eV-Scale Sterile Neutrino Search Using Eight Years of Atmospheric Muon Neutrino Data from the IceCube Neutrino Observatory

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

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, 13.6×10^{20} protons on target, doubles that previously analyzed by NOvA, and the analysis is the first to use ν_{μ} 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 Δm_{41}^{2} range extending 2 (3) orders of magnitude above (below) 1  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 ν_{τ} appearance for Δm_{41}^{2}≤3  eV^{2}.

Searches for beyond-standard-model physics with astroparticle physics instruments

Many instruments for astroparticle physics are primarily geared towards multi-messenger astrophysics, to study the origin of cosmic rays and to understand high-energy astrophysical processes. Since these instruments observe the Universe at extreme energies and in kinematic ranges not accessible at accelerators these experiments provide also unique and complementary opportunities to search for particles and physics beyond the standard model of particle physics. In particular, the reach of IceCube, Fermi and KATRIN to search for and constrain Dark Matter, Axions, heavy Big Bang relics, sterile neutrinos and Lorentz invariance violation will be discussed. The contents of this article are based on material presented at the Humboldt-Kolleg 'Clues to a mysterious Universe-exploring the interface of particle, gravity and quantum physics' in June 2022. This article is part of the theme issue 'The particle-gravity frontier'.

Evolution of Bulk Nanobubbles under Different Freezing Conditions

The freezing process of aqueous solutions plays a crucial role in various applications including cryopreservation, glaciers, and frozen materials. However, less research has focused on the influence of nanoscale gas bubble formation or collapse in water during freezing, which may significantly impact the formation of ice crystals. Herein, we report for the first time that the freezing process can produce nanobubbles in aqueous solutions, and their size and number concentration could be changed by different cooling rates, i.e., the size would decrease as the cooling rate increased, and the maximum number concentration was found at the -80 °C system. Furthermore, increasing the dissolved gas content in the solution enhanced the production of nanobubbles, whereas for preexisting nanobubbles, the freezing resulted in a decrease in their number concentration, which was negatively correlated with the cooling rate. Our results indicated that a moderate cooling rate of -80 °C favored nanobubble generation, whereas a higher cooling rate was preferable for maintaining preexisting nanobubbles. Conversely, a lower cooling rate could be employed to eliminate preexisting nanobubbles. This study explored the evolution and stability of nanobubbles during the freezing process, providing valuable insights into the application or elimination of nanobubbles.

First Constraints on Light Sterile Neutrino Oscillations from Combined Appearance and Disappearance Searches with the MicroBooNE Detector

We present a search for eV-scale sterile neutrino oscillations in the MicroBooNE liquid argon detector, simultaneously considering all possible appearance and disappearance effects within the 3+1 active-to-sterile neutrino oscillation framework. We analyze the neutrino candidate events for the recent measurements of charged-current ν_{e} and ν_{μ} interactions in the MicroBooNE detector, using data corresponding to an exposure of 6.37×10^{20} protons on target from the Fermilab booster neutrino beam. We observe no evidence of light sterile neutrino oscillations and derive exclusion contours at the 95% confidence level in the plane of the mass-squared splitting Δm_{41}^{2} and the sterile neutrino mixing angles θ_{μe} and θ_{ee}, excluding part of the parameter space allowed by experimental anomalies. Cancellation of ν_{e} appearance and ν_{e} disappearance effects due to the full 3+1 treatment of the analysis leads to a degeneracy when determining the oscillation parameters, which is discussed in this Letter and will be addressed by future analyses.

Search for Unstable Sterile Neutrinos with the IceCube Neutrino Observatory

We present a search for an unstable sterile neutrino by looking for a resonant signal in eight years of atmospheric ν_{μ} data collected from 2011 to 2019 at the IceCube Neutrino Observatory. Both the (stable) three-neutrino and the 3+1 sterile neutrino models are disfavored relative to the unstable sterile neutrino model, though with p values of 2.8% and 0.81%, respectively, we do not observe evidence for 3+1 neutrinos with neutrino decay. The best-fit parameters for the sterile neutrino with decay model from this study are Δm_{41}^{2}=6.7_{-2.5}^{+3.9}  eV^{2}, sin^{2}2θ_{24}=0.33_{-0.17}^{+0.20}, and g^{2}=2.5π±1.5π, where g is the decay-mediating coupling. The preferred regions of the 3+1+decay model from short-baseline oscillation searches are excluded at 90% C.L.

Sterile Neutrino Search with MicroBooNE's Electron Neutrino Disappearance Data

A sterile neutrino is a well motivated minimal new physics model that leaves an imprint in neutrino oscillations. Over the last two decades, a number of hints pointing to a sterile neutrino have emerged, many of which are pointing near m_{4}∼1  eV. Here, we show how MicroBooNE data can be used to search for electron neutrino disappearance using each of their four analysis channels. We find a hint for oscillations with the highest single channel significance of 2.4σ (using the Feldman-Cousins approach) coming from the Wire-Cell analysis and a simplified treatment of the experimental systematics. The preferred parameters are sin^{2}(2θ_{14})=0.35_{-0.16}^{+0.19} and Δm_{41}^{2}=1.25_{-0.39}^{+0.74}  eV^{2}. This region of parameter space is in good agreement with existing hints from source experiments, is at a similar frequency but higher mixing than indicated by reactor antineutrinos, and is at the edge of the region allowed by solar neutrino data. Existing unanalyzed data from MicroBooNE could increase the sensitivity to the >3σ level.

Strong Constraints on Neutrino Nonstandard Interactions from TeV-Scale ν_{μ} Disappearance at IceCube

We report a search for nonstandard neutrino interactions (NSI) using eight years of TeV-scale atmospheric muon neutrino data from the IceCube Neutrino Observatory. By reconstructing incident energies and zenith angles for atmospheric neutrino events, this analysis presents unified confidence intervals for the NSI parameter ε_{μτ}. The best-fit value is consistent with no NSI at a p value of 25.2%. With a 90% confidence interval of -0.0041≤ε_{μτ}≤0.0031 along the real axis and similar strength in the complex plane, this result is the strongest constraint on any NSI parameter from any oscillation channel to date.

MicroBooNE and the ν_{e} Interpretation of the MiniBooNE Low-Energy Excess

A new generation of neutrino experiments is testing the 4.7σ anomalous excess of electronlike events observed in MiniBooNE. This is of huge importance for particle physics, astrophysics, and cosmology, not only because of the potential discovery of physics beyond the standard model, but also because the lessons we will learn about neutrino-nucleus interactions will be crucial for the worldwide neutrino program. MicroBooNE has recently released results that appear to disfavor several explanations of the MiniBooNE anomaly. Here, we show quantitatively that MicroBooNE results, while a promising start, unquestionably do not probe the full parameter space of sterile neutrino models hinted at by MiniBooNE and other data, nor do they probe the ν_{e} interpretation of the MiniBooNE excess in a model-independent way.

Two Sides of the Same Coin: Sterile Neutrinos and Dark Radiation, Status and Perspectives

The presence of light sterile neutrinos is one of the unanswered questions of particle physics. The cosmological counterpart is represented by dark radiation, i.e., any form of radiation present in the early Universe besides photons and standard (active) neutrinos. This short review provides a comprehensive overview of the two problems and of their connection. We review the status of neutrino oscillation anomalies, commenting on the most recent oscillation data and their mutual tensions, and we discuss the constraints from other terrestrial probes. We show the shortcomings of translating light sterile neutrinos in cosmology as additional thermalised relativistic species, produced by neutrino oscillations, and we detail alternative solutions, specifically focusing on neutrino nonstandard interactions, and on their link to the Hubble constant problem. The impact of a new force leading to dark radiation–dark matter interactions is also discussed in the realm of new physics in the dark sector.