Parity Conservation in the weak (beta decay) interaction
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Transcript Parity Conservation in the weak (beta decay) interaction
Parity Conservation in
the weak (beta decay) interaction
The parity operation
The parity operation involves the transformation
In rectangular coordinates --
x x y y z z
In spherical polar coordinates --
r r
In quantum mechanics
For states of definite (unique & constant) parity -
ˆ x, y,z 1x,y,z
If the parity operator commutes with hamiltonian -
ˆ , Hˆ 0
The parity is a
“constant of the motion”
Stationary states must be states of constant parity
e.g., ground state of 2H is s (l=0) + small d (l=2)
In quantum mechanics
To test parity conservation - Devise an experiment that could be done:
(a) In one configuration
(b) In a parity “reflected” configuration
- If both experiments give the “same” results,
parity is conserved -- it is a good symmetry.
Parity operations -Parity operation on a scaler quantity -
ˆ E
ˆ r r r r
E
Parity operation on a polar vector quantity -
ˆ r r
ˆ p p
Parity operation on a axial vector quantity -
ˆ L
ˆ r p r p L
Parity operation on a pseudoscaler quantity -
ˆ p L p L
If parity is a good symmetry…
• The decay should be the same whether the process
is parity-reflected or not.
• In the hamiltonian, V must not contain terms that
are pseudoscaler.
• If a pseudoscaler dependence is observed - parity
symmetry is violated in that process - parity is
therefore not conserved.
T.D. Lee and C. N. Yang, Phys. Rev. 104, 254 (1956).
http://publish.aps.org/
puzzle
Parity experiments (Lee & Yang)
Original
I
Parity reflected
p
I
p
Look at the angular distribution of decay particle
(e.g., red particle). If this is symmetric
above/below the mid-plane, then --
p
I 0
If parity is a good symmetry…
• The p I 0 the decay intensity should not
depend on p I .
p I 0 there is a dependence on p I
• If
and parity is not conserved in beta decay.
Discovery of parity nonconservation (Wu, et al.)
Consider the decay of 60Co
Conclusion: G-T, allowed
C. S. Wu, et al., Phys. Rev 105, 1413 (1957)
http://publish.aps.org/
GT : I 1
1v
A :1
cos
3c
v
H
c
Measure
Trecoil
v
H
c
H 1
F:I 0I 0
v
V :1 cos
c
H 1
H 1
Not
observed
H 1
Conclusions
GT : I 1
1v
A :1
cos
3c
GT is an axial-vector
Violates parity
v
H
c
F:I 0I 0
v
V :1 cos
c
F is a vector
Conserves parity
H 1
Implications
Inside the nucleus, the N-N interaction is
VN N Vs Vw
Conserves
parity
Can violate
parity
The nuclear state functions are a superposition
s Fw ; F 107
Nuclear spectroscopy not affected by Vw
Generalized -decay
The hamiltonian for the vector and axial-vector weak
interaction is formulated in Dirac notation as --
H g
H g
h.c. gV
h.c. gA
*
p n
*
p 5
*
e
n
*
e 5
Or a linear combination of these two --
H gCV V C A A
Generalized -decay
The generalized hamiltonian for the weak interaction
that includes parity violation and a two-component
neutrino theory is --
*
Ci p Oi n
Hg
iA,V
*
e Oi 1 5
h.c.
OV ; OV 5
Empirically, we need to find --
g and C A CV
Study: n p (mixed F and GT),
and: 14O 14N* (I=0 I=0; pure F)
Generalized -decay
14O
14N* (pure F)
2 3
ft
7
5 4
m c MF
log 2
2
g 2 CV2
g 2 CV2 1.4029 0.0022 10 49 erg cm 3
n p (mixed F and GT)
2 3
ft
2
5 4
g m c
7
MF
2
CV
log 2
2
M GT
2
CA
2
Generalized -decay
Assuming simple (reasonable) values for the
square of the matrix elements, we can get (by
taking the ratio of the two ft values -2CV2
CV2 3C A2
0.3566
CV2
C A2
1.53
Experiment shows that CV and CA have opposite signs.
Universal Fermi Interaction
In general, the fundamental weak interaction is of
the form --
H gV A
n p e
e e
p
o
semi-leptonic weak decay
pure-leptonic weak decay
semi-leptonic weak decay
Pure hadronic weak decay
All follow the (V-A) weak decay. (c.f. Feynman’s CVC)
Universal Fermi Interaction
In general, the fundamental weak interaction is of
the form --
H gV A
BUT -- is it really that way - absolutely?
How would you proceed to test it?
e e
pure-leptonic weak decay
The Triumf Weak Interaction Symmetry Test - TWIST
Other symmetries
Charge symmetry - C
n p e
C
n p e
All vectors unchanged
Time symmetry - T
n p e
e n p
T
n p e
(Inverse -decay)
All time-vectors changed (opposite)
Symmetries in weak decay
s 0
Note helicities
of neutrinos
at rest
P
C
Symmetries in weak decay
s 0
at rest
Note helicities
of neutrinos
P
C
Conclusions
1. Parity is not a good symmetry in the weak
interaction. (P)
2. Charge conjugation is not a good symmetry in the
weak interaction. (C)
3. The product operation is a good symmetry in the
weak interaction. (CP) - except in the kaon system!
4. Time symmetry is a good symmetry in the weak
interaction. (T)
5. The triple product operation is also a good
symmetry in the weak interaction. (CPT)