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Antonio Palazzo

Publications and source records attributed to Antonio Palazzo.

31 records · Page 2Linked to original sources
arXiv2017

Octant of $θ_{23}$ in danger with a light sterile neutrino

Present global fits of world neutrino data hint towards non-maximal $θ_{23}$ with two nearly degenerate solutions, one in the lower octant ($θ_{23} <π/4$), and the other in the higher octant ($θ_{23} >π/4$). This octant ambiguity of $θ_{23}$ 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 $ν_μ\to ν_e$ 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 $θ_{23}$ octant, even if one uses the information from both neutrino and antineutrino channels. As a consequence, the sensitivity to the octant of $θ_{23}$ can be completely lost and this may have serious implications in our understanding of neutrinos from both the experimental and theoretical perspectives.

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arXiv2016

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$σ$ if the three new mixing angles ($θ_{14}, θ_{24}, θ_{34}$) are all close to $θ_{13}$. In contrast, it can decrease to 4$σ$ if the least constrained mixing angle $θ_{34}$ is close to its upper limit $\sim 30^0$. We also assess the sensitivity to the CPV induced both by the standard CP-phase $δ_{13} \equiv δ$, and the new CP-phases $δ_{14}$ and $δ_{34}$. In the 3+1 scheme, the discovery potential of CPV induced by $δ_{13}$ gets deteriorated compared to the 3$ν$ case. In particular, the maximal sensitivity (reached around $δ_{13}$ $\sim$ $\pm$ $90^0$) decreases from $5σ$ to $4σ$ if all the three new mixing angles are close to $θ_{13}$. It can further diminish to almost $3σ$ if $θ_{34}$ is large ($\sim 30^0$). The sensitivity to the CPV due to $δ_{14}$ can reach 3$σ$ for an appreciable fraction of its true values. Interestingly, $θ_{34}$ and its associated phase $δ_{34}$ can influence both the $ν_e$ appearance and $ν_μ$ disappearance channels via matter effects, which in DUNE are pronounced. Hence, DUNE can also probe CPV induced by $δ_{34}$ provided $θ_{34}$ is large. We also reconstruct the two phases $δ_{13}$ and $δ_{14}$. The typical 1$σ$ uncertainty on $δ_{13}$ ($δ_{14}$) is $\sim20^0$ ($30^0$) if $θ_{34} =0$. The reconstruction of $δ_{14}$ (but not that of $δ_{13}$) degrades if $θ_{34}$ is large.

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arXiv2016

Degeneracy between $θ_{23}$ octant and neutrino non-standard interactions at DUNE

We expound in detail the degeneracy between the octant of $θ_{23}$ and flavor-changing neutral-current non-standard interactions (NSI's) in neutrino propagation, considering the Deep Underground Neutrino Experiment (DUNE) as a case study. In the presence of such NSI parameters involving the $e-μ$ ($\varepsilon_{eμ}$) and $e-τ$ ($\varepsilon_{eτ}$) flavors, the $ν_μ\to ν_e$ and $\barν_μ\to \barν_e$ appearance probabilities in long-baseline experiments acquire an additional interference term, which depends on one new dynamical CP-phase $ϕ_{eμ/eτ}$. This term sums up with the well-known interference term related to the standard CP-phase $δ$ creating a source of confusion in the determination of the octant of $θ_{23}$. We show that for values of the NSI coupling (taken one at-a-time) as small as $few\,\%$ (relative to the Fermi coupling constant $G_{\mathrm F}$), and for unfavorable combinations of the two CP-phases $δ$ and $ϕ_{eμ/eτ}$, the discovery potential of the octant of $θ_{23}$ gets completely lost.

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arXiv2016

3-flavor and 4-flavor implications of the latest T2K and NO$ν$A electron (anti-)neutrino appearance results

The two long-baseline experiments T2K and NO$ν$A have recently presented new findings. T2K has shown the first $\bar ν_e$ appearance data while NO$ν$A has released the first $ν_e$ appearance results. These data are of particular importance because they allow us to probe for the first time in a direct (or manifest) way the leptonic CP-violation. In fact, it is the first time that a hint of CP-violation arises from the comparison of the observations of neutrinos and antineutrinos. We consider the implications of such new results both for the standard 3-flavor framework and for the non-standard 3+1 scheme involving one sterile neutrino species. The 3-flavor analysis shows a consolidation of the previous trends, namely a slight preference for $\sin δ<0$, disfavoring CP conservation ($δ=0, π$) with a statistical significance close to $90\%$ C.L., and a mild preference (at more than 68\% C.L.) for the normal hierarchy. In a 3+1 framework, the data constrain two CP-phases ($δ_{13} \equiv δ$ and $δ_{14}$), which exhibit a slight preference for the common value $δ_{13} \simeq δ_{14} \simeq -π/2 $. Interestingly, in the enlarged four neutrino scheme the preference for the normal hierarchy found within the 3-flavor framework completely disappears. This indicates that light sterile neutrinos may constitute a potential source of fragility in the capability of the two LBL experiments of discriminating the neutrino mass hierarchy.

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arXiv2016

Discovery Potential of T2K and NOvA in the Presence of a Light Sterile Neutrino

We study the impact of one light sterile neutrino on the prospective data expected to come from the two presently running long-baseline experiments T2K and NOvA when they will accumulate their full planned exposure. Introducing for the first time, the bi-probability representation in the 4-flavor framework, commonly used in the 3-flavor scenario, we present a detailed discussion of the behavior of the numu to nue and numubar to nuebar transition probabilities in the 3+1 scheme. We also perform a detailed sensitivity study of these two experiments (both in the stand-alone and combined modes) to assess their discovery reach in the presence of a light sterile neutrino. For realistic benchmark values of the mass-mixing parameters (as inferred from the existing global short-baseline fits), we find that the performance of both these experiments in claiming the discovery of the CP-violation induced by the standard CP-phase delta13 equivalent to delta, and the neutrino mass hierarchy get substantially deteriorated. The exact loss of sensitivity depends on the value of the unknown CP-phase delta14. Finally, we estimate the discovery potential of total CP-violation (i.e., induced simultaneously by the two CP-phases delta13 and delta14), and the capability of the two experiments of reconstructing the true values of such CP-phases. The typical (1 sigma level) uncertainties on the reconstructed phases are approximately 40 degree for delta13 and 50 degree for delta14.

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arXiv2015

Consistent analysis of the $ν_μ\to ν_e$ 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 $ν_μ\to ν_e$ 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 $Δm^2 _{14}$ with the atmospheric one. The inclusion in the data analysis of such effects weakens the upper bounds on the effective appearance amplitude $\sin^2 2θ_{μe}$ approximately by a factor of two. In addition, we evidence that, in a 4-flavor scheme, the flavor oscillations involve also the $ν_e$ 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 $ν_e$ disappearance effects leads to a further weakening of the upper bounds on $\sin^2 2θ_{μe}$, which overall are relaxed by a factor of three with respect to those obtained in the effective 2-flavor description.

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arXiv2013

Constraints on very light sterile neutrinos from $θ_{13}$-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 $θ_{13}$ exploiting a multiple baseline comparison of $ν_e \to ν_e$ 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$ν$) 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 $θ_{13}$ 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$ν$'s on the rest of the neutrino oscillation phenomenology and emphasize their potential role in the cosmological "dark radiation" anomaly.

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arXiv2013

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.

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arXiv2012

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.

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arXiv2012

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.

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arXiv2011

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.

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arXiv2011

Hint of non-standard dynamics in solar neutrino conversion

Motivated by the recent low-threshold measurements of the solar 8B neutrino spectrum performed by Borexino, Super-Kamiokande and the Sudbury Neutrino Observatory -- all now monitoring the transition regime between low-energy (vacuum-like) and high-energy (matter-dominated) flavor conversions -- we consider the role of sub-dominant dynamical terms induced by new flavor-changing interactions. We find that the presence of such perturbations with strength ~ 0.1 G_F is now favored, offering a better description of the anomalous behavior suggested by the new results, whose spectrum shows no sign of the typical low-energy upturn predicted by the standard MSW mechanism. Our findings, if interpreted in a 2-flavor scheme, provide a hint of such new interactions at the ~ 2 sigma level, which is rather robust with respect to 3-flavor effects possibly induced by non-zero theta_13.

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