Origin of the Reactor Antineutrino Anomalies in Light of a New Summation Model with Parametrized ${\beta }^{-}$ Transitions

We investigate the possible origins of the reactor antineutrino anomalies in norm and shape within the framework of a summation model where ${\beta }^{-}$ transitions are simulated by a phenomenological model of Gamow-Teller decay strength. The general trends of divergence from the Huber-Mueller model on the antineutrino side can be reproduced in both norm and shape. From the exact electron-antineutrino correspondence of the summation model, we predict similar distortions in the electron spectra, suggesting that biases on the reference spectra of fission electrons could be the cause of the anomalies.

Figure 1

Top: electron kinetic energy spectra for ${}^{90}\mathrm{Br}$ generated with the summation model using the G-T strength model (in blue) with three $\alpha$ values and the ENSDF nuclear database (in red) as inputs and compared to the experimental spectrum from Ref. [48]. Bottom: cumulative electron kinetic energy spectra calculated with the summation model using the G-T strength model as input (in blue) or the TAGS experimental data (in yellow) for the same three $\alpha$ values. The summation was done for the 66 isotopes measured in TAGS experiments using ${}^{235}\mathrm{U}$ fission yields for the weighting. Only the uncertainties due to the stochastic process are plotted.

Figure 2

Average IBD cross section per fission versus the number of electrons per fission calculated with the G-T model for the different values of $\alpha$ marked in the right plot and for three fissioning systems. The blue band results from the uncertainties due to the $\beta$-strength model. The uncertainties in the fission yields are lower than this band and amount to 0.4%, 0.5%, and 0.7% for ${}^{235}\mathrm{U}$, ${}^{239}\mathrm{Pu}$, and ${}^{241}\mathrm{Pu}$, respectively. The cross lines represent the Huber-Mueller model. Experimental results from Daya Bay [18] (orange band) and STEREO [7] (green hatch band) are also shown. Note that $N_e$ has been integrated on the same energy range as the HM data.

Figure 3

Ratios of the kinetic energy spectra for ${}^{235}\mathrm{U}$, ${}^{239}\mathrm{Pu}$, and ${}^{241}\mathrm{Pu}$, calculated with our model for different values of $\alpha$, to the Huber-Mueller model for antineutrinos (top) and electrons (bottom). The widths of the lines indicate the standard deviation of the G-T strength model due to the stochastic process. The gray band on the antineutrino side corresponds to the uncertainties in the fission yields, fully correlated between antineutrinos and electrons, while the gray band on the electron side corresponds to the systematic uncertainties in the BILL spectrometer efficiency. For comparison, the ratio to HM (scaled by a factor of 0.95 [7]) constructed with the unfolded antineutrino spectrum of Ref. [51] is added.

Figure 4

Ratios of the electron kinetic energy spectra for ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ calculated with our model for $\alpha =0.7$ and compared to experimental BILL data [27, 28] (top plot). The light blue band corresponds to the total uncertainty, including fission yield uncertainties, and the dark blue band shows the uncertainty of the G-T model only. For comparison, the data from Kopeikin et al. [20] are added. The bottom plot is the double ratio to BILL data. All the uncertainties are propagated.