1 / 22

Universality of weak interactions?

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. Weak decays of quarks.

urian
Download Presentation

Universality of weak interactions?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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

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

  4. 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

  5. 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 eigenstatesd, s Intro. to elementary particle physics Y. Kwon 11/24/2003

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

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

  12. 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

  13. t b' W W+ Extension of Cabibbo theory to 3 quark generations • Leptons are not involved in the strong interactions • Quarks & Leptons do notchange 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 Intro. to elementary particle physics Y. Kwon 11/24/2003

  14. very important for CP study responsible for most b decays 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 mass eigenstates weak interaction eigenstates Intro. to elementary particle physics Y. Kwon 11/24/2003

  15. 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 CKM matrix • How do we determine the CKM matrix elements? Intro. to elementary particle physics Y. Kwon 11/24/2003

  16. 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

  17. 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

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

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

  20. 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

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

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

More Related