Neutrino Background Research Articles

Relic neutrino decay solution to the excess radio background

The excess radio background detected by ARCADE 2 represents a puzzle within the standard cosmological model. There is no clear viable astrophysical solution, and therefore, it might indicate the presence of new physics. Radiative decays of a relic neutrino ν_i (either i=1, or i=2, or i=3) into a sterile neutrino ν_ s, assumed to be quasi-degenerate, provide a solution that currently evades all constraints posed by different cosmological observations and reproduces very well the ARCADE 2 data. We find a very good fit to the ARCADE 2 data with best fit values τ_i = 1.46 × 10^21 s and Δ m_i = 4.0 × 10^-5 eV, where τ_i is the lifetime and Δ m_i is the mass difference between the decaying active neutrino and the sterile neutrino. On the other hand, if relic neutrino decays do not explain ARCADE 2 data, then these place a stringent constraint Δ m_i^3/2τ_i ≳ 2 × 10^14 eV^3/2 s in the range 1.4 × 10^-5 eV<Δ m_i < 2.5 × 10^-4 eV. The solution also predicts a stronger 21 cm absorption global signal than the predicted one from the ΛCDM model, with a contrast brightness temperature T_21 = -238^+21_-20 mK (99% C.L.) at redshift z≃ 17. This is in mild tension with the even stronger signal found by the EDGES collaboration, T_21 = - 500^+200_-500 mK, suggesting that this might have been overestimated, possibly receiving a contribution from some unidentified foreground source.

The neutrino background from non-jetted active galactic nuclei

Aims. We calculate the contribution to the neutrino background from the non-jetted active galactic nuclei (AGN) population following the recent IceCube association of TeV neutrinos with NGC 1068. Methods. We exploited our robust knowledge of the AGN X-ray luminosity function and evolution and converted it to the neutrino band by using NGC 1068 as a benchmark, together with a theoretically motivated neutrino spectrum. Results. The resulting neutrino background up to redshift 5 does not violate either the IceCube diffuse flux or the upper bounds for non-jetted AGN, although barely so. This is consistent with a scenario in which the latter class makes a substantial contribution mostly below 1 PeV, while jetted AGN, that is, blazars, dominate above this energy, in intriguing agreement with the dip in the neutrino data at ∼300 TeV. More and better IceCube data on Seyfert galaxies will allow us to constrain the fraction of neutrino emitters among non-jetted AGN.

Supernova Burst and Diffuse Supernova Neutrino Background Simulator for Water Cherenkov Detectors

If a Galactic core-collapse supernova explosion occurs in the future, it will be critical to rapidly alert the community to the direction of the supernova by utilizing neutrino signals in order to enable the initiation of follow-up optical observations. In addition, there is anticipation that observation of the diffuse supernova neutrino background will yield discoveries in the near future, given that experimental upper limits are approaching theoretical predictions. We have developed a new supernova event simulator for water Cherenkov neutrino detectors, such as the highly sensitive Super-Kamiokande. This simulator calculates the neutrino interaction in water for two simulation purposes, individual core-collapse supernova bursts and diffuse supernova neutrino background. Based on this simulator, we can evaluate the precision in determining the location of supernovae and estimate the expected number of events related to the diffuse supernova neutrino background in Super-Kamiokande. In this paper, we describe the basic structure of the simulator and its demonstration.

Spin-flavor oscillations of relic neutrinos in primordial magnetic field

Spin-flavor oscillations of relic neutrinos in primordial magnetic field

XENONnT and LUX-ZEPLIN constraints on DSNB-boosted dark matter

We consider a scenario in which dark matter particles are accelerated to semi-relativistic velocities through their scattering with the Diffuse Supernova Neutrino Background. Such a subdominant, but more energetic dark matter component can be then detected via its scattering on the electrons and nucleons inside direct detection experiments. This opens up the possibility to probe the sub-GeV mass range, a region of parameter space that is usually not accessible at such facilities. We analyze current data from the XENONnT and LUX-ZEPLIN experiments and we obtain novel constraints on the scattering cross sections of sub-GeV boosted dark matter with both nucleons and electrons. We also highlight the importance of carefully taking into account Earth's attenuation effects as well as the finite nuclear size into the analysis. By comparing our results to other existing constraints, we show that these effects lead to improved and more robust constraints.

Measuring neutrino mass and asymmetry with matter pairwise velocities

ABSTRACT Neutrinos are believed to be the most abundant fermions in the Universe, but their masses are unknown, except for being non-zero but much smaller than other fermions. Cosmological relic neutrinos could also have non-zero chemical potentials (or asymmetries). Using neutrino-involved N-body simulations, we investigate the neutrino effects on the matter pairwise velocity, which itself is an interesting probe of cosmology. We find that for light-halo ([1011, 1013] M⊙) mean pairwise velocity, in the transition range ([4, 15] Mpc), the effects of neutrino masses overwhelm the effects of neutrino asymmetries, while in the two-halo-group range ([25, 50] Mpc), for both light and heavy haloes ([1013, 1015] M⊙), the effects of neutrino asymmetries dominate, making it possible to disentangle the two effects. We provide fitting formulae to quantify the effects of neutrino mass and asymmetry on halo–halo pairwise velocities.

Solar neutrino background in high-pressure gaseous 82SeF6 TPC neutrinoless double beta decay experiments* *Supported in part by the National Natural Science Foundation of China (12105110) and National Key Research and Development Program of China (2022YFA1604703)

In this study, the possibility of observing a solar neutrino background in a future neutrinoless double beta decay experiment using a high-pressure gaseous 82SeF6 TPC is investigated. Various contributions are simulated, and possible features that could be used for event classification are discussed; two types of backgrounds are identified. The rate of multi-site background events is approximately 0.63 events/(ton·yr) in a 30 keV ROI window. This background could be effectively reduced to less than 0.0001 events/(ton·yr) (95% C.L.) while maintaining a high signal efficiency of 93% by applying a selection based on the number of clusters and energy of the leading cluster. The rate of the single-electron background events is approximately 0.01 events/(ton·yr) in the ROI. Assuming a reduction factor of 10 for the single-electron background events obtained via the algorithms developed for radioactive background rejection, the total background induced by the solar neutrino would be 0.001 events/(ton·yr), which is sufficiently small for conducting ton-level experiments.

Emergent Nucleosynthesis from a 1.2 s Long Simulation of a Black Hole Accretion Disk

We simulate a black hole accretion disk system with full-transport general relativistic neutrino radiation magnetohydrodynamics for 1.2 s. This system is likely to form after the merger of two compact objects and is thought to be a robust site of r-process nucleosynthesis. We consider the case of a black hole accretion disk arising from the merger of two neutron stars. Our simulation time coincides with the nucleosynthesis timescale of the r-process (∼1 s). Because these simulations are time-consuming, it is common practice to run for a “short” duration of approximately 0.1–0.3 s. We analyze the nucleosynthetic outflow from this system and compare the results of stopping at 0.12 and 1.2 s. We find that the addition of mass ejected in the longer simulation as well as more favorable thermodynamic conditions from emergent viscous ejecta greatly impacts the nucleosynthetic outcome. We quantify the error in nucleosynthetic outcomes between short and long cuts.

Oscillations of high-energy cosmic neutrinos in the copious MeV neutrino background

Oscillations of high-energy cosmic neutrinos in the copious MeV neutrino background

From Dirac to Majorana: The cosmic neutrino background capture rate in the minimally extended Standard Model

From Dirac to Majorana: The cosmic neutrino background capture rate in the minimally extended Standard Model

Diraxiogenesis

The family of Dirac Seesaw models offers an intriguing alternative explanation for the smallness of neutrino masses without necessarily requiring microscopic lepton number violation, when compared to the more familiar class of Majorana Seesaws. A global U(1)D symmetry, that is explicitly broken by a higher dimensional scalar operator, ensures that the right handed neutrino does not couple directly to the Standard Model like Higgs and an exact gauged or residual lepton number symmetry prohibits all Majorana masses. We demonstrate that all three Dirac Seesaws possess a Pseudo-Nambu-Goldstone boson associated with the U(1)D symmetry, that we call the Diraxion, whose cosmological dynamics have so far been left unexplored. Furthermore we illustrate that a Dirac-Leptogenesis version of the recently proposed Lepto-Axiogenesis scenario can be realized in this class of models, leading to a unified origin of the observed baryon asymmetry and dark matter relic abundance. Explaining only the baryon asymmetry can lead to potentially observable amounts of right handed neutrino dark radiation with ∆Neff. ≲ 0.028. On the other hand, if we only fix the dark matter abundance via the kinetic misalignment mechanism, this set-up could lead to detectable signatures in proposed cosmic neutrino background experiments via decays of eV-scale Diraxions to neutrinos. Here there is no domain wall problem, since topological defects decay to a subleading fraction of relic Diraxions. A key ingredient of all Axiogenesis scenarios is the dynamics of relatively light scalar called the Saxion, that in our case has a mass at the GeV-scale and which might reveal itself in heavy meson decays or collider searches. Our setup predicts isocurvature perturbations in baryons, dark matter and dark radiation sourced by fluctuations of the Saxion.

Can we explain cosmic birefringence without a new light field beyond Standard Model?

The recent analysis of the Planck 2018 polarization data shows a nonzero isotropic cosmic birefringence (ICB) that is not explained within the ΛCDM paradigm. We then explore the question of whether the nonzero ICB is interpreted by the framework of the Standard Model Effective Field Theory (SMEFT), or at the energy scales of the cosmic microwave background, the low-energy EFT (LEFT) whose dynamical degrees of freedom are five SM quarks and all neutral and charged leptons. Our systematic study reveals that any operator in the EFT on a cosmological background would not give the reported ICB angle, which is observationally consistent with frequency independence. In particular, we estimate the size of the ICB angle generated by the effect that the cosmic microwave background photons travel through the medium of the cosmic neutrino background with parity-violating neutrino-photon interactions and find that it would be too small to explain the data. If the reported ICB angle should be confirmed, then our result would indicate the existence of a new particle that is lighter than the electroweak scale and feebly interacting with the SM particles.

Measurement of the neutrino-oxygen neutral-current quasielastic cross section using atmospheric neutrinos in the SK-Gd experiment

We report the first measurement of the atmospheric neutrino-oxygen neutral-current quasielastic (NCQE) cross section in the gadolinium-loaded Super-Kamiokande (SK) water Cherenkov detector. In June 2020, SK began a new experimental phase, named SK-Gd, by loading 0.011% by mass of gadolinium into the ultrapure water of the SK detector. The introduction of gadolinium to ultrapure water has the effect of improving the neutron-tagging efficiency. Using a 552.2 day dataset from August 2020 to June 2022, we measure the NCQE cross section to be 0.74±0.22(stat)−0.15+0.85(syst)×10−38 cm2/oxygen in the energy range from 160 MeV to 10 GeV, which is consistent with the atmospheric neutrino-flux-averaged theoretical NCQE cross section and the measurement in the SK pure-water phase within the uncertainties. Furthermore, we compare the models of the nucleon-nucleus interactions in water and find that the binary cascade model and the Liège intranuclear cascade model provide a somewhat better fit to the observed data than the Bertini cascade model. Since the atmospheric neutrino-oxygen NCQE reactions are one of the main backgrounds in the search for diffuse supernova neutrino background (DSNB), these new results will contribute to future studies—and the potential discovery—of the DSNB in SK. Published by the American Physical Society 2024

Cosmic background neutrinos deflected by gravity: DEMNUni simulation analysis

The local supercluster acts as a gravity deflection source for cosmic background neutrinos. This deflection by gravity changes the neutrino helicity and therefore has important consequences for ground based tritium capture experiments aimed at determining if the neutrino is Dirac or Majorana. Here we explore the deflection effect of the local supercluster using two simulations from the DEMNUni suite characterised by very different mass resolutions, as they are both filled with 20483 dark matter particles (and an equal number of massive neutrino particles) but have comoving volumes of (2 h -1Gpc)3 and (500 h -1Mpc)3, respectively. We reaffirm our previous results and show that the lightest neutrinos are ultra-relativistic enough to suffer little deflection by gravity and at the same time not relativistic enough to achieve the same capture rate for Dirac and Majorana cases. This means that the capture rate in Ptolemy-like experiments will be sensitive to the neutrino nature and that gravity deflection enlarges the difference between Majorana and Dirac rates. Moreover, using the relation between mass and momentum of the neutrinos frozen Fermi-Dirac distribution, but taking into consideration gravity corrections, we are able to calculate the deflection angle for different neutrino masses from the same set of neutrinos obtained from the simulation. Doing so, we provide a formula to compute the deflection angle for any neutrino mass, such that when cosmology detects an absolute neutrino mass, precise predictions can be made for tritium ground-based detectors on Earth aimed to determine neutrinos nature.

The Milky Way revealed to be a neutrino desert by the IceCube Galactic plane observation

The Galactic diffuse emission (GDE) is formed when cosmic rays leave the sources where they were accelerated, diffusively propagate in the Galactic magnetic field and interact with the interstellar medium and interstellar radiation field. GDE in γ-rays (GDE-γ) has been observed up to subpetaelectronvolt energies, although its origin may be explained by either cosmic-ray nuclei or electrons. Here we show that the γ-rays accompanying the high-energy neutrinos recently observed by the IceCube Observatory from the Galactic plane have a flux that is consistent with the GDE-γ observed by the Fermi-LAT and Tibet ASγ experiments around 1 TeV and 0.5 PeV, respectively. The consistency suggests that the diffuse γ-ray emission above ~1 TeV could be dominated by hadronuclear interactions, although a partial leptonic contribution cannot be excluded. Moreover, by comparing the fluxes of the Galactic and extragalactic diffuse emission backgrounds, we find that the neutrino luminosity of the Milky Way is one-to-two orders of magnitude lower than the average of distant galaxies. This finding implies that our Galaxy has not hosted the type of neutrino emitters that dominates the isotropic neutrino background at least in the past few tens of kiloyears.

Gravitational wave memory produced by cosmic background radiation

It is well known that energy fluxes will produce gravitational wave memory. The gravitational wave memory produced by background including cosmic microwave background (CMB), cosmic neutrino background (CνB), and gravitational wave background is investigated in this work. We construct a theory relating the gravitational wave memory strength to the energy flux of a stochastic background. We find that the resulted gravitational wave memory behaves as a constantly varying metric tensor. Such a varying metric tensor will introduce a quadrupole structure to the universe expansion. The gravitational wave memory due to the CMB is too small to be detected. But the gravitational wave memory due to the CνB and the gravitational wave background is marginally detectable. Interestingly, such detection can be used to estimate the neutrino masses and the properties of the gravitational wave background.

Influence of local structure on relic neutrino abundances and anisotropies

Gravitational potentials of the Milky Way and extragalactic structures can influence the propagation of the cosmic neutrino background (CNB). Of particular interest to future CNB observatories, such as PTOLEMY, is the CNB number density on Earth. In this study, we have developed a simulation framework that maps the trajectories of relic neutrinos as they move through the local gravitational environment. The potentials are based on the dark matter halos found in state-of-the-art cosmological N-body simulations, resulting in a more nuanced and realistic input than the previously employed analytical models. We find that the complex dark matter distributions, along with their dynamic evolution, influence the abundance and anisotropies of the CNB in ways unaccounted for by earlier analytical methods. Importantly, these cosmological simulations contain multiple instances of Milky Way-like halos that we employ to model a variety of gravitational landscapes. Consequently, we notice a variation in the CNB number densities that can be primarily attributed to the differences in the masses of these individual halos. For neutrino masses between 0.01 and 0.3 eV, we note clustering factors within the range of 1 + 𝒪(10-3) to 1 + 𝒪(1). Furthermore, the asymmetric nature of the underlying dark matter distributions within the halos results in not only overdense, but intriguingly, underdense regions within the full-sky anisotropy maps. Gravitational clustering appears to have a significant impact on the angular power spectra of these maps, leading to orders of magnitude more power on smaller scales beyond multipoles of ℓ = 3 when juxtaposed against predictions by primordial fluctuations. We discuss how our results reshape our understanding of relic neutrino clustering and how this might affect observability of future CNB observatories such as PTOLEMY.GitHub: our simulation code will be made visible here.

Neutrinos from GRB 221009A: producing ALPs and explaining LHAASO anomalous γ event

We propose a novel explanation for the 18 TeV gamma ray from GRB 221009A observed by LHAASO. High-energy neutrinos are converted into axion-like particles (ALPs) via their interaction with the cosmic neutrino background. Subsequently, ALPs are converted into high-energy photons in the magnetic field of our galaxy. We compute the fluxes of neutrinos, ALPs, and photons reaching Earth. IceCube's constraints on the neutrino flux from GRB 221009A translate into a severe upper bound on the photon flux. We find a range of parameters where all existing bounds are satisfied and the 18 TeV LHAASO photon can be explained. In the future, the specific correlation between the photon and neutrino flux reaching Earth from powerful neutrino sources with energies larger than 10 TeV such as GRBs or AGNs, can be used as a tool to differentiate our explanation from the alternatives suggested in the literature. We discuss how the interactions of our scenario can be embedded within electroweak gauge-invariant models, avoiding various cosmological and terrestrial bounds. We comment on the possibility of explaining the 251 TeV photon observed by the Carpet-2 detector, taking into account the bounds from the observation of high-energy neutrinos from TXS 0506+056.

Neutrino winds on the sky

We develop a first-principles formalism to compute the distortion to the relic neutrino density field caused by the peculiar motions of large-scale structures. This distortion slows halos down due to dynamical friction, causes a local anisotropy in the neutrino-CDM cross-correlation, and reduces the global cross-correlation between neutrinos and CDM. The local anisotropy in the neutrino-CDM cross-spectrum is imprinted in the three point cross-correlations of matter and galaxies, or the bispectrum in Fourier space, producing a signal peaking at squeezed triangle configurations. This bispectrum signature of neutrino masses is not limited by cosmic variance or potential inaccuracies in the modeling of complicated nonlinear and galaxy formation physics, and it is not degenerate with the optical depth to reionization. We show that future surveys have the potential to detect the distortion bispectrum.

Neff constraints on light mediators coupled to neutrinos: the dilution-resistant effect

We investigate the impact of new light particles, carrying significant energy in the early universe after neutrino decoupling, on the cosmological effective relativistic neutrino species, Neff. If the light particles are produced from decoupled neutrinos, Neff is predominantly modified through the dilution-resistant effect. This effect arises because the energy stored in the mass of new particles is less diluted than the photon and neutrino energy as the universe expands. Our study comprehensively explores this effect, deriving Neff constraints on the couplings of light mediators with neutrinos, encompassing both scalar and vector mediators. We find that the dilution-resistant effect can increase Neff by 0.118 and 0.242 for scalar and vector mediators, respectively. These values can be readily reached by forthcoming CMB experiments. Upon reaching these levels, future Neff constraints on the couplings will be improved by many orders of magnitude.