Vedran Brdar

Neutrino telescopes have detected astrophysical neutrinos with energies up to 𝒪(100) PeV. Several current and proposed experiments aim to observe neutrinos at even higher energies, with the goal of detecting cosmogenic neutrinos. This increase in neutrino energy makes tests of Lorentz invariance violation (LIV) particularly appealing, since the effects of higher-dimension LIV operators on neutrino propagation grow rapidly with energy. In this work, we investigate the potential of the upcoming experiments GRAND and POEMMA to probe LIV in the neutrino sector through the detection of ultra-high-energy tau neutrinos. We generate the cosmogenic neutrino flux using SimProp and interface it with a calculation of neutrino flavor transition probabilities in the presence of LIV effects. Deviations from standard flavor transition probabilities manifest as changes in the expected tau neutrino event rates at GRAND and POEMMA. We first consider the case with a single nonzero LIV operator of various dimensions, and find that the projected sensitivities exceed existing limits from lower-energy probes by orders of magnitude. We then explore scenarios with multiple nonzero LIV parameters and show that their interplay can significantly modify the sensitivities compared to the single-parameter case. Overall, we find that upcoming observations of ultra-high-energy tau neutrinos will place some of the most stringent constraints on LIV.

A bstract: Violations of classicality can be probed through measurements performed on a system at different times, as proposed by Leggett and Garg. Specifically, violations of Leggett-Garg inequalities suggest the presence of quantum effects in macroscopic systems. Long-baseline neutrino experiments provide some of the longest available propagation distances over which such tests can be performed. Previous studies of Leggett-Garg tests in the neutrino sector have largely focused on showing that the oscillation probabilities can violate classical bounds for certain parameter choices. In this work, we develop a more complete and data-driven framework that treats both the distributions representing the classical and quantum behavior, as well as the experimental uncertainties. We consider MINOS, T2K, NOvA, as well as the upcoming DUNE, and present the respective statistical significance for distinguishing quantum behavior from classical scenarios at these long-baseline neutrino experiments. Among them, we find that T2K yields the most significant violation of classicality, at the level of ∼ 14 σ , with NOvA and projections for DUNE also resulting in a significance of more than 5 σ .

In light-shining-through-walls experiments, axions and axion-like particles (ALPs) are searched for by exposing an optically thick barrier to a laser beam. In a magnetic field, photons could convert into ALPs in front of the barrier and reconvert behind it, giving rise to a signal that can occur only in the presence of such hidden particles. In this work, we utilize the light-shining-through-walls concept and apply it to astrophysical scales. Namely, we consider eclipsing binary systems, consisting of a neutron star, which is a bright source of X-rays, and a companion star with a much larger radius. Space observatories such as XMM-Newton and NuSTAR have performed extensive measurements of such systems, obtaining data on both out-of-eclipse photon rates and those during eclipses. The latter are typically 𝒪(10 ² -10 ³ ) times smaller, due to the fact that X-rays propagating along the line of sight from the neutron star to the X-ray observatory do not pass through the barrier that is the companion star. Using this attenuation, we derive a constraint on ALP-photon coupling of g _aγ ≤ 1.44 × 10 ^-10 GeV ^-1 (at 90% C.L.) for the LMC X-4 eclipsing binary system, surpassing current bounds from light-shining-through-walls experiments. We also present future prospects that could realistically improve this limit by an order of magnitude in g_aγ, making it competitive with some of the strongest limits derived to date.

The KM3NeT collaboration recently reported the observation of KM3-230213A, a neutrino event with an energy exceeding 100 PeV, more than an order of magnitude higher than the most energetic neutrino in IceCube’s catalog. Given its longer data-taking period and larger effective area relative to KM3NeT, IceCube should have observed events around that energy. This tension has recently been quantified to lie between 2 σ and 3.5 σ , depending on the neutrino source. A O ( 100 ) PeV neutrino detected at KM3NeT has traversed approximately 147 km of rock and sea en route to the detector, whereas neutrinos arriving from the same location in the sky would have only traveled through about 14 km of ice before reaching IceCube. We use this difference in propagation distance to address the tension between KM3NeT and IceCube. Specifically, we consider a scenario in which the source emits sterile neutrinos that partially convert to active neutrinos through oscillations. We scrutinize two such realizations, one where a new physics matter potential induces a resonance in sterile-to-active transitions and another one where off-diagonal neutrino nonstandard interactions are employed. In both cases, sterile-to-active neutrino oscillations become relevant at length scales of ∼ 100 km , resulting in increased active neutrino flux near the KM3NeT detector, alleviating the tension between KM3NeT and IceCube. Overall, we propose the exciting possibility that neutrino telescopes may have started detecting new physics.

We propose heavy decaying dark matter (DM) as a new probe of the cosmic neutrino background ( C ν B ). Heavy DM, with mass ≳ 10 9 GeV , decaying into neutrinos can be a new source of ultrahigh-energy (UHE) neutrinos. Including this contribution along with the measured astrophysical and predicted cosmogenic neutrino fluxes, we study the scattering of UHE neutrinos with the C ν B via standard weak interactions mediated by the Z boson. We solve the complete neutrino transport equation, taking into account both absorption and reinjection effects, to calculate the expected spectrum of UHE neutrino flux at future neutrino telescopes, such as the IceCube-Gen2 radio. We argue that such observations can be used to probe the C ν B properties and, in particular, local C ν B clustering. We find that, depending on the absolute neutrino mass and the DM mass and lifetime, a local C ν B overdensity ≳ 10 6 can be probed at the IceCube-Gen2 radio within ten years of data taking.

In this work, motivated by several beyond the Standard Model signal topologies, we perform detailed background mitigation analyses for the DUNE near detector. Specifically, we investigate e + e − , e − γ , γ , and γ γ final states that may arise from long-lived particles, including light mediators, dark matter, heavy neutral leptons, and axionlike particles (ALPs), decaying or scattering inside the liquid argon detector. To this end, we employ both photophilic and leptophilic ALPs as phenomenological benchmarks. The aforementioned final states leave a hard electromagnetic signature with no hadronic activity above the detector energy thresholds. Nevertheless, such signatures are not immune to backgrounds from neutrino scattering in the detector, which are in the focus of our study. In order to model realistic experimental analyses, we take into account particle misidentification rates, cross-contamination effects, and detector responses. We calculate confidence limit projections for DUNE, thereby presenting realistic capabilities for constraining or discovering new physics manifested through electromagnetic showers.