leptons are not involved in the strong interactions Quarks & Leptons do not change its flavor when interacting with neutral gauge bosons quarks do not change flavor under strong int. leptons & quarks do not change flavor when interacting with g or Z0 leptons & quarks change flavor only when interacting with W, and only within its family t b' W+ W Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 Decays of b quark Then how does b decay at all? Note: b  W- t but m(t) >> m(b) For quarks, mass eigenstates  weak interaction eigenstates flavor mixing through CKM matrix very important for CP study responsible for most b decays weak interaction eigenstates mass eigenstates Intro. to elementary particle physics Y. Kwon 11/24/2003

Intro. to elementary particle physics Y. Kwon 11/24/2003 CKM matrix CKM is 3x3 and unitary only 3 generations in the SM CKM is almost 1, but not exactly Vii 1, Vij 0 for i j How do we determine the CKM matrix elements? Intro. to elementary particle physics Y. Kwon 11/24/2003

Expt’l determination of CKM elements Vud from nuclear b-decay Vus from results of K+ and K0 decays agree Vcd from charm meson production via neutrino scattering Intro. to elementary particle physics Y. Kwon 11/24/2003

Expt’l determination of CKM elements Vcs from semileptonic decay of D meson unitarity constraint assuming only 3 generations gives a much tighter bound Intro. to elementary particle physics Y. Kwon 11/24/2003

Expt’l determination of CKM elements Vcb: from and HQET Intro. to elementary particle physics Y. Kwon 11/24/2003

Expt’l determination of CKM elements Vub: from Intro. to elementary particle physics Y. Kwon 11/24/2003

Expt’l determination of CKM elements Vtb: from t-quark decay, assuming only 3 generations at the Tevatron collider at Fermi Lab, top quarks are produced mainly in pairs Assuming one could obtain a pure sample of (Ref. hep-ex/9707026) Intro. to elementary particle physics Y. Kwon 11/24/2003

Expt’l determination of CKM elements Vtd and Vts (1) Vtd (s) (2) Intro. to elementary particle physics Y. Kwon 11/24/2003

Exp. determination of CKM elements & phase (7) Intro. to elementary particle physics Y. Kwon 11/24/2003