Post on 26-Jan-2021
• Main points of today’s lecture:– Fermi theory of Beta decay
• relation to Feynman diagram, etc.
– Types of Beta decay• allow• forbidden • superallowed
– Kurie plot and ft value– Parity violation
• Main points of last lecture:– Neutrino Oscillations
Physics 492 Lecture 39
Fermi theory of Beta decay
• consider eepn ν++→−
Beta decay
• calculate decay per unit electron momentum:
( ))p(I
c2EEcpG
e672
2e
22e
2F ≡
−⋅=
hπ
eEEEcP −== νν
( )eEI≡
diagrammatically
• GF, g, e, sin(θw) and Mw are related:
Types of beta decay
• Allowed transitions:
Types of beta decay
• forbidden transitions:
• Coulomb effects
Kurie plot
0
0.5
1
1.5
2
0 4 8 12 16
[I(p e
)/(p e
2 K(Z
,pe)
]1/2
K.E.electron
(keV)
Computation of beta decay rates
• Role of matrix element:
• superallowed decays
Ft value
~
Particle decays:
• tau decay
• Main points of today’s lecture:– Kurie plot and ft value– Parity violation– CP symmetry and violation
• Main points of last lecture:– Fermi theory of Beta decay
• relation to Feynman diagram, etc.
– Types of Beta decay• allow• forbidden • superallowed
– Kurie plot and ft value
Physics 492 Lecture 40
Z0 decay branching ratios• Naive decay:
• Actual decay:
WS. Theory
ActualNaiveB.R.
14.9%15%14%Z0→
11.5%12%14%Z0→
6.7%6.7%4.8%Z0→3.4%3.4%4.8%Z0→e+e-
νν
ccbb
Ft value
~
Particle decays:
• tau decay
Fermi coupling constant
Parity violation
• K+ decay
Parity violation
• 60Co decay example:
Parity violation in beta decay• Basic properties
C violation
• Existence of coupling (mainly) to L.H. neutrinos, and electrons and R.H. anti-neutrinos and positrons is parity violating– It also violates charge conjugation.
C symmetry
• Charge conjugation:
C and fermions
CP symmetry
• Does CP transform beta decay processes into something possible?