Testing the very-short-baseline neutrino anomalies at the solar sector

Motivated by the accumulating hints of new sterile neutrino species at the eV scale, we explore the consequences of such an hypothesis on the solar sector phenomenology. After introducing the theoretical formalism needed to describe the MSW conversion of solar neutrinos in the presence of one (or more) sterile neutrino state(s) located "far" from the (nu_1,nu_2) "doublet", we perform a quantitative analysis of the available experimental results, focusing on the electron neutrino mixing. We find that the present data posses a sensitivity to the amplitude of the lepton mixing matrix element U_e4 --- encoding the admixture of the electron neutrino with a new mass eigenstate --- which is comparable to that achieved on the standard matrix element U_e3. In addition, and more importantly, our analysis evidences that, in a 4-flavor framework, the current preference for |U_e3|>0 is indistinguishable from that for |U_e4|>0, having both a similar statistical significance (which is ~ 1.3 sigma adopting the old reactor fluxes determinations, and ~ 1.8 sigma using their new estimates). We also point out that, differently from the standard 3-flavor case, in a 3+1 scheme the Dirac CP-violating phases cannot be eliminated from the description of solar neutrino conversions.Comment: 16 pages, 3 figures; minor changes; matches version published in PR

Phenomenology of light sterile neutrinos: a brief review

An increasing number of anomalous experimental results are emerging, which cannot be described within the standard 3-neutrino framework. We present a concise discussion of the most popular phenomenological interpretation of such findings, based on a hypothetical flavor conversion phenomenon of the ordinary "active" neutrinos into new light "sterile" species having mass m ~ O(1) eV.Comment: 12 pages, 5 figures, invited review for MPLA, added references, matches published versio

Consistent analysis of the νμ→νe\nu_\mu \to \nu_e sterile neutrinos searches of ICARUS and OPERA

The two long-baseline experiments ICARUS and OPERA have recently provided bounds on light ($\sim$ eV) sterile neutrinos exploiting the negative results of the $\nu_\mu \to \nu_e$ appearance searches. Both collaborations have performed the data analysis using an effective 2-flavor description. We show that such a simplified treatment neglects sizable genuine 4-flavor effects, which are induced by the interference of the new large squared-mass splitting $\Delta m^2 _{14}$ with the atmospheric one. The inclusion in the data analysis of such effects weakens the upper bounds on the effective appearance amplitude $\sin^2 2\theta_{\mu e}$ approximately by a factor of two. In addition, we evidence that, in a 4-flavor scheme, the flavor oscillations involve also the $\nu_e$ component of the CNGS beam and can suppress the theoretical expectation of the background in a substantial way. The inclusion in the data analysis of the $\nu_e$ disappearance effects leads to a further weakening of the upper bounds on $\sin^2 2\theta_{\mu e}$, which overall are relaxed by a factor of three with respect to those obtained in the effective 2-flavor description.Comment: 5 pages, 5 figures; references updated; to appear in PRD as a Rapid Communicatio

A conceptual issue on the statistical determination of the neutrino velocity

We discuss a conceptual issue concerning the neutrino velocity measurement, in connection with the statistical method employed by the OPERA collaboration for the inference of the neutrino time of flight. We expound the theoretical framework that underlies the delicate statistical procedure illustrating its salient aspects. In particular, we show that the order of the two operations of sum and normalization used to combine the single waveforms so as to build the global PDF is a crucial point. We also illustrate how a consistency check able to test the correctness of the PDF-composing procedure should be designed.Comment: 12 pages, 2 figures, discussion enlarged and divided in seven sections; matches version accepted for publication in NIMA (Nuclear Instruments and Methods in Physics Research, Section A

Constraints on very light sterile neutrinos from θ13\theta_{13}-sensitive reactor experiments

Three dedicated reactor experiments, Double Chooz, RENO and Daya Bay, have recently performed a precision measurement of the third standard mixing angle $\theta_{13}$ exploiting a multiple baseline comparison of $\nu_e \to \nu_e$ disappearance driven by the atmospheric mass-squared splitting. In this paper we show how the same technique can be used to put stringent limits on the oscillations of the electron neutrino into a fourth very light sterile species (VLS$\nu$) characterized by a mass-squared difference lying in the range [$10^{-3} - 10^{-1}$] eV$^2$. We present accurate constraints on the admixture $|U_{e4}|^2$ obtained by a 4-flavor analysis of the publicly available reactor data. In addition, we show that the estimate of $\theta_{13}$ obtained by the combination of the three reactor experiments is rather robust and substantially independent of the 4-flavor-induced perturbations provided that the new mass-squared splitting is not too low ($\gtrsim 6 \times 10^{-3}$ eV$^2$). We briefly comment on the possible impact of VLS$\nu$'s on the rest of the neutrino oscillation phenomenology and emphasize their potential role in the cosmological "dark radiation" anomaly.Comment: 13 pages, 4 figures, added references, version published in JHE

Physics Reach of DUNE with a Light Sterile Neutrino

We investigate the implications of one light eV scale sterile neutrino on the physics potential of the proposed long-baseline experiment DUNE. If the future short-baseline experiments confirm the existence of sterile neutrinos, then it can affect the mass hierarchy (MH) and CP-violation (CPV) searches at DUNE. The MH sensitivity still remains above 5$\sigma$ if the three new mixing angles ($\theta_{14}, \theta_{24}, \theta_{34}$) are all close to $\theta_{13}$. In contrast, it can decrease to 4$\sigma$ if the least constrained mixing angle $\theta_{34}$ is close to its upper limit $\sim 30^0$. We also assess the sensitivity to the CPV induced both by the standard CP-phase $\delta_{13} \equiv \delta$, and the new CP-phases $\delta_{14}$ and $\delta_{34}$. In the 3+1 scheme, the discovery potential of CPV induced by $\delta_{13}$ gets deteriorated compared to the 3$\nu$ case. In particular, the maximal sensitivity (reached around $\delta_{13}$ $\sim$ $\pm$ $90^0$) decreases from $5\sigma$ to $4\sigma$ if all the three new mixing angles are close to $\theta_{13}$. It can further diminish to almost $3\sigma$ if $\theta_{34}$ is large ($\sim 30^0$). The sensitivity to the CPV due to $\delta_{14}$ can reach 3$\sigma$ for an appreciable fraction of its true values. Interestingly, $\theta_{34}$ and its associated phase $\delta_{34}$ can influence both the $\nu_e$ appearance and $\nu_\mu$ disappearance channels via matter effects, which in DUNE are pronounced. Hence, DUNE can also probe CPV induced by $\delta_{34}$ provided $\theta_{34}$ is large. We also reconstruct the two phases $\delta_{13}$ and $\delta_{14}$. The typical 1$\sigma$ uncertainty on $\delta_{13}$ ($\delta_{14}$) is $\sim20^0$ ($30^0$) if $\theta_{34} =0$. The reconstruction of $\delta_{14}$ (but not that of $\delta_{13}$) degrades if $\theta_{34}$ is large.Comment: 26 pages, 8 figures, 2 tables. Minor revisions. Accepted in JHE

Octant of θ23\theta_{23} in danger with a light sterile neutrino

Present global fits of world neutrino data hint towards non-maximal $\theta_{23}$ with two nearly degenerate solutions, one in the lower octant ($\theta_{23} <\pi/4$), and the other in the higher octant ($\theta_{23} >\pi/4$). This octant ambiguity of $\theta_{23}$ is one of the fundamental issues in the neutrino sector, and its resolution is a crucial goal of next-generation long-baseline (LBL) experiments. In this letter, we address for the first time, the impact of a light eV-scale sterile neutrino towards such a measurement, taking the Deep Underground Neutrino Experiment (DUNE) as a case study. In the so-called 3+1 scheme involving three active and one sterile neutrino, the $\nu_\mu \to \nu_e$ transition probability probed in the LBL experiments acquires a new interference term via active-sterile oscillations. We find that this novel interference term can mimic a swap of the $\theta_{23}$ octant, even if one uses the information from both neutrino and antineutrino channels. As a consequence, the sensitivity to the octant of $\theta_{23}$ can be completely lost and this may have serious implications in our understanding of neutrinos from both the experimental and theoretical perspectives.Comment: Brief note added; accepted for publication in Physical Review Letter

An estimate of theta_14 independent of the reactor antineutrino flux determinations

In a previous paper [Phys. Rev. D 83, 113013 (2011)] we have shown that the solar sector data (solar and KamLAND) are sensitive to the parameter theta_14, encoding the admixture of the electron neutrino with a fourth (essentially) sterile mass eigenstate. In that work we evidenced that such data prefer a non-zero value of theta_14 and that such a preference is completely degenerate with that of non-zero theta_13. In this report we show how the evidence of theta_13 > 0, recently emerged from global neutrino data analyses, lifts such a degeneracy and disfavors the case of sterile neutrino mixing. By excluding from our analysis the total rate information coming from the reactor experiments we untie our results from any assumption on their flux normalization. In this way, we establish the robust upper bound sin^2 (theta_14) < 0.04 at the 90% C.L.Comment: 4 pages, 1 figure, minor changes, matches version accepted by PR