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1. Dual-Baseline Search for Active-to-Sterile Neutrino Oscillations in NOvA

   https://doi.org/10.1103/PhysRevLett.134.081804  

   Acero, M A; Acharya, B; Adamson, P; Anfimov, N; Antoshkin, A; Arrieta-Diaz, E; Asquith, L; Aurisano, A; Back, A; Balashov, N; et al (February 2025, Physical Review Letters)

   We report a search for neutrino oscillations to sterile neutrinos under a model with three active and one sterile neutrinos ( $3+1$model). This analysis uses the NOvA detectors exposed to the NuMI beam, running in neutrino mode. The data exposure, $13.6\times {10}^{20}$protons on target, doubles that previously analyzed by NOvA, and the analysis is the first to use ${\nu }_{\mu }$charged-current interactions in conjunction with neutral-current interactions. Neutrino samples in the near and far detectors are fitted simultaneously, enabling the search to be carried out over a $\mathrm{\Delta }{m}_{41}^2$range extending 2 (3) orders of magnitude above (below) $1{\mathrm{eV}}^2$. NOvA finds no evidence for active-to-sterile neutrino oscillations under the $3+1$model at 90% confidence level. New limits are reported in multiple regions of parameter space, excluding some regions currently allowed by IceCube at 90% confidence level. We additionally set the most stringent limits for anomalous ${\nu }_{\tau }$appearance for $\mathrm{\Delta }{m}_{41}^2\le 3{\mathrm{eV}}^2$. Published by the American Physical Society2025 

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2. Search for an eV-Scale Sterile Neutrino Using Improved High-Energy ${\nu }_{\mu }$ Event Reconstruction in IceCube

   https://doi.org/10.1103/PhysRevLett.133.201804  

   Abbasi, R; Ackermann, M; Adams, J; Agarwalla, S K; Aguilar, J A; Ahlers, M; Alameddine, J M; Amin, N M; Andeen, K; Argüelles, C; et al (November 2024, Physical Review Letters)

   This Letter presents the result of a $3+1$sterile neutrino search using 10.7 yr of IceCube data. We analyze atmospheric muon neutrinos that traverse the Earth with energies ranging from 0.5 to 100 TeV, incorporating significant improvements in modeling neutrino flux and detector response compared to earlier studies. Notably, for the first time, we categorize data into starting and throughgoing events, distinguishing neutrino interactions with vertices inside or outside the instrumented volume, to improve energy resolution. The best-fit point for a $3+1$model is found to be at ${\sin }^2\left(2{\theta }_{24}\right)=0.16$and $\mathrm{\Delta }{m}_{41}^2=3.5{\mathrm{eV}}^2$, which agrees with previous iterations of this Letter. The result is consistent with the null hypothesis of no sterile neutrinos with a $p$value of 3.1%. Published by the American Physical Society2024 

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3. Search for a light sterile neutrino with 7.5 years of IceCube DeepCore data

   https://doi.org/10.1103/PhysRevD.110.072007  

   Abbasi, R; Ackermann, M; Adams, J; Agarwalla, S K; Aguilar, J A; Ahlers, M; Alameddine, J M; Amin, N M; Andeen, K; Argüelles, C; et al (October 2024, Physical Review D)

   We present a search for an eV-scale sterile neutrino using 7.5 years of data from the IceCube DeepCore detector. The analysis uses a sample of 21,914 events with energies between 5 and 150 GeV to search for sterile neutrinos through atmospheric muon neutrino disappearance. Improvements in event selection and treatment of systematic uncertainties provide greater statistical power compared to previous DeepCore sterile neutrino searches. Our results are compatible with the absence of mixing between active and sterile neutrino states, and we place constraints on the mixing matrix elements $|U_{\mu 4}{|}^2<0.0534$and $|U_{\tau 4}{|}^2<0.0574$at 90% CL under the assumption that $\mathrm{\Delta }{m}_{41}^2\ge 1{\mathrm{eV}}^2$. These null results add to the growing tension between anomalous appearance results and constraints from disappearance searches in the $3+1$sterile neutrino landscape. Published by the American Physical Society2024 

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4. Constraints on metastable superheavy dark matter coupled to sterile neutrinos with the Pierre Auger Observatory

   https://doi.org/10.1103/PhysRevD.109.L081101  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, G A; et al (April 2024, Physical Review D)

   Dark matter particles could be superheavy, provided their lifetime is much longer than the age of the Universe. Using the sensitivity of the Pierre Auger Observatory to ultrahigh energy neutrinos and photons, we constrain a specific extension of the Standard Model of particle physics that meets the lifetime requirement for a superheavy particle by coupling it to a sector of ultralight sterile neutrinos. Our results show that, for a typical dark coupling constant of 0.1, the mixing angle ${\theta }_m$between active and sterile neutrinos must satisfy, roughly, ${\theta }_m\lesssim 1.5\times {10}^{-6}(M_X/{10}^9\mathrm{GeV}{)}^{-2}$for a mass $M_X$of the dark-matter particle between ${10}^8\mathrm{GeV}$and ${10}^{11}\mathrm{GeV}$. Published by the American Physical Society2024 

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5. Solar neutrinos and ${\nu }_2$ visible decays to ${\nu }_1$

   https://doi.org/10.1103/PhysRevD.109.013003  

   de_Gouvêa, André; Weill, Jean; Sen, Manibrata (January 2024, Physical Review D)

   Experimental bounds on the neutrino lifetime depend on the nature of the neutrinos and the details of the potentially new physics responsible for neutrino decay. In the case where the decays involve active neutrinos in the final state, the neutrino masses also qualitatively impact how these manifest themselves experimentally. In order to further understand the impact of nonzero neutrino masses, we explore how observations of solar neutrinos constrain a very simple toy model. We assume that neutrinos are Dirac fermions and there is a new massless scalar that couples to neutrinos such that a heavy neutrino— ${\nu }_2$with mass $m_2$—can decay into a lighter neutrino— ${\nu }_1$with mass $m_1$—and a massless scalar. We find that the constraints on the new physics coupling depend, sometimes significantly, on the ratio of the daughter-to-parent neutrino masses and that, for large-enough values of the new physics coupling, the “dark side” of the solar neutrino parameter space— ${\sin }^2{\theta }_{12}\sim 0.7$—provides a reasonable fit to solar neutrino data, if only ${}^8\mathrm{B}$or ${}^7\mathrm{Be}$neutrino data alone are considered, but no allowed region is found in the combined analysis. Our results generalize to other neutrino-decay scenarios, including those that mediate ${\nu }_2\to {\nu }_1{\overline{\nu }}_3{\nu }_3$when the neutrino mass ordering is inverted mass and $m_2>m_1\gg m_3$, the mass of ${\nu }_3$. Published by the American Physical Society2024 

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