Chronology of Milestone Events in Particle Physics - GARWIN 1957
Chronology of Milestone Events in Particle Physics


Garwin, R.L.; Lederman, L.M.; Weinrich, M.;
Observation of the Failure of Conservation of Parity and Charge Conjugation in Meson Decays: The Magnetic Moment of the Free Muon
Phys. Rev. 105 (1957) 1415;

Reprinted in
R. N. Cahn and G. Goldhaber, The Experimental Foundations of Particle Physics, Cambridge Univ. Press (1991) 173.
The Physical Review - the First Hundred Years, AIP Press (1995) 863.

Lee and Yang have proposed that the long held space-time principles of invariance under charge conjugation, time reversal, and space reflection (parity) are violated by the "weak'' interactions responsible for decay of nuclei, mesons, and strange particles. Their hypothesis, born out of the - puzzle, was accompanied by the suggestion that confirmation should be sought (among other places) in the study of the successive reactions + + ,(1) + e+ 2 .(2) They have pointed out that parity nonconservation implies a polarization of the spin of the muon emitted from stopped pions in (1) along the direction of motion and that furthermore, the angular distribution of electrons in (2) should serve as an analyzer for the muon polarization. They also point out that the longitudinal polarization of the muons offers a natural way of determining the magnetic moment. Confirmation of this proposal in the form of preliminary results on decay of oriented nuclei by Wu et al. reached us before this experiment was begun.
By stopping, in carbon, the + beam formed by forward decay in flight of + mesons inside the cyclotron, we have performed the meson experiment, which establishes the following facts:
I. A large asymmetry is found for the electrons in (2), establishing that our + beam is strongly polarized.
II. The angular distribution of the electrons is given by 1 + a cos , where is measured from the velocity vector of the incident 's. We find a = - 1 / 3 with an estimated error of 10%.
III. In reactions (1) and (2), parity is not conserved.
IV. By a theorem of Lee, Oehme, and Yang, the observed asymmetry proves that invariance under charge conjugation is violated.
V. The g value (ratio of magnetic moment to spin) for the (free) + particle is found to be + 2.00 0.10.
VI. The measured g value and the angular distribution in (2) lead to the very strong probability that the spin of the + is 1 / 2.
VII. The energy dependence of the observed asymmetry is not strong.
VIII. Negative muons stopped in carbon show an asymmetry (also peaked backwards) of a ~ -1/20, i.e., about 1.5% of that for +.
IX. The magnetic moment of the -, bound in carbon, is found to be negative and agrees within limited accuracy with that of the +.
X. Large asymmetries are found for the electrons from polarized + beams stopped in polyethylene and calcium. Nuclear emulsion yields an asymmetry of about half that observed in carbon.
(Extracted from the introductory part of the paper.).

Accelerator NEVI-CYC Detectors CNTR

Related references
More (later) information appears in
T. D. Lee, R. Oehme, and C. N. Yang, Phys. Rev. 106 (1957) 340;
See also
R. H. Dalitz, Phil. Mag. 44 (1953) 1068;
M. Weinrich and L. M. Lederman, Proceedings of the CERN Symposium, Geneva (1956);
T. D. Lee and C. N. Yang, Phys. Rev. 104 (1956) 254;
F. J. Belinfante, Phys. Rev. 92 (1953) 997;
Analyse data from
C. S. Wu et al., Phys. Rev. 105 (1957) 1413;
V. L. Fitch and J. Rainwater, Phys. Rev. 92 (1953) 789;

  + e+ 2 asym, qnc
  e 2 asym, qnc

Particles studied
  + qn, mom
  qn, mom

Record comments
Confirmation of parity violation in weak decays. Evidence of charge conjugation parity violation in weak interactions. Measurement of the magnetic moment.
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