P461 - decays II1 Parity Violation in Beta Decays The Parity operator is the mirror image and is NOT conserved in Weak decays (is conserved in EM and strong)

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P461 - decays II1 Parity Violation in Beta Decays The Parity operator is the mirror image and is NOT conserved in Weak decays (is conserved in EM and strong) non-conservation is on the lepton side, not the nuclear wave function side spin 1/2 electrons and neutrinos are (nominally) either right-handed (spin and momentum in same direction) or left-handed (opposite) Parity changes LH to RH RH LH

P461 - decays II2 “Handedness” of Neutrinos “handedness” is call chirality. If the mass of a neutrino = 0 then: all neutrinos are left-handed all antineutrinos are right-handed Parity is maximally violated As the mass of an electron is > 0 can have both LH and RH. But RH is suppressed for large energy (as electron speed approaches c) fraction RH vs LH can be determined by solving the Dirac equation which naturally incorporates spin

P461 - decays II3 Polarized Beta Decays Some nuclei have non-zero spin and can be polarized by placing in a magnetic field magnetic moments of nuclei are small (1/M factor) and so need low temperature to have a high polarization (see Eq 144 and 14-5) Gamow-Teller transition with S(e-nu) = 1 if Co polarized, look at angular distribution of electrons. Find preferential hemisphere (down) Co Pnu pe Spin antinu-RH Spin e - LH

P461 - decays II4 Discovery of Parity Violation in Beta Decay by C.S. Wu et al. Test parity conservation by observing a dependence of a decay rate (or cross section) on a term that changes sign under the parity operation. If decay rate or cross section changes under parity operation, then the parity is not conserved. Parity reverses momenta and positions but not angular momenta (or spins). Spin is an axial vector and does not change sign under parity operation. neutron PePe PePe     mirror Beta decay of a neutron in a real and mirror worlds: If parity is conserved, then the probability of electron emission at  is equal to that at 180 o - . Selected orientation of neutron spins - polarisation.

P461 - decays II5 Wu’s experiment Beta-decay of 60 Co to 60 Ni *. The excited 60 Ni * decays to the ground state through two successive  emissions. Nuclei polarised through spin alignment in a large magnetic field at 0.01 o K. At low temperature thermal motion does not destroy the alignment. Polarisation was transferred from 60 Co to 60 Ni nuclei. Degree of polarisation was measured through the anisotropy of gamma-rays. Beta particles from 60 Co decay were detected by a thin anthracene crystal (scintillator) placed above the 60 Co source. Scintillations were transmitted to the photomultiplier tube (PMT) on top of the cryostat.

P461 - decays II6 Wu’s results Graphs: top and middle - gamma anisotropy (difference in counting rate between two NaI crystals) - control of polarisation; bottom -  asymmetry - counting rate in the anthracene crystal relative to the rate without polarisation (after the set up was warmed up) for two orientations of magnetic field. Similar behaviour of gamma anisotropy and beta asymmetry. Rate was different for the two magnetic field orientations. Asymmetry disappeared when the crystal was warmed up (the magnetic field was still present): connection of beta asymmetry with spin orientation (not with magnetic field). Beta asymmetry - Parity not conserved