Highly-forbidden non-unique beta-decays are known to have electron-spectrum shapes which depend on the effective value of the axial vector coupling constant $g_{\rm A}$ [1, 2]. Resent calculations show that this is also the case for many first-forbidden non-unique decays. Moreover, the spectral shapes of first-forbidden $J^{+}\leftrightarrow \, J^{-}$ decays with $J\neq 0$ are found to depend also on the value of the axial-charge matrix element, which is known to be enhanced in nuclear medium due to meson-exchange effects [3, 4].
In the reactor-antineutrino analysis the beta decays contributing to the cumulative electron spectrum are usually assumed to have allowed spectral shapes. However, about 30 \% of these decays are actually first-forbidden. In some cases, like in the case of the ground-state-to-ground-state decay of $^{140}\rm Cs$ (see figure), this is found to be a rather poor approximation. Based on the recent results, the use of the allowed-approximation can at least partially explain the so called reactor antineurtino anomaly.
![Electron spectrum of $^{140}\rm Cs$ for $g_{\rm A}=0.8-1.2$.][1]
\[1\] M. Haaranen, P. C. Srivastava, and J. Suhonen, Phys. Rev. C 93, 034308 (2016).
[2] J. Kostensalo, M. Haaranen, and J. Suhonen, Phys. Rev. C 95, 044313 (2017).
[3] K. Kubodera, J. Delorme, and M. Rho, Phys. Rev. Lett. 40, 755 (1978).
[4] P. Guichon, M. Giffon, J. Joseph, R. Laverriere, and C. Samour, Z. Phys. A 285, 183 (1978).
[1]: http://i65.tinypic.com/2w66qkw.png