Effect of first-forbidden decays on the shape of neutrino spectra

We examine the effect of first forbidden (FF) decays on $\beta$-decay neutrino spectra by performing microscopic nuclear structure calculations. By analyzing the FF decay branches of even-even nuclei, we conclude that FF decays may be responsible for part of the missing neutrinos in the so-called reactor neutrino anomaly. Further calculations and more experimental data are needed for a firm conclusion.

Figure 1

Neutrino spectra of low-lying FF decay branches for ${}^{136}\mathrm{Te}$ (left) and ${}^{140}\mathrm{Xe}$ (right) from SM [dashed lines in panels (a) and (b)] and QRPA (thick lines) calculations. The meanings of different line styles are illustrated in the graph. For ${}^{140}\text{Xe}$, “Full FF” means this calculation with matrix elements and phase space stated above in the text, meanwhile the “Simp. FF” means the simplified matrix element used in Ref. [5].

Figure 2

Dependence of the changes for the percentage of neutrino number $\delta$ (defined in text) on the endpoint energies for several FF decay branches of ${}^{136}\mathrm{Te}$ (bold lines for QRPA calculations and dashed lines for SM calculations) and ${}^{140}\mathrm{Xe}$ (dash-dotted lines for QRPA calculation). Here we vary the $Q$ values of the two nuclei above to see how the changes are related to the endpoint energies of the decay branches.