New Clues about light sterile neutrinos: preference for models with damping effects in global fits

Abstract

This article reports global fits of short-baseline neutrino data to oscillation models involving light sterile neutrinos. In the commonly-used 3+1 plane wave model, there is a well-known 4.9σ tension between data sets sensitive to appearance versus disappearance of neutrinos. We find that models that damp the oscillation prediction for the reactor data sets, especially at low energy, substantially improve the fits and reduce the tension. We consider two such scenarios. The first scenario introduces the quantum mechanical wavepacket effect that accounts for the source size in reactor experiments into the 3+1 model. We find that inclusion of the wavepacket effect greatly improves the overall fit compared to a three-neutrino model by ∆χ2/dof = 61.1/4 (7.1σ improvement) with best-fit ∆m2 = 1.4 eV2 and wavepacket length of 67 fm. The internal tension is reduced to 3.4σ. If reactor-data only is fit, then the wavepacket preferred length is 91 fm (> 20 fm at 99% CL). The second model introduces oscillations involving sterile flavor and allows the decay of the heaviest, mostly sterile mass state, ν4. This model introduces a damping term similar to the wavepacket effect, but across all experiments. Compared to a three-neutrino fit, this has a ∆χ2/dof = 60.6/4 (7σ improvement) with preferred ∆m2 = 1.4 eV2 and decay Γ = 0.35 eV. The internal tension is reduced to 3.7σ.For many years, the reactor event rates have been observed to have structure that deviates from prediction. Community discussion has focused on an excess compared to prediction observed at 5 MeV; however, other deviations are apparent. This structure has L dependence that is well-fit by the damped models. Before assuming this points to new physics, we urge closer examination of systematic effects that could lead to this L dependence.

A decoherence explanation of the gallium neutrino anomaly

“…Recent discussions of decoherence in oscillations of eV-scale sterile neutrinos can be found in refs. [37,38]. Let us stress that the decoherence that we postulate here requires exotic new physics which modifies the standard quantum mechanical evolution; conventional decoherence based on particle localisation leads only to tiny effects which are negligible for all oscillation experiments considered here [39,40].…”

Wilks’s theorem, global fits, and neutrino oscillations

“…In the sterile neutrino case, it is natural to separate appearance and disappearance as they are expected to be the most independent. A very serious problem in neutrino physics related to the 3 + 1 models is that the PG test p-values are consistently very low [1], which is leading to controversy in the field. However, in this case, as we are using toy-models, we know that the data are internally consistent, so we can evaluate the applicability of this test.…”

“…Experimental data from oscillation experiments can have many complex features arising from systematic uncertainties. For a review of the many issues involved with actual data, see [1]. To avoid distraction from these issues, and for educational purposes, it is simpler to use 'toy models', where we control all aspects of the 'experiments.'…”

“…where x  is the set of data, PDF is the probability density function representing the probability of a given event, q  are the parameters of that function, and L is the likelihood of the set of data. 1 L is the number that contains the most information about the data's relation to the model, though its interpretation often requires approximations. L is often unwieldy and computationally difficult to deal with, and so we almost always compute the likelihood in log-space, producing the log-likelihood:…”

Decoherence effects in reactor and Gallium neutrino oscillation experiments: a QFT approach

“…For early papers on the topic see [1][2][3][4][5][6][7][8][9][10][11][12], some examples of further investigations are e.g., [13][14][15][16][17][18][19]. Recently, this discussion received further attention in the context of short-baseline reactor [20][21][22][23] and radioactive source Gallium experiments [24][25][26][27] searching for sterile neutrino oscillations [28][29][30][31][32]. These papers discuss the question whether quantum decoherence could help to reduce tension in the data [28,32] which arises in standard sterile neutrino explanations, see e.g., [33][34][35][36].…”