Mirror Symmetry in theβDecay of theA=20and 25 Systems

Abstract

10 The importance of imaginary contributions to the phase shifts by dispersion corrections has been recog-The simplest expectation for mirror /3 decays is that they should have identical/£ values. It has, however, been known for some time that the ft value for N 12 decay to the ground state of C 12 is some 10% greater than that for the mirror decay of B 12 . Careful analysis 1 suggests that the discrepancy may well be a significant one, not explicable in terms of electromagnetic, secondforbidden, isospin-mixing, and binding-energy corrections, and that its resolution may lie in the reality of second-class currents, specifically the induced tensor interaction, 2 ' 3 i.e., that the 0 interaction does not respect G parity. 2 Before accepting this fundamental conclusion one must be sure both that the known corrections have been properly evaluated and that there are not others of a "structural" nature that may fluctuate from case to case.It is therefore important to investigate other cases of mirror decay to see whether or not they fall systematically into line with A = 12. If the discrepancy in ft values was due solely to an induced tensor term its magnitude, for light nuclei and W Q * » 1, should be proportional to W 0 + + W 0~ and approximately state independent. 3 * 4 We have carried out measurements on the systems A = 20 and 25 that enable us to extend the mirror test to nized previously by Rawitscher (Ref. 6). Using a model of pure monopole excitation he was able to show that the imaginary contributions to the phase shifts are bigger than the corrections of the real part by about one order of magnitude. Assuming then that the real part of the correction may be neglected, our Eq. (1) may be derived directly from the Appendix of Ref. 6. n F. Wo J. Olver, Phil. Trans 0 Roy. Soc D London A247, 307 (1954), and to be published. these cases. We find that A = 25 shows a large departure from mirror symmetry in the same sense as for A = 12; the evidence on A = 20 is conflicting but suggests that the departure from mirror symmetry could have the opposite sense. These results emphasize the caution that must be used in interpreting the failure of mirror symmetry in terms of second-class currents.The P~ decay of F 20 is 99.92% to the first excited state of Ne 20 at 1.63 MeV. 5 The /3 + decay of Na 20 has been extensively investigated 6 by measurement of its j3 rays and the subsequent y decay and a decay of Ne 20 . The Na 20 half-life is reported 6 as 408 ± 6 msec and the /3 branch to the first excited state of Ne 20 as 90.0%. However, in view of technical problems in the j3-ray measurements we have preferred to make our analysis in terms of the superallowed transition of Na 20 to its analog in Ne 20 at 10.270±0.009 MeV. 7 For the Fermi part of this transition we have taken 8 ft-3060 ± 20 sec; for the Gamow-Teller part we have based ourselves on local systematics and taken the range ft = 6.3 xl0 4 -°° sec; this leads toft = 2990 ± 70 sec for the two together.The half-life of F 20 has been determined as 11.03 ±0.06 sec and J3...

Beta-decay properties of 25 Si and 26 P

“…However, two systematic deviations from theoretical predictions show the limitation of our theoretical understanding and treatment of fundamental interactions. They are reported as the mirror asymmetry anomaly in β decay [7,8,9,10] and the quenching of the Gamow-Teller strength [11,12,13].…”

Second class currents and mirror beta decays

“…Is a mirror asymmetry evidence for second- We do not discuss whether the present observations as summarized by Wilkinson [1] indicate the existence of such a mirror asymmetry or can alternqtively be explained as due to electromagnetic effects. J~ Weinberg's statement of the theorem is somewhat ambiguous, but the conclusions of the theorem are not; our formulation is the same as that of Cabibbo [6] except that he uses the U transformation rather than (; to distinguish first-and second-class currents.…”

“…A recent analysis by Wilkinson [1] and experiments by Wilkinson and Alburger [2] concerning mirror beta-decays have revived interest in the question of the existence of secondclass currents. Such currents, first discussed by Weinberg [3], have opposite signs under the G transformation from the "normal" strangeness-conserving currents included in the standard treatments of weak interactions.…”

Resolving the discrepancy in the half-life of F20

“…Questions arose from discrepancies in f t values of the A=12 isobars 12 N and 12 B [6]. This disagreement gave incentive to measure the f t value of other isobars, such as the A=20 system, and test the Conserved Vector Current (CVC) hypothesis [7]. This required a stronger constraint of experimental quantities needed to determine f t values, such as the half-life [8].…”