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A recent publication by Scielzo et al.[1] reported a measurement of the β-ν angular correlation coefficient, αβν, for the β-decay transition between Na and the ground state of its mirror, Ne. The authors compare their result with the standard-model prediction for αβν, with a view to testing for scalar or tensor currents, the presence of which would signal the need for an extension of the standard model. Although they found a significant discrepancy – the measured value, αβν = 0.524(9) disagrees with the standard-model prediction of 0.558 – they stop short of claiming a fundamental disagreement with the standard model. Scielzo et al. [1] offer two alternative explanations that would have to be eliminated before their result could begin to raise questions about the need for an extension to the standard model. One is that some Na2 dimers formed by cold photoassociation could also have been present in their trap, thus distorting the result; they themselves propose to do further measurements to test that possibility. The other is that the branching-ratio value they used for β decay to the first excited state of Ne might not be correct. Because Scielzo’s measurement could not distinguish between positrons from the two predominant β-decay branches from Na (see Fig. 1), the adopted branching ratio for the β transition to the first excited state not only affects their data analysis but also helps determine the theoretical prediction for αβν itself, since the axial-vector component of the ground-state branch can only be determined from its ft value, which also depends on the branching ratio. This branching ratio is a key component of their standard-model test, yet the five published values [2-6] are between 25 and 45 years old, are quite inconsistent with one another and range from 2.2(3) to 5.1(2)%. To remedy this problem, we measured the excited-state branching ratio, for which we quote ±0.8% relative precision, five times better than the best precision claimed in any previous measurement. This experiment has been published [7]. We produced 22.5-s Na using a 28A-MeV Ne to initiate the H(Ne, 2n)Na reaction on a LN2-cooled hydrogen gas target. The ejectiles from the reaction were fully stripped and, after passing through the MARS spectrometer, produced a Na secondary beam of >99% purity at the extraction slits in the MARS focal plane. This beam, containing ~3×10 atoms/s at 24.4A MeV, then exited the vacuum system through a 50-μm-thick Kapton window, passed successively through a 0.3-mm-thick BC-404 scintillator and a stack of aluminum degraders, finally stopping in the 76-μm-thick aluminized Mylar tape of a tape transport system. Since the few impurities remaining in the beam had ranges different from that of Na, most were not collected on the tape; residual collected impurities were concluded to be less than 0.1% of the Na content. In a typical measurement, we collected Na on the tape for a few seconds, then interrupted the beam and triggered the tape-transport system to move the sample in 180 ms to a shielded counting station located 90 cm away, where the sample was positioned between a 1-mm-thick BC404 scintillator to detect β particles, and a 70% HPGe detector for γ rays. We then recorded β singles and β-γ coincidences for a pre-determined counting period. Two timing modes were used: in one, the collection and detection

Branching ratios for the beta decay of 21Na