Atmospheric Neutrino Background Research Articles

JUNO sensitivity to the annihilation of MeV dark matter in the galactic halo

We discuss JUNO sensitivity to the annihilation of MeV dark matter in the galactic halo via detecting inverse beta decay reactions of electron anti-neutrinos resulting from the annihilation. We study possible backgrounds to the signature, including the reactor neutrinos, diffuse supernova neutrino background, charged- and neutral-current interactions of atmospheric neutrinos, backgrounds from muon-induced fast neutrons and cosmogenic isotopes. A fiducial volume cut, as well as the pulse shape discrimination and the muon veto are applied to suppress the above backgrounds. It is shown that JUNO sensitivity to the thermally averaged dark matter annihilation rate in 10 years of exposure would be significantly better than the present-day best limit set by Super-Kamiokande and would be comparable to that expected by Hyper-Kamiokande.

Search for Cosmic-Ray Boosted Sub-GeV Dark Matter Using Recoil Protons at Super-Kamiokande.

We report a search for cosmic-ray boosted dark matter with protons using the 0.37 megaton×years data collected at Super-Kamiokande experiment during the 1996-2018period (SKI-IV phase). We searched for an excess of proton recoils above the atmospheric neutrino background from the vicinity of the Galactic Center. No such excess is observed, and limits are calculated for two reference models of dark matter with either a constant interaction cross section or through a scalar mediator. This is the first experimental search for boosted dark matter with hadrons using directional information. The results present the most stringent limits on cosmic-ray boosted dark matter and exclude the dark matter-nucleon elastic scattering cross section between 10^{-33}cm^{2} and 10^{-27}cm^{2} for dark matter mass from 1 MeV/c^{2} to 300 MeV/c^{2}.

Combined significance of spatial coincidence of high energy neutrinos from PSR B1509-58 by Super-Kamiokande and MACRO

In their searches for astrophysical point sources of high energy neutrinos, both the Super-Kamiokande and MACRO neutrino detectors saw the largest angular excess from the same source, viz. PSR B1509-58. We estimate the probability for the observed number of events by both Super-Kamiokande and MACRO to be a chance coincidence due to atmospheric neutrino background. We find that this probability is about 0.4%, corresponding to 2.6σ significance. We also propose some additional tests to ascertain if this excess corresponds to an astrophysical signal or is only a background event.

Search for neutrino emission from cores of active galactic nuclei

The sources of the majority of the high-energy astrophysical neutrinos observed with the IceCube neutrino telescope at the South Pole are unknown. So far, only a gamma-ray blazar was compellingly associated with the emission of high-energy neutrinos. In addition, several studies suggest that the neutrino emission from the gamma-ray blazar population only accounts for a small fraction of the total astrophysical neutrino flux. In this work we probe the production of high-energy neutrinos in the cores of Active Galactic Nuclei (AGN), induced by accelerated cosmic rays in the accretion disk region. We present a likelihood analysis based on eight years of IceCube data, searching for a cumulative neutrino signal from three AGN samples created for this work. The neutrino emission is assumed to be proportional to the accretion disk luminosity estimated from the soft X-ray flux. Next to the observed soft X-ray flux, the objects for the three samples have been selected based on their radio emission and infrared color properties. For the largest sample in this search, an excess of high-energy neutrino events with respect to an isotropic background of atmospheric and astrophysical neutrinos is found, corresponding to a post-trial significance of 2.60$\sigma$. Assuming a power-law spectrum, the best-fit spectral index is $2.03^{+0.14}_{-0.11}$, consistent with expectations from particle acceleration in astrophysical sources. If interpreted as a genuine signal with the assumptions of a proportionality of X-ray and neutrino fluxes and a model for the sub-threshold flux distribution, this observation implies that at 100 TeV, 27$\%$ - 100$\%$ of the observed neutrinos arise from particle acceleration in the core of AGN.

Multiband Emission up to PeV Energy from the Crab Nebula in a Spatially Dependent Lepto-hadronic Model

Abstract Multiband emissions from radio to ultra-high-energy gamma-rays in the Crab Nebula have been detected. To explain the observed results, nonthermal photon production in the Crab Nebula is carefully studied in a spatially dependent lepto-hadronic model. In our model, the dynamical evolution of the pulsar wind nebula (PWN) is simulated in a spherically symmetric system. Both electrons and protons are accelerated at the termination shock. The relevant particle propagation equations, as well as the photon evolving equation, are simultaneously solved. For the Crab Nebula, our results reveal that the observed multiband photon spectra can be reproduced with reasonable model parameters. In particular, the photons with energy ≳200 TeV are mainly contributed to by the hadronic component via proton–proton interaction. The contribution of the hadronic component depends on both proton spectral index α p and number density n H of the medium within the PWN. Additionally, high-energy neutrino fluxes are predicted with variable proton spectral indices. The predicted fluxes are not only far below the sensitivities of current neutrino observatories but also beneath the atmospheric neutrino background with energy less than ∼40 TeV. Moreover, the calculated radial profiles of surface brightness and spectral index are presented.

Sensitivity estimates for diffuse, point-like, and extended neutrino sources with KM3NeT/ARCA

The identification of cosmic objects emitting high energy neutrinos could provide new insights about the Universe and its active sources. The existence of these cosmic neutrinos has been proven by the IceCube Collaboration, but the big question of which sources these neutrinos originate from, remains unanswered. The KM3NeT detector for Astroparticle Research with Cosmics in the Abyss (ARCA), with a cubic kilometer instrumented volume, is currently being built in the Mediterranean Sea. It will excel at identifying cosmic neutrino sources due to its unprecedented angular resolution for muon neutrinos (< 0.2 degree for E > 10 TeV events). KM3NeT has a view of the sky complementary to IceCube, and is sensitive to neutrinos across a wide range of energies. In order to identify the signature of cosmic neutrino sources in the background of atmospheric neutrinos and muons, statistical methods are being developed and tested with Monte-Carlo pseudo-experiments. This contribution presents the most recent sensitivity estimates for diffuse, point-like, and extended neutrino sources with KM3NeT/ARCA.

Astrophysical neutrino source searches with IceCube starting track events

Muons and neutrinos created by cosmic rays interacting in the atmosphere create a significant background for IceCube astrophysical neutrinos in the Southern Sky. However, looking for neutrino events that start in the detector can reduce both the atmospheric muon and atmospheric neutrino background while retaining the astrophysical neutrino signal. The method presented here results in a higher astrophysical neutrino purity for IceCube events at declinations less than -25°. We specifically select for track events, which results in better directional reconstruction due to the long signature the muon leaves in the detector than for cascade events. Due to the improved signal-background ratio and good pointing resolution, we will discuss how this event selection will improve the IceCube sensitivity to southern neutrino sources. We will focus on its impact on searches for galactic plane point sources and diffuse galactic plane neutrino emission. This selection also allows IceCube to send out high astrophysical purity realtime alert events with neutrino energies in the tens of TeV to the multimessenger community.

Neutron-antineutron oscillation search using a 0.37 megaton-years exposure of Super-Kamiokande

As a baryon number violating process with ΔB=2, neutron-antineutron oscillation (n→n¯) provides a unique test of baryon number conservation. We have performed a search for n→n¯ oscillation with bound neutrons in Super-Kamiokande, with the full dataset from its first four run periods, representing an exposure of 0.37 Mton−years. The search used a multivariate analysis trained on simulated n→n¯ events and atmospheric neutrino backgrounds and resulted in 11 candidate events with an expected background of 9.3 events. In the absence of statistically significant excess, we derived a lower limit on n¯ appearance lifetime in O16 nuclei of 3.6×1032 years and on the neutron-antineutron oscillation time of τn→n¯>4.7×108 s at 90% C.L.1 MoreReceived 6 December 2020Accepted 24 December 2020DOI:https://doi.org/10.1103/PhysRevD.103.012008Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBaryogenesis & leptogenesisHadron mixingPhysical SystemsNeutronsPropertiesC-symmetryTechniquesArtificial neural networksCherenkov detectorsParticles & Fields

Search for proton decay via p→e+π0 and p→μ+π0 with an enlarged fiducial volume in Super-Kamiokande I-IV

We have searched for proton decay via $p\to e^+\pi^0$ and $p\to \mu^+\pi^0$ modes with the enlarged fiducial volume data of Super-Kamiokande from April 1996 to May 2018, which corresponds to 450 kton$\cdot$years exposure. We have accumulated about 25% more livetime and enlarged the fiducial volume of the Super-Kamiokande detector from 22.5 kton to 27.2 kton for this analysis, so that 144 kton$\cdot$years of data, including 78 kton$\cdot$years of additional fiducial volume data, has been newly analyzed. No candidates have been found for $p\to e^+\pi^0$ and one candidate remains for $p\to \mu^+\pi^0$ in the conventional 22.5 kton fiducial volume and it is consistent with the atmospheric neutrino background prediction. We set lower limits on the partial lifetime for each of these modes: $\tau/B(p\to e^+\pi^0) > 2.4 \times 10^{34}$ years and $\tau/B(p\to \mu^+\pi^0) > 1.6 \times 10^{34}$ years at 90% confidence level.

Can we overcome the neutrino floor at high masses?

The neutrino floor is a barrier in the parameter space of weakly interacting massive particles (WIMPs) below which discovery is impeded due to an almost irreducible background of neutrinos. Directional gas time projection chambers could discriminate against solar neutrinos, relevant for WIMP masses $\lesssim$10 GeV. At higher masses $\gtrsim$100 GeV the floor is set by the background of atmospheric neutrinos. Probing below this part of the floor would require very large target exposures. Since gas-based detectors would be prohibitively large at this scale, we instead reevaluate the prospects for liquid noble experiments to probe below the neutrino floor. We combine all potential methods of subtracting the neutrino background to determine how much of this difficult to reach, but well-motivated, parameter space it is feasible to reach. Most notably, we quantify whether a proposed directional signal in xenon and argon experiments called "columnar recombination" can help in this task. We find that even if the strength of this effect is amplified beyond current experimental results, the quantity of directional information contained in the recombination signal is too low to realistically discriminate against the atmospheric neutrino background. Instead, benefiting from the refined measurements of neutrino fluxes by experiments such as DUNE and JUNO will be the most practical means to push direct WIMP searches below the neutrino floor. For an ultimate global coordination of xenon and argon experiments, we show that the neutrino floor is a surmountable barrier. The direct detection of 100 GeV-scale supersymmetric WIMPs may, eventually, be within reach.

Characteristics of the Diffuse Astrophysical Electron and Tau Neutrino Flux with Six Years of IceCube High Energy Cascade Data.

We report on the first measurement of the astrophysical neutrino flux using particle showers (cascades) in IceCube data from 2010-2015. Assuming standard oscillations, the astrophysical neutrinos in this dedicated cascade sample are dominated (∼90%) by electron and tau flavors. The flux, observed in the sensitive energy range from 16TeV to 2.6PeV, is consistent with a single power-law model as expected from Fermi-type acceleration of high energy particles at astrophysical sources. We find the flux spectral index to be γ=2.53±0.07 and a flux normalization for each neutrino flavor of ϕ_{astro}=1.66_{-0.27}^{+0.25} at E_{0}=100 TeV, in agreement with IceCube's complementary muon neutrino results and with all-neutrino flavor fit results. In the measured energy range we reject spectral indices γ≤2.28 at ≥3σ significance level. Because of high neutrino energy resolution and low atmospheric neutrino backgrounds, this analysis provides the most detailed characterization of the neutrino flux at energies below ∼100 TeV compared to previous IceCube results. Results from fits assuming more complex neutrino flux models suggest a flux softening at high energies and a flux hardening at low energies (p value ≥0.06). The sizable and smooth flux measured below ∼100 TeV remains a puzzle. In order to not violate the isotropic diffuse gamma-ray background as measured by the Fermi Large Area Telescope, it suggests the existence of astrophysical neutrino sources characterized by dense environments which are opaque to gamma rays.

The Sun at TeV energies: Gammas, neutrons, neutrinos and a cosmic ray shadow

High energy cosmic rays reach the surface of the Sun and start showers with thousands of secondary particles. Most of them will be absorbed by the Sun, but a fraction of the neutral ones will escape and reach the Earth. Here we incorporate a new ingredient that is essential to understand the flux of these solar particles: the cosmic ray shadow of the Sun. We use Liouville’s theorem to argue that the only effect of the solar magnetic field on the isotropic cosmic ray flux is to interrupt some of the trajectories that were aiming to the Earth and create a shadow. This shadow reveals the average solar depth crossed by cosmic rays of a given rigidity. The absorbed cosmic ray flux is then processed in the thin Solar surface and, assuming that the emission of neutral particles by low-energy charged particles is isotropic, we obtain (i)a flux of gammas that is consistent with Fermi-LAT observations, (ii) a flux of 100–300 neutrons/(year m2) produced basically in the spallation of primary He nuclei, and (iii) a neutrino flux that is above the atmospheric neutrino background at energies above 0.1–0.6 TeV (depending on the solar phase and the zenith inclination). More precise measurements of the cosmic ray shadow and of the solar gamma flux, together with the possible discovery of the neutron and neutrino signals, would provide valuable information about the magnetic field, the cycle, and the interior of the Sun.

Time-Integrated Neutrino Source Searches with 10Years of IceCube Data.

This Letter presents the results from pointlike neutrino source searches using ten years of IceCube data collected between April 6, 2008 and July 10, 2018. We evaluate the significance of an astrophysical signal from a pointlike source looking for an excess of clustered neutrino events with energies typically above ∼1 TeV among the background of atmospheric muons and neutrinos. We perform a full-sky scan, a search within a selected source catalog, a catalog population study, and three stacked Galactic catalog searches. The most significant point in the northern hemisphere from scanning the sky is coincident with the Seyfert II galaxy NGC 1068, which was included in the source catalog search. The excess at the coordinates of NGC 1068 is inconsistent with background expectations at the level of 2.9σ after accounting for statistical trials from the entire catalog. The combination of this result along with excesses observed at the coordinates of three other sources, including TXS 0506+056, suggests that, collectively, correlations with sources in the northern catalog are inconsistent with background at 3.3σ significance. The southern catalog is consistent with background. These results, all based on searches for a cumulative neutrino signal integrated over the 10years of available data, motivate further study of these and similar sources, including time-dependent analyses, multimessenger correlations, and the possibility of stronger evidence with coming upgrades to the detector.

Proton Decay Search Sensitivity with the Enlarged Fiducial Mass of Super-Kamiokande

We have studied enlarging the fiducial mass of the Super-Kamiokande detector from 22.5 kton to 27.2 kton to improve the proton decay search sensitivity. For the p → e +π0 (μ +π0) decay mode, the estimated efficiency and the expected number of atmospheric neutrino background events in the enlarged region (78 kton⋅years) are 25.8% (25.2%) and 0.10 (0.20), respectively while in the conventional region (372 kton⋅years) they are 39.8% (36.3%) and 0.49 (0.74). As a result, enlarging the fiducial mass increases the proton decay search sensitivity by 12%.

Dark Matter Searches at Super-Kamiokande

This work presents indirect searches for dark matter (DM) as WIMPs (Weakly Interacting Massive Particles) using neutrino data collected with the Super-Kamiokande detector from 1996 until 2016. The results of the search for WIMP-induced neutrinos from the Sun, the Earth’s core and the Milky Way are discussed. We looked for an excess of neutrinos related to a given source as compared to the expected atmospheric neutrino background. No excess of the WIMP-induced neutrinos is observed in any of the analyses. Limits on the WIMP-nucleon spin-dependent/-independent cross sections (Solar & Earth analysis) and on the WIMP self-annihilation cross section 〈σAV〉 (Galactic analysis) are derived assuming various annihilation modes and masses of the relic particles.

Atmospheric Cherenkov Telescopes as a potential veto array for neutrino astronomy

The IceCube Neutrino Observatory has revealed the existence of sources of high-energy astrophysical neutrinos. However, identification of the sources is challenging because astrophysical neutrinos are difficult to separate from the background of atmospheric neutrinos produced in cosmic-ray-induced particle cascades in the atmosphere. The efficient detection of air showers in coincidence with detected neutrinos can greatly reduce those backgrounds and increase the sensitivity of neutrino telescopes. Imaging Air Cherenkov Telescopes (IACTs) are sensitive to gamma-ray-induced (and cosmic-ray-induced) air showers in the 50 GeV to 50 TeV range, and can therefore be used as background-identifiers for neutrino observatories. This paper describes the feasibility of an array of small scale, wide field-of-view, cost-effective IACTs as an air shower veto for neutrino astronomy. A surface array of 250 to 750 telescopes would significantly improve the performance of a cubic kilometer-scale detector like IceCube, at a cost of a few percent of the original investment. The number of telescopes in the array can be optimized based on astronomical and geometrical considerations.

Monochromatic dark neutrinos and boosted dark matter in noble liquid direct detection

If dark matter self-annihilates into neutrinos or a second component of ("boosted") dark matter that is nucleophilic, the annihilation products may be detected with high rates via coherent nuclear scattering. A future multi-ten-tonne liquid xenon detector such as DARWIN, and a multi-hundred-tonne liquid argon detector, ARGO, would be sensitive to the flux of these particles in complementary ranges of $10-1000$ MeV dark matter masses. We derive these sensitivities after accounting for atmospheric and diffuse supernova neutrino backgrounds, and realistic nuclear recoil acceptances. We find that their constraints on the dark neutrino flux may surpass neutrino detectors such as Super-Kamiokande, and that they would extensively probe parametric regions that explain the missing satellites problem in neutrino portal models. The XENON1T and Borexino experiments currently restrict the effective baryonic coupling of thermal boosted dark matter to $\lesssim 10-100 \ \times$ the weak interaction, but DARWIN and ARGO would probe down to couplings 10 times smaller. Detection of boosted dark matter with baryonic couplings $\sim 10^{-3}-10^{-2} \ \times$ the weak coupling could indicate that the dark matter density profile in the centers of galactic halos become cored, rather than cuspy, through annihilations. This work demonstrates that, alongside liquid xenon, liquid argon direct detection technology would emerge a major player in dark matter searches within and beyond the WIMP paradigm.

Search for solar dark matter annihilation with JUNO

We explore the electron neutrino signals from light dark matter (DM) annihilation in the Sun for the large liquid scintillator detector JUNO. In terms of the spectrum features of three typical DM annihilation channels , we take two sets of selection conditions to calculate the expected signals and atmospheric neutrino backgrounds based on the Monte Carlo simulation data. Then the JUNO sensitivities to the spin independent DM- nucleon and spin dependent DM-proton cross sections are presented. It is found that the JUNO projected sensitivities are much better than the current spin dependent direct detection experimental limits for the and τ+ τ− channels. In the spin independent case, the JUNO will give the better sensitivity to the DM-nucleon cross section than the LUX and CDMSlite limits for the channel with the DM mass lighter than 6.5 GeV.

Search for Boosted Dark Matter Interacting with Electrons in Super-Kamiokande.

A search for boosted dark matter using 161.9kt yr of Super-Kamiokande IV data is presented. We search for an excess of elastically scattered electrons above the atmospheric neutrino background, with a visible energy between 100MeV and 1TeV, pointing back to the Galactic center or the Sun. No such excess is observed. Limits on boosted dark matter event rates in multiple angular cones around the Galactic center and Sun are calculated. Limits are also calculated for a baseline model of boosted dark matter produced from cold dark matter annihilation or decay. This is the first experimental search for boosted dark matter from the Galactic center or the Sun interacting in a terrestrial detector.

Prospects of indirect searches for dark matter at INO

The annihilation of Weakly Interactive Massive Particles (WIMP) in the centre of the sun could give rise to neutrino fluxes. We study the prospects of searching for these neutrinos at the upcoming Iron CALorimeter (ICAL) detector to be housed at the India-based Neutrino Observatory (INO). We perform ICAL simulations to obtain the detector efficiencies and resolutions in order to simulate muon events in ICAL due to neutrinos coming from annihilation of WIMP in the mass range mχ = (3−100) GeV . The atmospheric neutrinos pose a major background for these indirect detection studies and can be reduced using the fact that the signal comes only from the direction of the sun. For a given WIMP mass, we find the opening angle θ90 such that 90 % of the signal events are contained within this angle and use this cone-cut criteria to reduce the atmospheric neutrino background. The reduced background is then weighted by the solar exposure function at INO to obtain the final background spectrum for a given WIMP mass. We perform a χ2 analysis and present expected exclusion regions in the σSD−mχ and σSI−mχ, where σSD and σSI are the WIMP-nucleon Spin-Dependent (SD) and Spin-Independent (SI) scattering cross-section, respectively. For a 10 years exposure and mχ=25 GeV, the expected 90 % C.L. exclusion limit is found to be σSD < 6.87× 10−41 cm2 and σSI < 7.75× 10−43 cm2 for the τ+ τ− annihilation channel and σSD < 1.14× 10−39 cm2 and σSI < 1.30× 10−41 cm2 for the b b̄ channel, assuming 100 % branching ratio for each of the WIMP annihilation channel.