Neutrino Event Rates Research Articles

Solar neutrinos. Astrophysical aspects

This paper is a short pedagogical introduction to some aspects of the solar neutrino problem. The basic attention is concentrated on a qualitative consideration of the pp and CNO reactions responsible for hydrogen burning in the Sun, starting from an elementary derivation of the formula for the non-resonant thermonuclear reaction rate. We outline the physical content of the standard solar models, the problem of chemical composition of the Sun, expected neutrino energy spectrum, radial distributions of the neutrino fluxes in the Sun, and uncertainties in the predicted neutrino event rates.

A study of reactor neutrino monitoring at the experimental fast reactor JOYO

We carried out a study of neutrino detection at the experimental fast reactor JOYO using a 0.76 tons gadolinium loaded liquid scintillator detector. The detector was set up on the ground level at 24.3 m from the JOYO reactor core of 140 MW thermal power. The measured neutrino event rate from reactor on–off comparison was 1.11±1.24(stat.)±0.46(syst.) events/day. Although the statistical significance of the measurement was not enough, backgrounds in such a compact detector at the ground level were studied in detail and MC simulations were found to describe the data well. A study for improvement of the detector for future such experiments is also shown.

Gamma-rays and neutrinos from accreating neutron stars

AbstractDense wind of a massive star can be partially captured by a companion neutron star (NS) creating a very turbulent and magnetized transition region at some distance from the NS surface. We consider the consequences of electron and hadron acceleration at such a transition region. Electrons lose energy on the synchrotron process and the inverse Compton (IC) scattering of thermal radiation from the NS surface and/or the massive star. We calculate the synchrotron spectra (from X-rays to soft γ-rays) and IC spectra in the case of sources accreting the matter under the accretor and propeller scenarios. It is argued that a population of accreting massive binaries, recently discovered by the INTEGRAL observatory, can be detectable by the Fermi LAT telescope. On the other hand, TeV γ-ray emission from other class of massive binaries can be interpreted in terms of a magnetar accreting matter in the propeller scenario. We also calculate the expected neutrino event rates in a km2 detector produced by relativistic hadrons accelerated in such scenario.

High-energy neutrino emission from low-mass microquasars

In this paper we study the high-energy neutrino emission from low-mass microquasars (LMMQs) by assuming that an acceleration region for particles is located in the inner jet, based on the framework of a hadronic jet model. Adopting the parametrized formulae for inelastic pp interactions and the recent pγ parametrizations, we calculate the spectral energy distributions (SEDs) of various photon and neutrino spectra. Furthermore, we also consider the electromagnetic cascade processes due to γγ absorption. We then apply the jet model to the microquasar (MQ) GX 339-4, and calculate the expected neutrino event rate for three years of observations using the newest effective area of the KM3NeT detector in the Mediterranean Sea. The resulting results indicate that (1) photon spectra can roughly reproduce observations from radio to X-ray energies, and can also predict emission fluxes at high and very high energies (VHE); (2) GX 339-4 is a potential neutrino source, neutrinos from which are likely to be identified with some years of observations from the next-generation km 3 -scale neutrino telescopes.

Neutrino oscillation and expected event rate of supernova neutrinos in the adiabatic explosion model

We study how the influence of the shock wave appears in neutrino oscillations and the neutrino spectrum using density profile of adiabatic explosion model of a core-collapse supernova which is calculated in an implicit Lagrangian code for general relativistic spherical hydrodynamics. We calculate expected event rates of neutrino detection at SK and SNO for various theta_{13} values and both normal and inverted hierarchies. The predicted event rates of bar{nu}_e and nu_e depend on the mixing angle theta_{13} for the inverted and normal hierarchies, respectively, and the influence of the shock appears for about 2 - 8 s when sin^2 2 theta_{13} is larger than 10^{-3}. These neutrino signals for the shock propagation is decreased by < 30 % for bar{nu}_e in inverted (SK) or by < 15 % for nu_e in normal hierarchy (SNO) compared with the case without shock. The obtained ratio of the total event for high-energy neutrinos (20 MeV < E_{nu} < 60 MeV) to low-energy neutrinos (5 MeV < E_{nu} < 20 MeV) is consistent with the previous studies in schematic semi-analytic or other hydrodynamic models of the shock propagation. The time dependence of the calculated ratio of the event rates of high-energy to low-energy neutrinos is a very useful observable which is sensitive to theta_{13} and hierarchies. Namely, time-dependent ratio shows clearer signal of the shock propagation that exhibits remarkable decrease by at most factor \sim 2 for bar{nu}_e in inverted (SK), whereas it exhibits smaller change by \sim 10 % for nu_e in normal hierarchy (SNO). Observing time-dependent high-energy to low-energy ratio of the neutrino events thus would provide a piece of very useful information to constrain theta_{13} and mass hierarchy, and eventually help understanding the propagation how the shock wave propagates inside the star.

TeV neutrinos from accreting x-ray pulsars

Pulsars inside binary systems can accrete matter that arrives up to the pulsar surface provided that its period is long enough. During the accretion process, matter has to be accelerated to the rotational velocity of the pulsar magnetosphere at the distance where the balance between the pressure of matter and the magnetic field is achieved. At this distance, a very turbulent and magnetized region is formed in which hadrons can be accelerated to relativistic energies. These hadrons can interact with the very strong radiation field coming from the hot polar cap on the neutron star surface created by the in-falling matter. We calculate the neutrino event rates produced in an km$^2$ detector that can be expected from accreting millisecond and classical X-ray pulsars at a typical distance within our own Galaxy.

Dark matter in the Solar System. II. WIMP annihilation rates in the Sun

We calculate the annihilation rate of weakly interacting massive particles (WIMPs) in the Sun as a function of their mass and elastic scattering cross section. One byproduct of the annihilation, muon neutrinos, may be observed by the next generation of neutrino telescopes. Previous estimates of the annihilation rate assumed that any WIMPs from the Galactic dark halo that are captured in the Sun by elastic scattering off solar nuclei quickly reach thermal equilibrium in the Sun. Using simulations of WIMP orbits in the Solar System in the case that spin-independent scattering dominates in the Sun (and extrapolating to the case when spin-dependent scattering dominates), we show that the optical depth of the Sun to WIMPs and the gravitational forces from planets both serve to decrease the annihilation rate below these estimates. While we find that the sensitivity of upcoming km^3-scale neutrino telescopes to ~100 GeV WIMPs is virtually unchanged from previous estimates, the sensitivity of these experiments to ~10 TeV WIMPs may be an order of magnitude less than the standard calculations would suggest. The new estimates of the annihilation rates should guide future experiment design and improve the mapping from neutrino event rates to WIMP parameter space.

Solar neutrino measurements with Super-Kamiokande III

The third phase of Super-Kamiokande experiment (SK-III) has been running since 12th July, 2006. The SK-III detector is achieved 40% photo-cathode coverage with 11,129 20-inch PMTs. One of the physics goals in SK-III is observing the transition of solar neutrino oscillations between vacuum and matter oscillation around 4MeV. From 24th January, 2007 to 2nd March, 2008, we obtained data of live-time 288.9 days with energy threshold 6.5MeV (Full Final sample: FF sample). For data with a lower energy threshold 5.0MeV, we needed remove high radon contaminated period from the FF sample, then we obtained another data sample (Radon reduced sample: RR sample) with live-time 191.7 days. The current measurements show that SK-III has already achieved a similar signal to noise ratio as SK-I for energy range from 5.0 to 20.0MeV, and the solar angle distribution of FF sample shows that the solar neutrino event rate also looks consistent with SK-I for energy range from 6.5 to 20.0MeV. As for the RR sample, although the vertex distribution of low energy events is not uniform in the detector and there are more BG events in the edge of fiducial volume, it is clear that SK-III BG level is smaller than that of SK-I in the central region of the detector. Finally, the future plan for lowering the energy threshold shows a 2a discovery potential of the energy spectrum upturn with 3 years of observation after both software and hardware improvements.

Neutrinos from cosmic ray accelerators in the Cygnus region of the galaxy

While supernova remnants have been identified as the most likely sources of the galactic cosmic rays, no conclusive observational evidence for this association exists. We show here that IceCube has the possibility of producing incontrovertible evidence by detecting neutrinos produced by the cosmic ray beam interacting with the hydrogen in the vicinity of the supernova shock expanding into the interstellar medium. We show that the observational information on gamma ray fluxes from the Cygnus region, although limited, is sufficient to pinpoint the expected event rate of the neutrinos associated with a single source of 0.5 Crab at the TeV level to within a factor of 2, between 2 and 3.8 neutrinos per year. Finally, we note that recent gamma-ray observations reveal the presence of at least three and possibly up to eight such sources, raising the possibility of seeing more than 10 neutrinos per year from these sources alone.

The nylon scintillator containment vessels for the Borexino solar neutrino experiment

Borexino is a solar neutrino experiment designed to observe the 0.86 MeV Be 7 neutrinos emitted in the pp cycle of the sun. Neutrinos will be detected by their elastic scattering on electrons in 100 ton of liquid scintillator. The neutrino event rate in the scintillator is expected to be low ( ∼ 0.35 events per day per ton), and the signals will be at energies below 1.5 MeV, where background from natural radioactivity is prominent. Scintillation light produced by the recoil electrons is observed by an array of 2240 photomultiplier tubes. Because of the intrinsic radioactive contaminants in these PMTs, the liquid scintillator is shielded from them by a thick barrier of buffer fluid. A spherical vessel made of thin nylon film contains the scintillator, separating it from the surrounding buffer. The buffer region itself is divided into two concentric shells by a second nylon vessel in order to prevent inward diffusion of radon atoms. The radioactive background requirements for Borexino are challenging to meet, especially for the scintillator and these nylon vessels. Besides meeting requirements for low radioactivity, the nylon vessels must also satisfy requirements for mechanical, optical, and chemical properties. The present paper describes the research and development, construction, and installation of the nylon vessels for the Borexino experiment.

Extraction of the atmospheric neutrino fluxes from experimental event rate data

Precise knowledge of the atmospheric neutrino fluxes is a key ingredient in the interpretation of the results from any atmospheric neutrino experiment. In the standard atmospheric neutrino data analysis, these fluxes are theoretical inputs obtained from sophisticated numerical calculations. In this contribution, we present an alternative approach to the determination of the atmospheric neutrino fluxes based on the direct extraction from the experimental data on neutrino event rates. The extraction is achieved by means of a combination of artificial neural networks as interpolants and Monte Carlo methods.

Determination of the atmospheric neutrino flux from experimental data

The precise knowledge of the atmospheric neutrino fluxes is a key ingredient in the interpretation of the results from any atmospheric neutrino experiment. In the standard atmospheric neutrino data analysis, these fluxes are theoretical inputs obtained from sophisticated numerical calculations. In this work we present an alternative approach to the determination of the atmospheric neutrino fluxes based on the direct extraction from the experimental data on neutrino event rates.

Potential neutrino signals in a northern hemisphere neutrino telescope from galactic gamma-ray sources

Neutrino energy spectra have been calculated based on the recently measured energy spectra of Galactic very high energy γ-ray sources. Based on these neutrino spectra the expected event rates in the ANTARES neutrino telescope and KM3NeT, a future neutrino telescope in the Mediterranean Sea with an instrumented volume of one km3, have been calculated. For the brightest γ-ray sources we find event rates of the order of one neutrino per year. Although the neutrino event rates are comparable to the background from atmospheric neutrinos the detection of individual sources seems possible.

Determination of the atmospheric neutrino fluxes from atmospheric neutrino data

The precise knowledge of the atmospheric neutrino fluxes is a key ingredient in the interpretation of the results from any atmospheric neutrino experiment. In the standard atmospheric neutrino data analysis, these fluxes are theoretical inputs obtained from sophisticated numerical calculations based on the convolution of the primary cosmic ray spectrum with the expected yield of neutrinos per incident cosmic ray. In this work we present an alternative approach to the determination of the atmospheric neutrino fluxes based on the direct extraction from the experimental data on neutrino event rates. The extraction is achieved by means of a combination of artificial neural networks as interpolants and Monte Carlo methods for faithful error estimation

Physics and optimization of beta beams: From low to very high gamma

The physics potential of beta beams is investigated from low to very high gamma values and it is compared to superbeams and neutrino factories. The gamma factor and the baseline are treated as continuous variables in the optimization of the beta beam, while a fixed mass water Cherenkov detector or a totally active scintillator detector is assumed. We include in our discussion also the gamma dependence of the number of ion decays per year. For low gamma, we find that a beta beam could be a very interesting alternative to a superbeam upgrade, especially if it is operated at the second oscillation maximum to reduce correlations and degeneracies. For high gamma, we find that a beta beam could have a potential similar to a neutrino factory. In all cases, the sensitivity of the beta beams to $CP$ violation is very impressive if similar neutrino and antineutrino event rates can be achieved.

Cosmogenic neutrinos from cosmic ray interactions with extragalactic infrared photons

We discuss the production of cosmogenic neutrinos on extragalactic infrared photons in a model of its cosmological evolution. The relative importance of these infrared photons as a target for proton interactions is significant, especially in the case of steep injection spectra of the ultrahigh energy cosmic rays. For an E$^{-2.5}$ cosmic ray injection spectrum, for example, the event rate of neutrinos of energy above 1 PeV is more than doubled.

GeV gamma-rays and TeV neutrinos from very massive compact binary systems: the case of WR 20a

Abstract Massive Wolf—Rayet stars in a compact binary system are characterized by very strong winds which collide, creating a shock wave. If the wind nuclei, from helium up to oxygen, accelerated at the shock can reach large enough energies, they suffer disintegration in collisions with soft thermal radiation from the massive stars injecting relativistic protons and neutrons. Protons collide with the matter of the wind and a fraction of neutrons collide with the massive stars producing γ-rays and neutrinos via decay of pions. We calculate γ-ray fluxes from inverse Compton e± pair cascades initiated by primary γ-rays and leptons, which are produced by protons, and the neutrino fluxes, produced by protons and neutrons, for the example compact massive binary WR 20a. From normalization of these cascade γ-ray spectra to the fluxes of the EGRET sources 2EG J1021 − 5835 and 2EG J1049 − 5847 observed in the direction of WR 20a, we conclude that this massive binary can be detected by the IceCube type of neutrino detector. The estimated muon neutrino event rate inside the 1-km2 detector during 1 year is between a few and a few tens, depending on the spectrum of accelerated nuclei.

TeV Neutrinos from Microquasars in Compact Massive Binaries

We consider a compact binary system in which a Wolf-Rayet star supplies matter to a stellar mass black hole or a neutron star. This matter forms an accretion disk that ejects a jet like those observed in Galactic microquasars. A part of the jet kinetic energy, typically ~10%, can be transferred to relativistic nuclei. These nuclei lose nucleons as a result of the photodisintegration process in collisions with thermal photons from the accretion disk and the massive star. As a result of the head-on photon-nucleus collisions, most of the neutrons released from the nuclei move toward the surface of the accretion disk and/or the massive star producing neutrinos in collisions with the matter. We calculate the spectra of muon neutrinos and expected neutrino event rates for a 1 km3 neutrino detector of the IceCube type from a microquasar inside our Galaxy by applying, as an example, the parameters of the Cyg X-3 binary system. Such a model predicts up to several neutrino events per kilometer3 per year from Cyg X-3, provided that nuclei are accelerated to the Lorentz factors above 106 with a power-law spectrum with an index close to 2.

LMA and sterile neutrinos: a case for resonance spin flavour precession

Open questions remain after the confirmation of LMA as the dominant solution to the solar neutrino deficit. These are the apparent time modulation of the solar neutrino event rates in the Homestake, Gallium and SuperKamiokande experiments, possibly related to solar magnetic activity, the discrepancy between the event rate in the Homestake experiment and its theoretical prediction and the absence of the electron spectrum upturn in SuperKamiokande at energies below 6-8 MeV. We search for a possible understanding of these questions in the context of resonant spin flavour precession to sterile neutrinos, assuming a class of magnetic field profiles anchored in the upper radiation/lower convection zone. We consider the simplest such model beyond the standard 2$\nu$ flavour LMA one, with one single magnetic moment transition between active and sterile state and vanishing vacuum mixing. The preferred mass square difference is $\Delta m^2_{10}=O(10^{-8}eV^2)$. The Ga rate appears to be the most sensitive of all to solar activity. It is also found that a field profile extending within a longer region in the radial direction is favoured over another with a shorter span, and leads to a stronger suppression than in the LMA case of the intermediate energy neutrinos and some of the $^8 B$ ones.

Cosmic Star Formation History and the Future Observation of Supernova Relic Neutrinos

We investigate the flux and event rate of supernova relic neutrinos (SRNs) and discuss their implications for the cosmic star formation rate. As reference models, we adopt the supernova rate model based on recent observations and the supernova neutrino spectrum numerically calculated by several groups. In the detection energy range E_e>10 MeV, which will possibly be a background-free region in the near future, the SRN event rate is found to be 1-2 yr^{-1} at a water Cerenkov detector with a fiducial volume of 22.5 kton, depending on the adopted neutrino spectrum. We also simulate the expected signal with one set of the reference models by using the Monte Carlo method and then analyze these pseudodata with several free parameters, obtaining the distribution of the best-fit values for them. In particular, we use a parameterization such that R_{SN}(z)=R_{SN}^0 (1+z)^\alpha, where R_{SN}(z) is the comoving supernova rate density at redshift z and R_{SN}^0 and \alpha are free parameters, assuming that the supernova neutrino spectrum and luminosity are well understood by way of a future Galactic supernova neutrino burst or the future development of numerical supernova simulations. The obtained 1\sigma errors for these two parameters are found to be \delta\alpha /\alpha = 30% (7.8%) and \delta R_{SN}^0/ R_{SN}^0 = 28% (7.7%) for a detector with an effective volume of 22.5 kton 5 yr (440 kton 5 yr), where one of the parameters is fixed. On the other hand, if we fix neither of the values for these two parameters, the expected errors become rather large, \delta\alpha /\alpha = 37% and \delta R_{SN}^0/ R_{SN}^0 = 55%, even with an effective volume of 440 kton 5 yr.