Transcript PowerPoint Presentation - Parity Violation in Weak Interaction

Parity Violation in Weak
Interaction
The q-t puzzle : the beginning of doubt
Lee and Yang proposal : violation of PC in weak interaction
Wu experiment : proof of PC violation
The meson decay : confirmation of PC violation
October 24th, 2003
Lopez Bruno
The q-t puzzle
• In 1947 Powell identified the p-meson in his cloud chamber.
g Formulation of the weak interaction theory
• Two years later, he observed two decays of the k+ meson
which called into question the parity conservation
q+ g p+ + p0
t+ g p+ + p+ + p• Experimental data indicate indentical masses and life
times for q and t particules.
g q and t seemed to be the same particule
• In 1953, Dalitz argued that since the pion parity was (-1)
two pions would combine to produce a (+1) parity
three pions would combine to produce a (-1) parity
(-1).(-1) = (+1)
(-1).(-1).(-1) = (-1)
g
If parity is conserved, q and t can not be the
same particule
• The conclusion was either q and t are different particules or
parity is not conserved.
g this is the q-t puzzle
Lee and Yang proposal
• In 1956 Lee and Yang suggested a proposal for ending
the q-t puzzle.
g
Violation of parity in weak interaction
• « Existing experiment do indicates parity conservation in
strong and electromagnetic interactions to a high degree of
accuracy. »
• « Past experiments on the weak interactions had actually
no bearing on the question of parity conservation. »
• If parity is not stricly conserved atomic and nuclear states
become mixtures of the normal states with a small
percentage of states of opposite parity.
F is the fractional weight of these states.
g
F caracterizes the degree of violation of parity
conservation
• Experimental limits are F2 < 10– 4.
In a proton beam polarized perpendiculary to its momentum and
scattered by a nuclei, the scattered intensity in two direction A and B
are in the proportion:
( 1 + F ) / ( 1 - F)
if the scattering originates from a parity-conserving and a paritynonconserving interaction.
The experimental result reguires F < 10-2, or F2 < 10-4
• Experimental proof of parity conservation need an accuracy of
F2 < 10 – 24.
Parity violation implies states of opposite parity. It could therefore
possess an electric dipole moment of a magnetude:
M = e G2 (dimension of syst.)
Where G = F2 is the coupling strength of the decay interaction.
Since all the weak interactions are characterized by a coupling
strength G < 10-12, a violation of parity will introduce a parity mixing
characterized by an F2 < 10-24.
• Lee and Yang suggested possible experimental tests of parity
conservation:
g b-decay of the Cobalt 60
g p and m decay
Experimental test of parity conservation in
b-decay of CO60
•Observation of spacial asymmetry in emission of b-decay
electrons from CO60.
g Lead to a distinction between b-decay and it’s
mirror-image process.
•Angular distribution of electrons coming from b-decay of polarized
nuclei:
I(q) = cst ( 1 + a cos q ) sin q dq
Where a is proportionnal to the interference term between the parityconserving and the parity-nonconserving interactions, and q the angle
between the parent nuclei orientation and the momentum of the
electron.
• An asymmetry of ditribution between q and 1800-q implies that
parity is not conserved.
• a is obtained by mesuring the
fractionnal asymmetry between
q<900 and q>900 :
p/2
a=
[  I(q) dq - p/ I(q)
0
If a = 0
If a ≠ 0
p
p
2
dq
] /  I(q) dq
0
parity is conserved.
parity is not conserved.
The magnetic field used for
orienting the nuclei cause a
spacial separation between the
electron emitted with q<900
and q>900
• A thin layer of CO60 is placed
inside a vacuum chambre.
• An anthracene crystal detect b
particules.
• CO60 nuclei is polarized by the
Rose-Gorter method.
• The degree of polarization is
detected by mesuring the
anisotropy the g-rays.
• Very low temperature were necessary to
align spin orientation.
g Adiabatic demagnetization
refrigerator
• It use the properties of heat and the
magnetc properties of atoms. Atoms have
internal magnetic field which will align
themself with an external magnetic field.
g
energy
Transformation of thermal
energy into magnetic
• Liquid helium remove the heat produced
by magnetisation.
• A large asymmetry was observed !
• The time for disappearance of the b
asymmetry coincides well with that of
g anisotropy.
• b and g distrbution are different
with reversal of the demagnetzation
field, so with reversed nuclei
orientation.
g Difference between the real
world and the mirror one.
• Indeed they found a = 0.4
g
Proof of violation of parity
conservation.
Experimental test of parity conservation in
the decay of p and m mesons
• Lee and Yang suggested that the violation of parity conservation
could be prooved in the study of the decays:
p+ g m+ + n+
m+ g e + + 2n
(1)
(2)
• If parity is not conserved in (1), the muon emitted from the
stopped pion will be polarized in its direction of motion.
• The angular distibution of electrons in (2) serves as a analyzer for
the muon polarization, and hence, indicates whether or not parity is
conserved.
• Polarization of the muons also offers a way of determining the
magnetic moment.
• The p-meson beam is extracted from
a cyclotron in the conventional way.
• Eight inches of carbon are used in the
entance to separate the muons.
• The stopping of a m is signalled by a
fast 1-2 coincidence count.
Registration time is about 1.25 msec with
a 0.75 msec delay.
• The b-decay of the muon is detected
by the electron telescope 3-4.
It register electrons > 25 Mev.
g
The system counts electrons of energy > 25 Mev which
are born between 0.75 and 2.0 msec after the muon
stopping.
• If a magnetic field is applied, the muons are created with a large
polarization in the direction of motion and the process of slowing
down and stopping do not depolarized them.
g the electrons emitted from
m decay have an angular
asymmetry about the polarization direction.
• The consequences of these observations are that in the reactions
(1) and (2), parity is not conserved.
• They also set the ratio of the magnetic moment of m+
particule to 2.00 -+ 0.10.
The violation of parity conservation have been
confirmed !!!
References:
• « Question of Parity Conservation in Weak Interactions »
T. D. Lee and C. N. Yang
Phy. Rev. 104 (1956)
• « Experimental Test of Parity Conservation in Beta Decay »
C. S. Wu
Phy. Rev. 105 (1957)
• « Observation of the Failyre of Parity Conservation of Parity and
Charge Conjugason in Meson Decays : the Magnetic Moment of Free
Muon »
R. L. Garwin, L. M. Lederman, and M. Weinrich
Phy. Rev. 105 (1957)