• 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?