Wednesday, June 1, 2022

Neutrino Reactor Anomalies Explained

Inconsistencies between the sterile neutrino predictions from reactor anomalies at different experiments and some other hints in the experimental data have disfavored sterile neutrinos or non-standard neutrino interactions for quite a while. But, anomalies are always more comfortably put to rest when one can figure out what has caused them.

A new paper builds upon previous hints that the mix of isotypes in a reactor's fuel is strongly predictive of these anomalies to provide a more theoretically well grounded explanation of the neutrino (including antineutrino) reactor anomalies which have been observed to date without resorting to beyond the Standard Model physics.

The authors caution that while their simple tweaks to the formula used to generate the expected mix of neutrinos emitted from reactors solves most of the reactor anomalies in a theoretically well grounded manner, that considering higher order corrections to their predictions which are also theoretically well grounds should allow for even more complete explanations for these anomalies.
We investigate the possible origins of the reactor antineutrino anomalies in the framework of a summation model (SM) where missing β transitions are simulated by a phenomenological Gamow-Teller β-decay strength model. 
We show that the general trends of the discrepancies between the measured antineutrinos energy spectra and the Huber-Mueller model can be reproduced both in norm and shape. 
Using the exact electron-antineutrino correspondence of the SM model, we predict similar distortions in the fission-electron spectra, suggesting a norm bias for the 235U ILL electron spectrum as being at the origin of the "Reactor Antineutrino Anomaly" and a shape bias in the measured electron spectra of 235U and Pu isotopes as being at the origin of the "5 MeV bump".
A. Letourneau, et al., "Anomalies in reactor antineutrino spectra in light of a new summation model with parameterized missing transitions" arXiv:2205.14954 (May 30, 2022).

A more complete explanation which also frames the problem is found in the introduction portion of the body text:
The reactor antineutrino anomalies are a several-years long standing problem in neutrino physics. They refer to an observed ∼6% deficit in the detected rate, known as "Reactor Antineutrino Anomaly" (RAA), and a ∼10% excess of event in the 4-6 MeV range, known as the “5- MeV bump", when comparing experimental data to the prediction of the state-of-the-art Huber-Mueller (HM) model. The RAA was first put in evidence by comparing to short baseline reactor experiments, and confirmed by all recent high precision reactor antineutrino experiments at distances of 300-500 m from the reactor and below 30 m. The “5-MeV bump” is observed in all the above-cited high precision reactor antineutrino experiments also with slightly different amplitudes and shapes. 

At present, no consensus has been reached concerning the origins of these anomalies. The RAA was first interpreted as the possibility of the existence of a hypothetical sterile neutrino state, mixing with the active electronic flavor. The best fit parameters for this sterile state to absorb the anomaly was found around 1 eV^2 for the oscillation frequency (∆m^2) and 0.14 for the amplitude sin^2(2θ). This best fit region of oscillation parameters is now rejected to high C.L. by several experiments that have tested the sterile neutrino hypothesis in a model independent way. 

On the other hand, the Daya Bay and RENO experiments have studied the dependence of the antineutrino yield to the fuel-composition. They concluded that a ∼8% bias of the 235U Inverse Beta Decay (IBD) yield could be solely responsible for the RAA. This result is slightly in tension with experiments at research reactors with pure 235U fuel showing a (5 ± 1.3) % deficit, not allowing to conc [sic] But the hypothesis of a normalization bias on 235U spectrum is reinforced by the recent measurement of the 235U to 239Pu electron energy spectra ratio reporting a constant ∼5% disagreement with respect to the HM prediction. 

Regarding the shape anomaly, extensive studies have been conducted to find explanations in the prediction modeling but none of them have succeeded to bring satisfactory solutions. 

The Huber-Mueller model is based on an improved method to convert the cumulative β spectra measured at ILL with the BILL spectrometer into antineutrino spectra. In this method, if experimental biases exist on the measured β spectra, they would be transferred to the converted antineutrino spectra and could be at the origins of the anomalies. The method itself is not guaranteed unbiased due to the contribution of first forbidden transitions. 

The present contribution proposes to use the exact electron-antineutrino correspondence of a refined summation model (SM) to test the consistency of the electron and antineutrino spectra predicted by the HM model and to search for biases in the original β spectra used as reference to construct the HM model.

The body text of the paper concludes as follows:

In summary, we have presented a phenomenological Gamow-Teller strength model able to simulate β-decay transition-intensities for fission fragments and to correct for Pandemonium effect and missing transitions in the ENSDF database. 
Despite the simplicity of the model, the main features and divergences observed in antineutrino experiments compared to the Huber-Mueller model can be reproduced by a summation model with tuned input parameters. It highlights the importance of missing transitions in the modeling of antineutrino fission spectra. Using the exact correspondence between electron and antineutrino in the summation approach, we have seen that equivalent deviations are expected on the electron side. 
The conclusions of this study suggest that the reactor antineutrino anomalies could find their origins in a norm bias for the measured 235U spectrum after 12h of irradiation and a shape bias for all measured electron spectra. Although these conclusions are supported by independent measurements, the origin of the biases are still unclear at this stage. 
Some biases on the neutron cross sections used to normalize the beta spectra could cover part of the RAA and part of the shape anomaly could be included in the envelope of systematic of the BILL spectrometer efficiency. This work tends to confirm the need for improving the accuracy of β fission spectra both on the experimental and theoretical sides. 

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