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 /24/2003

2. 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 /24/2003

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

5. 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 /24/2003

6. 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 /24/2003

7. 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 /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 /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 /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 /24/2003

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

12. 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 /24/2003

13. 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 /24/2003

14. 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 /24/2003

15. 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 /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 /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 /24/2003

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

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

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

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