A COMBINED MAXIMUM-LIKELIHOOD ANALYSIS OF THE HIGH-ENERGY ASTROPHYSICAL NEUTRINO FLUX MEASURED WITH ICECUBE

Observation of Seven Astrophysical Tau Neutrino Candidates with IceCube

We report on a measurement of astrophysical tau neutrinos with 9.7 yr of IceCube data. Using convolutional neural networks trained on images derived from simulated events, seven candidate ν_{τ} events were found with visible energies ranging from roughly 20 TeV to 1 PeV and a median expected parent ν_{τ} energy of about 200 TeV. Considering backgrounds from astrophysical and atmospheric neutrinos, and muons from π^{±}/K^{±} decays in atmospheric air showers, we obtain a total estimated background of about 0.5 events, dominated by non-ν_{τ} astrophysical neutrinos. Thus, we rule out the absence of astrophysical ν_{τ} at the 5σ level. The measured astrophysical ν_{τ} flux is consistent with expectations based on previously published IceCube astrophysical neutrino flux measurements and neutrino oscillations.

Searches for beyond-standard-model physics with astroparticle physics instruments

Many instruments for astroparticle physics are primarily geared towards multi-messenger astrophysics, to study the origin of cosmic rays and to understand high-energy astrophysical processes. Since these instruments observe the Universe at extreme energies and in kinematic ranges not accessible at accelerators these experiments provide also unique and complementary opportunities to search for particles and physics beyond the standard model of particle physics. In particular, the reach of IceCube, Fermi and KATRIN to search for and constrain Dark Matter, Axions, heavy Big Bang relics, sterile neutrinos and Lorentz invariance violation will be discussed. The contents of this article are based on material presented at the Humboldt-Kolleg 'Clues to a mysterious Universe-exploring the interface of particle, gravity and quantum physics' in June 2022. This article is part of the theme issue 'The particle-gravity frontier'.

Tau Appearance from High-Energy Neutrino Interactions

High-energy muon and electron neutrinos yield a non-negligible flux of tau neutrinos as they propagate through Earth. In this Letter, we address the impact of this additional component in the PeV and EeV energy regimes for the first time. Above 300 TeV, this contribution is predicted to be significantly larger than the atmospheric background, and it alters current and future neutrino telescopes' capabilities to discover a cosmic tau-neutrino flux. Further, we demonstrate that Earth-skimming neutrino experiments, designed to observe tau neutrinos, will be sensitive to cosmogenic neutrinos even in extreme scenarios without a primary tau-neutrino component.

Search for Relativistic Magnetic Monopoles with Eight Years of IceCube Data

We present an all-sky 90% confidence level upper limit on the cosmic flux of relativistic magnetic monopoles using 2886 days of IceCube data. The analysis was optimized for monopole speeds between 0.750c and 0.995c, without any explicit restriction on the monopole mass. We constrain the flux of relativistic cosmic magnetic monopoles to a level below 2.0×10^{-19}  cm^{-2} s^{-1} sr^{-1} over the majority of the targeted speed range. This result constitutes the most strict upper limit to date for magnetic monopoles with β≳0.8 and up to β∼0.995 and fills the gap between existing limits on the cosmic flux of nonrelativistic and ultrarelativistic magnetic monopoles.

Lorentz Symmetry and High-Energy Neutrino Astronomy

The search of the violation of Lorentz symmetry, or Lorentz violation (LV), is an active research field. The effects of LV are expected to be very small, and special systems are often used to search it. High-energy astrophysical neutrinos offer a unique system to search signatures of LV, due to the three factors: high neutrino energy, long propagation distance, and the presence of quantum mechanical interference. In this brief review, we introduce tests of LV and summarize existing searches of LV, using atmospheric and astrophysical neutrinos.

Gamma-Ray and Neutrino Signals from Accretion Disk Coronae of Active Galactic Nuclei

To explain the X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts have established the existence of weak non-thermal coronal activity. In addition, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10 year survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understanding of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma ray, and high-energy neutrino observations.

Active Galactic Nuclei Jets as the Origin of Ultrahigh-Energy Cosmic Rays and Perspectives for the Detection of Astrophysical Source Neutrinos at EeV Energies

We demonstrate that a population of active galactic nuclei (AGN) can describe the observed spectrum of ultra-high-energy cosmic rays (UHECRs) at and above the ankle, and that the dominant contribution comes from low-luminosity BL Lacertae objects. An additional, subdominant contribution from high-luminosity AGN is needed to improve the description of the composition observables, leading to a substantial neutrino flux that peaks at exaelectronvolt (EeV) energies. We also find that different properties for the low- and high-luminosity AGN populations are required; a possibly similar baryonic loading can already be excluded from current IceCube Neutrino Observatory observations. We also show that the flux of neutrinos emitted from within the sources should outshine the cosmogenic neutrinos produced during the propagation of UHECRs. This result has profound implications for the ultra-high-energy (∼EeV) neutrino experiments, since additional search strategies can be used for source neutrinos compared to cosmogenic neutrinos, such as stacking searches, flare analyses, and multimessenger follow-ups.

Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies

Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress.

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 16 TeV to 2.6 PeV, 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.

Hidden Cores of Active Galactic Nuclei as the Origin of Medium-Energy Neutrinos: Critical Tests with the MeV Gamma-Ray Connection

Mysteries about the origin of high-energy cosmic neutrinos have deepened by the recent IceCube measurement of a large diffuse flux in the 10-100 TeV range. Based on the standard disk-corona picture of active galactic nuclei (AGN), we present a phenomenological model enabling us to systematically calculate the spectral sequence of multimessenger emission from the AGN coronae. We show that protons in the coronal plasma can be stochastically accelerated up to PeV energies by plasma turbulence, and find that the model explains the large diffuse flux of medium-energy neutrinos if the cosmic rays carry only a few percent of the thermal energy. We find that the Bethe-Heitler process plays a crucial role in connecting these neutrinos and cascaded MeV gamma rays, and point out that the gamma-ray flux can even be enhanced by the reacceleration of secondary pairs. Critical tests of the model are given by its prediction that a significant fraction of the MeV gamma-ray background correlates with ∼10  TeV neutrinos, and nearby Seyfert galaxies including NGC 1068 are promising targets for IceCube, KM3Net, IceCube-Gen2, and future MeV gamma-ray telescopes.

A New Concept for High-Elevation Radio Detection of Tau Neutrinos

Cosmic neutrinos are expected to include a significant flux of tau neutrinos due to flavor mixing over astronomical length scales. However, the tau-neutrino content of astrophysical neutrinos is poorly constrained and a significant flux of cosmogenic tau neutrinos awaits discovery. Earth-skimming tau neutrinos undergo charged-current interactions that result in a tau lepton exiting the Earth. The tau lepton decay generates anextensive air shower and geomagnetic radio emission. To target the tau neutrinos, we present a new tau neutrino detector concept that uses phased antenna arrays placed on high elevation mountains. Simulation studies indicate that a modest array size and small number of stations can achieve competitive sensitivity, provided the receivers are at highs enough elevation.

Cosmic tau neutrino detection via Cherenkov signals from air showers from Earth-emerging taus

We perform a new, detailed calculation of the flux and energy spectrum of Earth-emerging τ-leptons generated from the interactions of tau neutrinos and antineutrinos in the Earth. A layered model of the Earth is used to describe the variable density profile of the Earth. Different assumptions regarding the neutrino charged- and neutral-current cross sections as well as the τ-lepton energy loss models are used to quantify their contributions to the systematic uncertainty. A baseline simulation is then used to generate the optical Cherenkov signal from upward-moving extensive air showers generated by the τ-lepton decay in the atmosphere, applicable to a range of space-based instruments. We use this simulation to determine the neutrino sensitivity for E ν ≳ 10 PeV for a space-based experiment with performance similar to that for the Probe of Extreme Multi-Messenger Astrophysics (POEMMA) mission currently under study.

Inferring the Flavor of High-Energy Astrophysical Neutrinos at Their Sources

The sources and production mechanisms of high-energy astrophysical neutrinos are largely unknown. A promising opportunity for progress lies in the study of neutrino flavor composition, i.e., the proportion of each flavor in the flux of neutrinos, which reflects the physical conditions at the sources. To seize it, we introduce a Bayesian method that infers the flavor composition at the neutrino sources based on the flavor composition measured at Earth. We find that the present data from the IceCube neutrino telescope favor neutrino production via the decay of high-energy pions and rule out production via the decay of neutrons. In the future, improved measurements of flavor composition and mixing parameters may single out the production mechanism with high significance.

The Self-Control of Cosmic Rays

Several independent pieces of information have recently hinted at a prominent role of cosmic rays in controlling their own transport, within and around the sources as well as throughout their propagation on Galactic scales and even possibly during their escape from the Galaxy. I will discuss this topic with special attention to the theoretical implications and possible additional observational evidence that we may seek with upcoming experiments.

IceCube High Energy Starting Events at 7.5 Years - New Measurements of Flux and Flavor

The IceCube Neutrino Observatory at the South Pole, which detects Cherenkov light from charged particles produced in neutrino interactions, firmly established the existence of an astrophysical high-energy neutrino component. Here I present IceCube’s High-Energy Starting Event sample and the new results obtained with a livetime of about 7.5 years. I will focus on the new measurement of the flavor composition performed using this sample. IceCube is directly sensitive to each neutrino flavor via the single cascade, track and double cascade event topologies, the latter being the topology produced in tauneutrino interactions above an energy threshold of ~100 TeV. A measurement of the flavor ratio on Earth can provide important constraints on sources and production mechanisms within the standard model, and also constrain various beyond-standard-model processes.

Echo Technique to Distinguish Flavors of Astrophysical Neutrinos

The flavor composition of high-energy astrophysical neutrinos is a rich observable. However, present analyses cannot effectively distinguish particle showers induced by ν_{e} vs ν_{τ}. We show that this can be accomplished by measuring the intensities of the delayed, collective light emission from muon decays and neutron captures, which are, on average, greater for ν_{τ} than for ν_{e}. This new technique would significantly improve tests of the nature of astrophysical sources and of neutrino properties. We discuss the promising prospects for implementing it in IceCube and other detectors.

Universe's Worth of Electrons to Probe Long-Range Interactions of High-Energy Astrophysical Neutrinos

Astrophysical searches for new long-range interactions complement collider searches for new short-range interactions. Conveniently, neutrino flavor oscillations are keenly sensitive to the existence of long-ranged flavored interactions between neutrinos and electrons, motivated by lepton-number symmetries of the standard model. For the first time, we probe them using TeV-PeV astrophysical neutrinos and accounting for all large electron repositories in the local and distant Universe. The high energies and colossal number of electrons grant us unprecedented sensitivity to the new interaction, even if it is extraordinarily feeble. Based on IceCube results for the flavor composition of astrophysical neutrinos, we set the ultimate bounds on long-range neutrino flavored interactions.

Escape of Cosmic Rays from the Galaxy and Effects on the Circumgalactic Medium

The escape of cosmic rays from the Galaxy is expected to shape their spectrum inside the Galaxy. Yet, this phenomenon is very poorly understood and, in the absence of a physical description, it is usually modeled as a free escape from a given boundary, typically located at a few kiloparsec distance from the Galactic disc. We show that the assumption of free escape leads to the conclusion that the cosmic ray current propagating in the circumgalactic medium is responsible for a nonresonant cosmic ray induced instability that in turn leads to the generation of a magnetic field of strength ∼2×10^{-8}  G on a scale ∼10  kpc around our Galaxy. The self-generated diffusion produces large gradients in the particle pressure that induce a displacement of the intergalactic medium with velocity ∼10-100  km/s. Cosmic rays are then carried away by advection. If the overdensity of the intergalactic gas in a region of size ∼10  kpc around our Galaxy is ≳100 with respect to the cosmological baryon density Ω_{b}ρ_{cr}, then the flux of high energy neutrinos due to pion production becomes comparable with the flux of astrophysical neutrinos recently measured by IceCube.

Constraints on Minute-Scale Transient Astrophysical Neutrino Sources

High-energy neutrino emission has been predicted for several short-lived astrophysical transients including gamma-ray bursts (GRBs), core-collapse supernovae with choked jets, and neutron star mergers. IceCube's optical and x-ray follow-up program searches for such transient sources by looking for two or more muon neutrino candidates in directional coincidence and arriving within 100 s. The measured rate of neutrino alerts is consistent with the expected rate of chance coincidences of atmospheric background events and no likely electromagnetic counterparts have been identified in Swift follow-up observations. Here, we calculate generic bounds on the neutrino flux of short-lived transient sources. Assuming an E^{-2.5} neutrino spectrum, we find that the neutrino flux of rare sources, like long gamma-ray bursts, is constrained to <5% of the detected astrophysical flux and the energy released in neutrinos (100 GeV to 10 PeV) by a median bright GRB-like source is <10^{52.5}  erg. For a harder E^{-2.13} neutrino spectrum up to 30% of the flux could be produced by GRBs and the allowed median source energy is <10^{52}  erg. A hypothetical population of transient sources has to be more common than 10^{-5}  Mpc^{-3}  yr^{-1} (5×10^{-8}  Mpc^{-3}  yr^{-1} for the E^{-2.13} spectrum) to account for the complete astrophysical neutrino flux.

Extracting the Energy-Dependent Neutrino-Nucleon Cross Section above 10 TeV Using IceCube Showers

Neutrinos are key to probing the deep structure of matter and the high-energy Universe. Yet, until recently, their interactions had only been measured at laboratory energies up to about 350 GeV. An opportunity to measure their interactions at higher energies opened up with the detection of high-energy neutrinos in IceCube, partially of astrophysical origin. Scattering off matter inside Earth affects the distribution of their arrival directions-from this, we extract the neutrino-nucleon cross section at energies from 18 TeV to 2 PeV, in four energy bins, in spite of uncertainties in the neutrino flux. Using six years of public IceCube High-Energy Starting Events, we explicitly show for the first time that the energy dependence of the cross section above 18 TeV agrees with the predicted softer-than-linear dependence, and reaffirm the absence of new physics that would make the cross section rise sharply, up to a center-of-mass energy sqrt[s]≈1  TeV.

Latest results on high-energy cosmic neutrino searches with the ANTARES neutrino telescope

The ANTARES detector is currently the largest undersea neutrino telescope. Located in the Mediterranean Sea at a depth of 2.5 km, 40 km off the Southern coast of France, it has been looking for cosmic neutrinos for more than 10 years. High-energy cosmic neutrino production is strongly linked with cosmic ray production. The latest results from IceCube Collaboration represent a step forward towards the confirmation of a highenergy cosmic ray source. The ANTARES location in the Northern Hemisphere is optimal for the observation of most of the Galactic Plane, including the Galactic Center. It has constrained the IceCube neutrino excess reports as well as, more recently, the flux from the source identified in the Blazar TXS 0506+056. The latest results of ANTARES on such analyses, including point-like and extended sources, diffuse fluxes, transient phenomena and multi-messenger studies, are presented.

Radio detection in the multi-messenger context

The present work discusses the development of the radio technique for detection of ultra-high energy air-showers induced by cosmic radiation, and the prospects of its application in the future multi-messenger activities, particularly for detection of ultra-high energy cosmic rays, gamma rays and neutrinos. It gives an overview of the results achieved by the modern digital radio arrays, as well as discuss present challenges and future prospects.

Recent Results from the Antares Neutrino Telescope

Antares, the first undersea neutrino telescope, has been continuously operating since 2007 in the Mediterranean Sea. The transparency of the water allows for a very good angular resolution in the reconstruction of neutrino events of all flavors. This results in an unmatched sensitivity for neutrino source searches, in a large fraction of the Southern Sky, at TeV energies. As a consequence, Antares provides valuable constraints on the origin of the cosmic neutrino flux discovered by the IceCube Collaboration. Based on an all-flavor dataset spanning nine years of operation of the detector, the latest results of Antares searches for neutrino point sources, and for diffuse neutrino emission from the entire sky as well as from several interesting regions such as the Galactic Plane, are presented. Several results have been obtained through a joint analysis with the IceCube Collaboration. Concerning the multi-messenger program, the focus is made on the follow-up searches of IceCube alerts, in particular the one related to the TXS 0506+056 blazar, thought to be the first extragalactic high-energy neutrino source identified so far.

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.

Search for tau neutrinos with the MAGIC telescopes: improving selection criteria*

MAGIC, a system of two Cherenkov telescopes located at the Roque de los Muchachos Observatory (2200 a.s.l.) in the Canary Island of La Palma, has lately been engaged in an unconventional task: the search for a signature of particle showers induced by earth-skimming cosmic tau neutrinos arising from the ocean, in the PeV to EeV energy range. When pointing at the sea, the MAGIC telescopes can collect data in a range of about 5 deg in zenith and 80 deg in azimuth: the analysis of the shower images from ~30 hours of data, together with the simulations of upward-going tau neutrino showers, shows that the air showers induced by tau neutrinos can be discriminated from the hadronic background coming from a similar direction. We have calculated the point source acceptance and the expected event rates, assuming an incoming v flux consistent with IceCube measurements, and for a sample of generic neutrino fluxes from photohadronic interactions in AGNs and GRBs. A 90% C.L. upper limit on the tau-neutrino point source flux of 2.0 × 10–4 GeV cm–2 s–1 has been obtained. The presented results can also be important for future Cherenkov experiments such as the Cherenkov Telescope Array. This next generation ground-based observatory can have a much better possibility to detect v, given its larger FOV and much larger effective area.

Invisible Neutrino Decay Could Resolve IceCube's Track and Cascade Tension

The IceCube Neutrino Observatory detects high energy astrophysical neutrinos in two event topologies: tracks and cascades. Since the flavor composition of each event topology differs, tracks and cascades can be used to test the neutrino properties and the mechanisms behind the neutrino production in astrophysical sources. Assuming a conventional model for the neutrino production, the IceCube data sets related to the two channels are in >3σ tension with each other. Invisible neutrino decay with lifetime τ/m=10^{2}  s/eV solves this tension. Noticeably, it leads to an improvement over the standard nondecay scenario of more than 3σ while remaining consistent with all other multimessenger observations. In addition, our invisible neutrino decay model predicts a reduction of 59% in the number of observed ν_{τ} events which is consistent with the current observational deficit.

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.

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.

Pulsar TeV Halos Explain the Diffuse TeV Excess Observed by Milagro

Milagro observations have found bright, diffuse TeV emission concentrated along the galactic plane of the Milky Way. The intensity and spectrum of this emission is difficult to explain with current models of hadronic γ-ray production, and has been named the "TeV excess." We show that TeV emission from pulsars naturally explains this excess. Recent observations have detected "TeV halos" surrounding pulsars that are either nearby or particularly luminous. Extrapolating this emission to the full population of Milky Way pulsars indicates that the ensemble of "subthreshold" sources necessarily produces bright TeV emission diffusively along the Milky Way plane. Models indicate that the TeV halo γ-ray flux exceeds that from hadronic γ rays above an energy of ∼500  GeV. Moreover, the spectrum and intensity of TeV halo emission naturally matches the TeV excess. Finally, we show that upcoming HAWC observations will resolve a significant fraction of the TeV excess into individual TeV halos, conclusively confirming, or ruling out, this model.

Imaging Galactic Dark Matter with High-Energy Cosmic Neutrinos

We show that the high-energy cosmic neutrinos seen by the IceCube Neutrino Observatory can be used to probe interactions between neutrinos and the dark sector that cannot be reached by current cosmological methods. The origin of the observed neutrinos is still unknown, and their arrival directions are compatible with an isotropic distribution. This observation, together with dedicated studies of Galactic plane correlations, suggests a predominantly extragalactic origin. Interactions between this isotropic extragalactic flux and the dense dark matter (DM) bulge of the Milky Way would thus lead to an observable imprint on the distribution, which would be seen by IceCube as (i) slightly suppressed fluxes at energies below a PeV and (ii) a deficit of events in the direction of the Galactic center. We perform an extended unbinned likelihood analysis using the four-year high-energy starting event data set to constrain the strength of DM-neutrino interactions for two model classes. We find that, in spite of low statistics, IceCube can probe regions of the parameter space inaccessible to current cosmological methods.

Measurement of the ν μ energy spectrum with IceCube-79: IceCube Collaboration

IceCube is a neutrino observatory deployed in the glacial ice at the geographic South Pole. The ν μ 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 ν -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 σ in four adjacent bins for neutrino energies E ν ≥ 177.8 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.

γ-ray Constraints on Decaying Dark Matter and Implications for IceCube

Utilizing the Fermi measurement of the γ-ray spectrum toward the Inner Galaxy, we derive some of the strongest constraints to date on the dark matter (DM) lifetime in the mass range from hundreds of MeV to above an EeV. Our profile-likelihood-based analysis relies on 413 weeks of Fermi Pass 8 data from 200 MeV to 2 TeV, along with up-to-date models for diffuse γ-ray emission within the Milky Way. We model Galactic and extragalactic DM decay and include contributions to the DM-induced γ-ray flux resulting from both primary emission and inverse-Compton scattering of primary electrons and positrons. For the extragalactic flux, we also calculate the spectrum associated with cascades of high-energy γ rays scattering off of the cosmic background radiation. We argue that a decaying DM interpretation for the 10 TeV-1 PeV neutrino flux observed by IceCube is disfavored by our constraints. Our results also challenge a decaying DM explanation of the AMS-02 positron flux. We interpret the results in terms of individual final states and in the context of simplified scenarios such as a hidden-sector glueball model.

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 [Formula: see text] 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 [Formula: see text] C.L. upper limit on the diffuse astrophysical neutrino flux with a value per neutrino flavour of [Formula: see text] is set, applicable to the energy range from 23 TeV to 7.8 PeV, assuming an unbroken [Formula: see text] spectrum and neutrino flavour equipartition at Earth.

Constraints on Ultrahigh-Energy Cosmic-Ray Sources from a Search for Neutrinos above 10 PeV with IceCube

We report constraints on the sources of ultrahigh-energy cosmic rays (UHECRs) above 10^{9}  GeV, based on an analysis of seven years of IceCube data. This analysis efficiently selects very high- energy neutrino-induced events which have deposited energies from 5×10^{5}  GeV to above 10^{11}  GeV. Two neutrino-induced events with an estimated deposited energy of (2.6±0.3)×10^{6}  GeV, the highest neutrino energy observed so far, and (7.7±2.0)×10^{5}  GeV were detected. The atmospheric background-only hypothesis of detecting these events is rejected at 3.6σ. The hypothesis that the observed events are of cosmogenic origin is also rejected at >99% CL because of the limited deposited energy and the nonobservation of events at higher energy, while their observation is consistent with an astrophysical origin. Our limits on cosmogenic neutrino fluxes disfavor the UHECR sources having a cosmological evolution stronger than the star formation rate, e.g., active galactic nuclei and γ-ray bursts, assuming proton-dominated UHECRs. Constraints on UHECR sources including mixed and heavy UHECR compositions are obtained for models of neutrino production within UHECR sources. Our limit disfavors a significant part of parameter space for active galactic nuclei and new-born pulsar models. These limits on the ultrahigh-energy neutrino flux models are the most stringent to date.