Astrophysical Neutrino Flux Research Articles

High-energy neutrino interaction physics with IceCube

Although they are best known for studying astrophysical neutrinos, neutrino telescopes like IceCube can study neutrino interactions, at energies far above those that are accessible at accelerators. In this writeup, I present two IceCube analyses of neutrino interactions at energies far above 1 TeV. The first measures neutrino absorption in the Earth, and, from that determines the neutrino-nucleon cross-section at energies between 6.3 and 980 TeV. We find that the cross-sections are 1.30 +0.21 -0.19 (stat.) +0.39 -0.43 (syst.) times the Standard Model crosssection. We also present a measurement of neutrino inelasticity, using νμ charged-current interactions that occur within IceCube. We have measured the average inelasticity at energies from 1 TeV to above 100 TeV, and found that it is in agreement with the Standard Model expectations. We have also performed a series of fits to this track sample and a matching cascade sample, to probe aspects of the astrophysical neutrino flux, particularly the flavor ratio.

Differential limit on the extremely-high-energy cosmic neutrino flux in the presence of astrophysical background from nine years of IceCube data

We report a quasi-differential upper limit on the extremely-high-energy (EHE) neutrino flux above $5\times 10^{6}$ GeV based on an analysis of nine years of IceCube data. The astrophysical neutrino flux measured by IceCube extends to PeV energies, and it is a background flux when searching for an independent signal flux at higher energies, such as the cosmogenic neutrino signal. We have developed a new method to place robust limits on the EHE neutrino flux in the presence of an astrophysical background, whose spectrum has yet to be understood with high precision at PeV energies. A distinct event with a deposited energy above $10^{6}$ GeV was found in the new two-year sample, in addition to the one event previously found in the seven-year EHE neutrino search. These two events represent a neutrino flux that is incompatible with predictions for a cosmogenic neutrino flux and are considered to be an astrophysical background in the current study. The obtained limit is the most stringent to date in the energy range between $5 \times 10^{6}$ and $5 \times 10^{10}$ GeV. This result constrains neutrino models predicting a three-flavor neutrino flux of $E_\nu^2\phi_{\nu_e+\nu_\mu+\nu_\tau}\simeq2\times 10^{-8}\ {\rm GeV}/{\rm cm}^2\ \sec\ {\rm sr}$ at $10^9\ {\rm GeV}$. A significant part of the parameter-space for EHE neutrino production scenarios assuming a proton-dominated composition of ultra-high-energy cosmic rays is excluded.

Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert

A high-energy neutrino event detected by IceCube on 22 September 2017 was coincident in direction and time with a gamma-ray flare from the blazar TXS 0506+056. Prompted by this association, we investigated 9.5 years of IceCube neutrino observations to search for excess emission at the position of the blazar. We found an excess of high-energy neutrino events, with respect to atmospheric backgrounds, at that position between September 2014 and March 2015. Allowing for time-variable flux, this constitutes 3.5σ evidence for neutrino emission from the direction of TXS 0506+056, independent of and prior to the 2017 flaring episode. This suggests that blazars are identifiable sources of the high-energy astrophysical neutrino flux.

High-energy Neutrinos from Galactic Superbubbles

Abstract We study the propagation of cosmic rays (CRs) generated by sources residing inside superbubbles. We show that the enhanced magnetic field in the bubble wall leads to an increase in the interior CR density. Because of the large matter density in the wall, the probability for CR interactions on gas peaks there. As a result, the walls of superbubbles located near young CR sources efficiently emit neutrinos. We apply this scenario to the Loop I and Local Superbubble: these bubbles are sufficiently near such that CRs from a young source such as Vela interacting in the bubble wall can generate a substantial fraction of the observed astrophysical high-energy neutrino flux below ∼few × 100 TeV.

Intrinsic charm contribution to the prompt atmospheric neutrino flux

In this work we investigate the impact of intrinsic charm on the prompt atmospheric neutrino flux. The color dipole approach to heavy quark production is generalized to include the contribution of processes initiated by charm quarks. The prompt neutrino flux is calculated assuming the presence of intrinsic charm in the wave function of the projetile hadron. The predictions are compared with previous color dipole results which were obtained taking into account only the process initiated by gluons. In addition, we estimate the atmospheric (conventional + prompt) neutrino flux and compare our predictions with the ICECUBE results for the astrophysical neutrino flux. Our results demonstrate that the contribution of the charm quark initiated process is non - negligible and that the prompt neutrino flux can be enhanced by a factor $\approx$ 2 at large neutrino energies if an intrinsic charm component is present in the proton wave function.

High Energy Neutrino Astronomy: Where Do We Stand, Where Do We Go?

With the identification of a diffuse flux of astrophysical ("cosmic") neutrinos in the TeV-PeV energy range, IceCube has opened a new window to the Universe. However, the corresponding cosmic landscape is still uncharted: so far, the observed flux does not show any clear association with known source classes. In the present talk, I sketch the way from Baikal-NT200 to IceCube and summarize IceCube's recent astrophysics results. Finally, I describe the present projects to build even larger detectors: GVD in Lake Baikal, KM3NeT in the Mediterranean Sea and IceCube-Gen2 at the South Pole. These detectors will allow studying the high-energy neutrino sky in much more detail than the present arrays permit.

Multi-PeV Signals from a New Astrophysical Neutrino Flux beyond the Glashow Resonance.

The IceCube neutrino discovery was punctuated by three showers with E_{ν}≈1-2 PeV. Interest is intense in possible fluxes at higher energies, though a deficit of E_{ν}≈6 PeV Glashow resonance events implies a spectrum that is soft and/or cutoff below ∼few PeV. However, IceCube recently reported a through-going track depositing 2.6±0.3 PeV. A muon depositing so much energy can imply E_{ν_{μ}}≳10 PeV. Alternatively, we find a tau can deposit this much energy, requiring E_{ν_{τ}}∼10× higher. We show that extending soft spectral fits from TeV-PeV data is unlikely to yield such an event, while an ∼E_{ν}^{-2} flux predicts excessive Glashow events. These instead hint at a new flux, with the hierarchy of ν_{μ} and ν_{τ} energies implying astrophysical neutrinos at E_{ν}∼100 PeV if a tau. We address implications for ultrahigh-energy cosmic-ray and neutrino origins.

PeV IceCube signals and Dark Matter relic abundance in modified cosmologies

The discovery by the IceCube experiment of a high-energy astrophysical neutrino flux with energies of the order of PeV, has opened new scenarios in astroparticles physics. A possibility to explain this phenomenon is to consider the minimal models of Dark Matter (DM) decay, the 4-dimensional operator sim y_{alpha chi }overline{{L_{L_{alpha }}}}, H, chi , which is also able to generate the correct abundance of DM in the Universe. Assuming that the cosmological background evolves according to the standard cosmological model, it follows that the rate of DM decay Gamma _chi sim |y_{alpha chi }|^2 needed to get the correct DM relic abundance (Gamma _chi sim 10^{-58}) differs by many orders of magnitude with respect that one needed to explain the IceCube data (Gamma _chi sim 10^{-25}), making the four-dimensional operator unsuitable. In this paper we show that assuming that the early Universe evolution is governed by a modified cosmology, the discrepancy between the two the DM decay rates can be reconciled, and both the IceCube neutrino rate and relic density can be explained in a minimal model.

Constraining high-energy neutrino emission from choked jets in stripped-envelope supernovae

There are indications that γ-ray dark objects such as supernovae (SNe) with choked jets, and the cores of active galactic nuclei may contribute to the diffuse flux of astrophysical neutrinos measured by the IceCube observatory. In particular, stripped-envelope SNe have received much attention since they are capable of producing relativistic jets and could explain the diversity in observations of collapsar explosions (e.g., gamma-ray bursts (GRBs), low-luminosity GRBs, and Type Ibc SNe). We use an unbinned maximum likelihood method to search for spatial and temporal coincidences between Type Ibc core-collapse SNe, which may harbor a choked jet, and muon neutrinos from a sample of IceCube up-going track-like events measured from May 2011–May 2012. In this stacking analysis, we find no significant deviation from a background-only hypothesis using one year of data, and are able to place upper limits on the total amount of isotropic equivalent energy that choked jet core-collapse SNe deposit in cosmic rays ℰcr and the fraction of core-collapse SNe which have a jet pointed towards Earth fjet. This analysis can be extended with yet to be made public IceCube data, and the increased amount of optically detected core-collapse SNe discovered by wide field-of-view surveys such as the Palomar Transient Factory and All-Sky Automated Survey for Supernovae. The choked jet SNe/high-energy cosmic neutrino connection can be more tightly constrained in the near future.

Baikal-GVD: status and prospects

Baikal-GVD is a next generation, kilometer-scale neutrino telescope under construction in Lake Baikal. It is designed to detect astrophysical neutrino fluxes at energies from a few TeV up to 100 PeV. GVD is formed by multi-megaton subarrays (clusters). The array construction started in 2015 by deployment of a reduced-size demonstration cluster named "Dubna" . The first cluster in it’s baseline configuration was deployed in 2016, the second in 2017 and the third in 2018. The full-scale GVD will be an array of ~10.000 light sensors with an instrumented volume about of 2 cubic km. The first phase (GVD-1) is planned to be completed by 2020-2021. It will comprise 8 clusters with 2304 light sensors in total. We describe the design of Baikal-GVD and present selected results obtained in 2015 - 2017.

Origin of the High-energy Neutrino Flux at IceCube

Abstract We discuss the spectrum of the different components in the astrophysical neutrino flux reaching the Earth, and the possible contribution of each component to the high-energy IceCube data. We show that the diffuse flux from cosmic ray (CR) interactions with gas in our galaxy implies just two events among the 54-event sample. We argue that the neutrino flux from CR interactions in the intergalactic (intracluster) space depends critically on the transport parameter δ describing the energy dependence in the diffusion coefficient of galactic CRs. Our analysis motivates a neutrino spectrum with a drop at PeV energies that fits the data well, including the non-observation of the Glashow resonance at 6.3 PeV. We also show that a CR flux described by an unbroken power law may produce a neutrino flux with interesting spectral features (bumps and breaks) related to changes in the CR composition.

Measurement of the nu _{mu } energy spectrum with IceCube-79

IceCube is a neutrino observatory deployed in the glacial ice at the geographic South Pole. The nu _mu energy unfolding described in this paper is based on data taken with IceCube in its 79-string configuration. A sample of muon neutrino charged-current interactions with a purity of 99.5% was selected by means of a multivariate classification process based on machine learning. The subsequent unfolding was performed using the software Truee. The resulting spectrum covers an E_nu -range of more than four orders of magnitude from 125 GeV to 3.2 PeV. Compared to the Honda atmospheric neutrino flux model, the energy spectrum shows an excess of more than 1.9,sigma in four adjacent bins for neutrino energies E_nu ge 177.8,text {TeV}. The obtained spectrum is fully compatible with previous measurements of the atmospheric neutrino flux and recent IceCube measurements of a flux of high-energy astrophysical neutrinos.

Astrophysical neutrinos: IceCube highlights

The IceCube Neutrino Observatory features approximately a cubic-kilometer volume of instrumented ice at the geographic South Pole. A high energy astrophysical neutrino flux has been observed, initially in 2013, as an excess of neutrinos above 10 TeV compared to the expectation from an atmospheric neutrino background. This signal, nowadays significant at >6 σ, has been observed both in neutrino interactions starting in the detector volume and in throughgoing muons generated in charged-current muon neutrino interactions outside the detector. No astrophysical objects have yet been identified as the flux source, which is currently consistent with an isotropic expectation. This paper reviews the evidence for the astrophysical flux and the status of the search for its sources. It also presents a discussion of proposed detector extensions being designed to solve the mystery of the astrophysical neutrino origin.

Search for Astrophysical Sources of Neutrinos Using Cascade Events in IceCube

Abstract The IceCube neutrino observatory has established the existence of a flux of high-energy astrophysical neutrinos, which is inconsistent with the expectation from atmospheric backgrounds at a significance greater than 5σ. This flux has been observed in analyses of both track events from muon neutrino interactions and cascade events from interactions of all neutrino flavors. Searches for astrophysical neutrino sources have focused on track events due to the significantly better angular resolution of track reconstructions. To date, no such sources have been confirmed. Here we present the first search for astrophysical neutrino sources using cascades interacting in IceCube with deposited energies as small as 1 TeV. No significant clustering was observed in a selection of 263 cascades collected from 2010 May to 2012 May. We show that compared to the classic approach using tracks, this statistically independent search offers improved sensitivity to sources in the southern sky, especially if the emission is spatially extended or follows a soft energy spectrum. This enhancement is due to the low background from atmospheric neutrinos forming cascade events and the additional veto of atmospheric neutrinos at declinations ≲−30°.

Enhanced Starting Track Event Selection for Astrophysical Neutrinos in IceCube

IceCube’s measurements of the astrophysical neutrino flux have applied veto techniques to suppress atmospheric neutrinos and muons. All the vetos thus far have used the outer regions of the detector to identify and reject penetrating muon tracks, leaving the inner parts of the detector available to observe the astrophysical neutrino flux. Here we discuss a method that is optimized for muon neutrinos which have a charged-current interaction with a contained vertex. This analysis exploits the high quality directional information of muons to determine a veto on an event by event basis. The final sample will contain astrophysical neutrinos with good purity starting around 10 TeV.

Study of the Galactic Center region with the ANTARES neutrino telescope

The measurements of astrophysical neutrinos by the IceCube collaboration contains some indications of a North/South asymmetry which could hint a Galactic contribution. The ANTARES neutrino telescope has a direct view of the Galactic Center region and can provide complementary information on the neutrino flux from this region thanks to its excellent angular resolution both for tracks and showers. The recent model KRAγ, characterized by radially-dependent cosmic-ray transport properties predicts a neutrino flux close to the ANTARES sensitivity. We present here a study on a possible Galactic contribution to the astrophysical neutrino flux using seven years of data from the ANTARES neutrino telescope. The results of this analysis are preliminary, the median upper limit at 90% confidence level is roughly two times larger than the flux predicted by the KRAγ model and the discovery probability at 3σ is 3% of the KRAγ flux.

The galactic contribution to IceCube's astrophysical neutrino flux

High energy neutrinos have been detected by IceCube, but their origin remains a mystery. Determining the sources of this flux is a crucial first step towards multi-messenger studies. In this work we systematically compare two classes of sources with the data: galactic and extragalactic. We assume that the neutrino sources are distributed according to a class of Galactic models. We build a likelihood function on an event by event basis including energy, event topology, absorption, and direction information. We present the probability that each high energy event with deposited energy Edep>60 TeV in the HESE sample is Galactic, extragalactic, or background. For Galactic models considered the Galactic fraction of the astrophysical flux has a best fit value of 1.3% and is <9.5% at 90% CL. A zero Galactic flux is allowed at <1σ.

Exploring the Universe with Neutrinos: Recent Results from IceCube

In 2013, the IceCube Neutrino Observatory located at the geographic South Pole detected evidence for a diffuse astrophysical neutrino flux above ∼60 TeV. To this day, IceCube has operated with full detector configuration for more than 6 years. The observed astrophysical neutrino flux has been confirmed with >6σ significance with both events starting within the detector (all flavor) and events traversing through the Earth (νμ charged-current). Somewhat equal flavor ratio of astrophysical neutrinos is expected at Earth assuming standard thorough oscillation. A search for tau neutrinos has been carried out but yielded null result. No neutrino sources have been found to contribute significantly to the diffuse flux at this point. In this paper, we will review the current status of the astrophysical neutrino flux, discuss the quest for neutrino point sources and overview the proposed design and physics potentials of the future IceCube-Gen2.

An algorithm for the reconstruction of high-energy neutrino-induced particle showers and its application to the ANTARES neutrino telescope

A novel algorithm to reconstruct neutrino-induced particle showers within the ANTARES neutrino telescope is presented. The method achieves a median angular resolution of 6^circ for shower energies below 100 TeV. Applying this algorithm to 6 years of data taken with the ANTARES detector, 8 events with reconstructed shower energies above 10 TeV are observed. This is consistent with the expectation of about 5 events from atmospheric backgrounds, but also compatible with diffuse astrophysical flux measurements by the IceCube collaboration, from which 2–4 additional events are expected. A 90% C.L. upper limit on the diffuse astrophysical neutrino flux with a value per neutrino flavour of text {E}^2cdot Phi ^{90%} = 4.9 cdot 10^{-8},mathrm {GeV} cdot mathrm {cm^{-2} cdot s^{-1}cdot sr^{-1}} is set, applicable to the energy range from 23 TeV to 7.8 PeV, assuming an unbroken text {E}^{-2} spectrum and neutrino flavour equipartition at Earth.

Astrophysical neutrinos and cosmic rays observed by IceCube

The core mission of the IceCube neutrino observatory is to study the origin and propagation of cosmic rays. IceCube, with its surface component IceTop, observes multiple signatures to accomplish this mission. Most important are the astrophysical neutrinos that are produced in interactions of cosmic rays, close to their sources and in interstellar space. IceCube is the first instrument that measures the properties of this astrophysical neutrino flux and constrains its origin. In addition, the spectrum, composition, and anisotropy of the local cosmic-ray flux are obtained from measurements of atmospheric muons and showers. Here we provide an overview of recent findings from the analysis of IceCube data, and their implications to our understanding of cosmic rays.