Parity Violation in Weak Interaction

1. 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

2. q+g p+ + p0 t+g p+ + p+ + p- 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 • Experimental data indicate indentical masses and life times for q and t particules. • g q and t seemed to be the same particule

3. 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, qand 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

4. 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. »

5. 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 parity-nonconserving interaction. The experimental result reguires F < 10-2, or F2< 10-4

6. 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

7. 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 cosq )sinq dq • Where a is proportionnal to the interference term between the parity-conserving and the parity-nonconserving interactions, and q the angle between the parent nuclei orientation and the momentum of the electron.

8. 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 p p a =[I(q) dq - I(q) dq]/ I(q) dq p/2 0 0 If a = 0 parity is conserved. If a≠ 0 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

9. 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.

10. 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 Transformation of thermal energy into magnetic energy • Liquid helium remove the heat produced by magnetisation.

11. 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.

12. 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+ (1) m+ge+ + 2n (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.

13. 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.

14. 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 !!!

15. 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)