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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)