Neutrino Background Research Articles

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%.

Geant4 based Simulation Study for Super-Kamiokande

Super-Kamiokande (SK) will start a new phase “SK-Gd” from next year. Currently, SK uses Geant3 based simulation, however, its development has already stopped. For the next 10 years of SK-Gd operation, updated detector simulation is essential. For diffuse supernova neutrino background search in SK-Gd, precise neutron simulation is crucial. Therefore, we developed Geant4 based SK simulation software, named’ SKG4’ . Starting from the construction of SK geometry, some comparisons with the current SK detector simulation has been done. In this presentation, the current status of the development for SKG4 and the result of the several physics simulations will be reported.

Neutrino clustering in the Milky Way and beyond

The standard cosmological model predicts the existence of a Cosmic Neutrino Background, which has not yet been observed directly. Some experiments aiming at its detection are currently under development, despite the tiny kinetic energy of the cosmological relic neutrinos, which makes this task incredibly challenging. Since massive neutrinos are attracted by the gravitational potential of our Galaxy, they can cluster locally. Neutrinos should be more abundant at the Earth position than at an average point in the Universe. This fact may enhance the expected event rate in any future experiment. Past calculations of the local neutrino clustering factor only considered a spherical distribution of matter in the Milky Way and neglected the influence of other nearby objects like the Virgo cluster, although recent N-body simulations suggest that the latter may actually be important. In this paper, we adopt a back-tracking technique, well established in the calculation of cosmic rays fluxes, to perform the first three-dimensional calculation of the number density of relic neutrinos at the Solar System, taking into account not only the matter composition of the Milky Way, but also the contribution of the Andromeda galaxy and the Virgo cluster. The effect of Virgo is indeed found to be relevant and to depend non-trivially on the value of the neutrino mass. Our results show that the local neutrino density is enhanced by 0.53% for a neutrino mass of 10 meV, 12% for 50 meV, 50% for 100 meV or 500% for 300 meV.

Analytical response function for the Borexino solar neutrino analysis

Borexino experiment is located at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, and its primary goal is detecting solar neutrinos, in particular those below 2 MeV, with unprecedentedly high sensitivity. Its technical distinctive feature is the ultra-low radioactive background of the inner scintillating core, which is the basis of the outstanding achievements obtained by the experiment (fluxes of 7Be, pep, pp, and limit on CNO). A spectral fit in the whole energy range from 200 keV up to 2 MeV has been performed for the first time, allowing to obtain simultaneously fluxes of all the solar neutrino components. To make such a fit possible, one requires the exact shapes of neutrino signals and backgrounds, as seen in the detector. Therefore, the transformation of the spectra from the original energy scale to the scale of the desired energy estimator, such as the number of hit PMTs or photoelectrons, is one of the key steps of the analysis. This conversion accounts for the energy scale non-linearity and the detector’s energy response, and can be performed using two approaches: the Monte Carlo simulation and the use of analytical models. The details and advantages of the analytical approach are presented in this contribution.

Nonlinear Dirac Neutrino Oscillations

Neutrino oscillations are a possible way to probe beyond Standard Model physics. The propagation of Dirac neutrinos in a massive medium is governed by the Dirac equation modified with an effective Hamiltonian that de- pends on the number density of surrounding matter fields. At the same time, quantum nonlinearities may contribute to neutrino oscillations by further mod- ifying the Dirac equation. A possible nonlinearity is computationally studied using Mathematica at low energies. We find that the presence of a uniform, static background matter distribution may significantly alter the oscillation am- plitude and wavelength; the considered nonlinearity may further reduce both oscillation amplitude and wavelength. In addition, the presence of matter al- lows the effects of the nonlinearity to be more readily observed for the chosen background densities and neutrino energy.

The R&D progress of the Jinping neutrino experiment

China JinPing underground Laboratory (CJPL) is located about 2400 m below Jinping mountain, providing a unique feature for studies of low-energy neutrinos. A neutrino experiment has been proposed to perform an in-depth research on solar neutrinos, geo-neutrinos and supernova relic neutrinos at Jinping. The physics motivations, the R&D efforts and the present status are reported for a proposed kilo-ton neutrino detector at Jinping. The future prospects are also given.

Local density of relic neutrinos with minimal mass

Local density of relic neutrinos with minimal mass

Astrophyisical Neutrino in Hyper-Kamiokande

Hyper-Kamiokande (Hyper-K) is a proposed next generation underground large water Cherenkov detector. We plan to build two cylindrical water tanks in our experimental period, filled with ultra pure water and surrounded with newly developed photo sensors. In total, it will provide the fiducial volume of 0.19-0.37 Mt. The energies, positions, directions and types of charged particles produced by neutrino interactions are detected using its Cherenkov light in water. Our detector will be located at deep underground to reduce the cosmic muon flux and its spallation products, which is a dominant background for the analysis of the low energy astrophysical neutrinos.With the fruitful physics research programs planned for the accelerator neutrinos, atomospheric neutrinos and nucleon decay, Hyper-K will play an important role in the next neutrino physics frontier, even in the neutrino astrophysics. It will provide remarkable information for both of particle physics and astrophysics with its large statistics of astrophysical neutrino measurements, i.e., solar neutrino, supernova burst neutrinos and supernova relic neutrino. Here, we will discuss about physics potential of Hyper-K neutrino astrophysics and expected performance of the detector.

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.

The Super Kamionade Gadolinium Project

Super-Kamiokande (SK) will be upgraded to Super-Kamiokande Gd (SK-Gd). This modification will enable it to identify low energy anti-neutrinos for the world’s first observation of the Diffuse Supernova Neutrino Background (DSNB). On average, there is one core-collapse supernova somewhere in the universe each second. The neutrinos emitted from all of these supernovae since the onset of stellar formation have suffused the universe. The flux of the DSNB is expected to be several tens per square centimeter per second. Theoretical models vary, but as many as five diffused supernova neutrinos per year above 10 MeV are expected to interact in SK. However, in order to separate these signals from the much more common solar and atmospheric neutrinos and other backgrounds, we need a new detection method. In 2015, the Super-Kamiokande Collaboration approved the SK-Gd project. It is the upgrade of the SK detector via the addition of water-soluble gadolinium (Gd) salt. Since then, we have been conducting many dedicated studies and developments for deploying Gd to SK.

The Diffuse Supernova Neutrino Background

The Diffuse Supernova Neutrino Background (DSNB) in the MeV regime represents the cumulative cosmic neutrino emission, predominantly due to core collapse supernovae. We estimate the DSNB flux for different Star Formation Rate Density (SFRD) models. We find that the DSNB flux estimated using the SFRD derived from Fermi-LAT Collaboration et al. (2018) is significantly higher ($\approx$ 32$\%$) relative to the flux estimated using the SFRD from Madau & Fragos (2017). This depicts the sensitivity between the DSNB flux and the SFRD estimates which shows that future detection of the DSNB can be used as a valuable tool to constrain the SFRD.

Measurement of neutrino and antineutrino neutral-current quasielasticlike interactions on oxygen by detecting nuclear deexcitation γ rays

Neutrino- and antineutrino-oxygen neutral-current quasielastic-like interactions are measured at Super-Kamiokande using nuclear de-excitation $\gamma$-rays to identify signal-like interactions in data from a $14.94 \ (16.35)\times 10^{20}$ protons-on-target exposure of the T2K neutrino (antineutrino) beam. The measured flux-averaged cross sections on oxygen nuclei are $\langle \sigma_{\nu {\rm -NCQE}} \rangle = 1.70 \pm 0.17 ({\rm stat.}) ^{+ {\rm 0.51}}_{- {\rm 0.38}} ({\rm syst.}) \times 10^{-38} \ {\rm cm^2/oxygen}$ with a flux-averaged energy of 0.82 GeV and $\langle \sigma_{\bar{\nu} {\rm -NCQE}} \rangle = 0.98 \pm 0.16 ({\rm stat.}) ^{+ {\rm 0.26}}_{- {\rm 0.19}} ({\rm syst.}) \times 10^{-38} \ {\rm cm^2/oxygen}$ with a flux-averaged energy of 0.68 GeV, for neutrinos and antineutrinos, respectively. These results are the most precise to date, and the antineutrino result is the first cross section measurement of this channel. They are compared with various theoretical predictions. The impact on evaluation of backgrounds to searches for supernova relic neutrinos at present and future water Cherenkov detectors is also discussed.

Can we detect PeV neutrinos from merging black hole binaries?

In the environment of a newly formed black hole, even small amounts of plasma can support a strong magnetic field up to 1011 G in a short time, and protons could be accelerated to ∼1017 eV which is limited by the curvature radiation. In the most optimistic estimation which consider no curvature cooling during the acceleration of protons, the maximum proton energy might be up to ∼1020 eV. After acceleration, in such extreme environment, the dominated energy loss process of proton is through their interaction with the strong magnetic fields, that is, the so-called pB process (Yuan and Shi, 2019). The maximum energy of neutrino produced due to the pB process can reach to ∼1019 eV. In the case of GW150914 detected by Advanced LIGO, the neutrino flux is estimated to be below the detection limit of IceCube. Neutrinos associating with the merging of nearby black hole binary with distance ≲ 5 Mpc could be detected by the current IceCube detector, and the diffuse neutrino background resulting from black hole mergers could be detected by the future IceCube-Gen2.

Neutrino Echoes from Multimessenger Transient Sources.

The detection of the high-energy neutrino event, IceCube-170922A, demonstrated that multimessenger particle astrophysics triggered by neutrino alerts is feasible. We consider time delay signatures caused by secret neutrino interactions with the cosmic neutrino background and dark matter and suggest that these can be used as a novel probe of neutrino interactions beyond the standard model (BSM). The tests with BSM-induced neutrino echoes are distinct from existing constraints from the spectral modification and will be enabled by multimessenger observations of bright neutrino transients with future experiments such as IceCube-Gen2, KM3Net, and Hyper-Kamiokande. The constraints are complementary to those from accelerator and laboratory experiments and powerful for testing various particle models that explain tensions prevailing in the cosmological data.

TES Microcalorimeters for PTOLEMY

The PTOLEMY project is devoted to directly detect the cosmic neutrino background. A key point for the project success is the development of a device which is capable of detecting electrons with an energy resolution lower than 0.05 eV. Microcalorimeters based on transition-edge sensors are among the best candidates since they already reach 0.11 eV of energy resolution for telecomm photons. To further improve the energy resolution, while maintaining a suitable saturation energy, it is necessary to reduce the transition temperature. This could be achieved by proximity effect of a normal-superconducting bilayer. To this aim, TiAu very thin films are under development to demonstrate the feasibility of reaching 0.05 eV energy resolution for light pulses of few eV. Thanks to the high electron stopping power of metals, the penetration depth of low energy incident electrons is limited to few nanometers and, with respect to visible light, we expect a high detection efficiency, while keeping similar dark counts and energy resolution.

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.

High efficiency resonance ionization of thorium

Multi-step resonance ionization spectroscopy of thorium has been performed with Ti:Sapphire lasers and a hot-cavity laser ion source. Resonance ionization schemes involving three-color, three-photon ionization processes are evaluated and a new three-photon scheme is demonstrated with measured overall ionization efficiency of 38.6(5)%. This high efficiency makes it possible to determine the thorium impurities in materials used for ultra-low background neutrino detectors and could also benefit ultra-trace thorium analysis using resonance ionization mass spectrometry for various radiochemical tracer applications.

Triangulation pointing to core-collapse supernovae with next-generation neutrino detectors

A core-collapse supernova releases the vast majority of the gravitational binding energy of its compact remnant in the form of neutrinos over an interval of a few tens of seconds. In the event of a core-collapse supernova within our galaxy, multiple current and future neutrino detectors would see a large burst in activity. Neutrinos escape a supernova hours before light does, so any prompt information about the supernova's direction that can be inferred via the neutrino signal will help to enable early electromagnetic observations of the supernova. While there are methods to determine the direction via intrinsic directionality of some neutrino-matter interaction channels, a complementary method which will reach maturity with the next generation of large neutrino detectors is the use of relative neutrino arrival times at different detectors around the globe. To evaluate this triangulation method for realistic detector configurations of the next few decades, we generate random supernova neutrino signals with realistic detector assumptions, and quantify the error in expected time delay between detections. We investigate a practical and robust method of estimating the time differences between burst detections, also correcting for detection efficiency bias. With this method, we determine the pointing precision of supernova neutrino triangulation as a function of supernova distance and location, detectors used, detector background level and neutrino mass ordering assumption. Under favorable conditions, the 1$\sigma$ supernova search area from triangulation could be reduced to a few percent of the sky. It should be possible to implement this method with low latency under realistic conditions.

MeV neutrino dark matter: relic density, lepton flavour violation and electron recoil

Right-handed neutrinos with MeV to GeV mass are very promising candidates for dark matter (DM). Not only can they solve the missing satellite puzzle, the cusp-core problem of inner DM density profiles, and the too-big-to fail problem, i.e. that the unobserved satellites are too big to not have visible stars, but they can also account for the Standard Model (SM) neutrino masses at one loop. We perform a comprehensive study of the right-handed neutrino parameter space and impose the correct observed relic density and SM neutrino mass differences and mixings. We find that the DM masses are in agreement with bounds from big-bang nucleosynthesis, but that these constraints induce sizeable DM couplings to the charged SM leptons. We then point out that previously overlooked limits from current and future lepton flavour violation experiments such as MEG and SINDRUM heavily constrain the allowed parameter space. Since the DM is leptophilic, we also investigate electron recoil as a possible direct detection signal, in particular in the XENON1T experiment. We find that despite the large coupling and low backgrounds, the energy thresholds are still too high and the predicted cross sections too low due to the heavy charged mediator, whose mass is constrained by LEP limits.

Multimessenger tests of cosmic-ray acceleration in radiatively inefficient accretion flows

The cores of active galactic nuclei (AGNs) have been suggested as the sources of IceCube neutrinos, and recent numerical simulations have indicated that hot AGN coronae of Seyfert galaxies and radiatively inefficient accretion flows (RIAFs) of low-luminosity AGNs (LLAGNs) may be promising sites of ion acceleration. We present detailed studies on detection prospects of high-energy multi-messenger emissions from RIAFs in nearby LLAGNs. We construct a model of RIAFs that can reproduce the observational features of the current X-ray observations of nearby LLAGNs. We then calculate the high-energy particle emissions from nearby individual LLAGNs, including MeV gamma rays from thermal electrons, TeV--PeV neutrinos produced by non-thermal protons, and sub-GeV to sub-TeV gamma rays from proton-induced electromagnetic cascades. We find that, although these are beyond the reach of current facilities, proposed future experiments such as e-ASTROGAM and IceCube-Gen2 should be able to detect the MeV gamma rays and the neutrinos, respectively, or else they can place meaningful constraints on the parameter space of the model. On the other hand, the detection of high-energy gamma rays due to the electromagnetic cascades will be challenging with the current and near-future experiments, such as Fermi and Cherenkov Telescope Array. In an accompanying paper, we demonstrate that LLAGNs can be a source of the diffuse soft gamma-ray and TeV--PeV neutrino backgrounds, whereas in the present paper, we focus on the prospects for multi-messenger tests which can be applied to reveal the nature of the high-energy neutrinos and photons from LLAGNs.