4. Weak Interactions + Quark Flavour Mixing Flashcards
When do hadrons usually undergo weak interaction
When the EM and strong interactions are suppressed or forbidden
Describe what characterises the weak interaction
A small cross section and a relatively long lifetime
Which symmetries does the weak interaction break?
Breaks parity (P), charge conjugation (C), time reversal (T) and CP
What are charged current weak interactions?
Weak interactions which include the W + or W - bosons
- The charged current refers to the fermion line that couples to the W boson and the fermions must differ by a charge of 1 unit
What are neutral current weak interactions?
Weak interactions that involve the Z-0 boson
- The neutral current refers to the fermion line which couples to the Z 0 and must involve fermions of the same flavour
Are neutral currents suppressed relative to charged currents and why?
Yes - Z boson mass is greater than the W boson mass
- Also the Z-0 coupling strength is not the same as the charged coupling strength
What experiment is used to measure the strength of the charged current weak interaction
From the rate of muon decay
How are interactions which conserve and change the strangeness differ in magnitude?
Strangeness changing interactions are suppressed relative to strangeness conserving ones
How did Cabibbo account for the difference in amplitudes between strangeness changing and strangeness conserving interactions
He said the weak interaction is universal but the quark states entering the hadronic weak currents are QM’icallly mixed
- They are not the same mass eigenstates which define flavour
What is a consequence of Cabibbo’s hypothesis of the weak interaction being universal?
All the W couplings to the charged currents have the same strength
- Difference in observed amplitudes is due to the mixing of the quark flavour states
What is the biggest problem with Cabibbo’s hypothesis
It predicts FCNCs at tree level
How was Cabibbo’s failure of FCNCs solved
Through the GIM mechanism which predicted the existence of the charm quark
- The amplitude contributions of the Δs = 1 terms cancel out
Briefly describe the structure of the CKM matrix
The diagonal elements are close to unity
- Off diagonal elements get smaller with the smallest being located in the bottom left two and upper right two
- This form is not predicted by the SM, but is however accommodated by it
