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Characterization of the astrophysical diffuse neutrino flux using starting track events in IceCube
Abstract
A measurement of the diffuse astrophysical neutrino spectrum is presented using IceCube data collected from 2011–2022 (10.3 years). We developed novel detection techniques to search for events with a contained vertex and exiting track induced by muon neutrinos undergoing a charged-current interaction. Searching for these starting track events allows us to not only more effectively reject atmospheric muons but also atmospheric neutrino backgrounds in the southern sky, opening a new window to the sub-100 TeV astrophysical neutrino sky. The event selection is constructed using a dynamic starting track veto and machine learning algorithms. We use this data to measure the astrophysical diffuse flux as a single power law flux (SPL) with a best-fit spectral index of γ=2.58−0.09+0.10 and per-flavor normalization of ϕper−flavorAstro=1.68−0.22+0.19×10−18×GeV−1 cm−2 s−1 sr−1 (at 100 TeV). The sensitive energy range for this dataset is 3–550 TeV under the SPL assumption. This data was also used to measure the flux under a broken power law, however we did not find any evidence of a low energy cutoff.
... Despite this idea could have not been experimental explored for many years, current γ-ray and neutrino telescopes have strongly improved our knowledge about astrophysical messengers, making it possible to quantitative constrain this scenario. Firstly, many analyses have highlighted a tension between the diffuse γ-ray [7] and neutrino [8][9][10][11] fluxes, especially because of the large neutrino flux below 100 TeV, exploring the potential role of hidden CR accelerators [12][13][14][15][16][17][18][19][20][21][22] (see also [23] for other details). Furthermore, the IceCube collaboration has recently found a 4.2 σ excess above the background-only hypothesis of 72 high-energy neutrino events coming from the direction of NGC 1068, a nearby AGN [24], inferring a power-law differential neutrino flux (∼ E −3.2 ) approximately ten times higher than expected from a γ-ray transparent source, taking into account the γ-ray emission observed by the Fermi-LAT telescope from this source [25][26][27]. ...
... We then extrapolate this information to the whole Seyfert population (diffuse neutrino flux), using the X-ray luminosity function (see [47]), showing that it slightly overproduce the diffuse neutrino flux inferred by the ICeCube collaboration through the starting track sample [11] in the ∼ 1−10 TeV range. Future observations from the upcoming neutrino telescopes, such as KM3NeT [48], Baikal-GVD [49][50][51], IceCube gen 2 [52], P-ONE [53] and the TRIDENT [54] telescopes will be fundamental to unveil the role of Seyfert galaxies to the diffuse neutrino spectrum as well as the role of other astrophysical accelerators. ...
... For the source density uncertainty, we take into account the variability of all parameters reported in tables 2 and 3. For F CT , we consider a uniform distribution between [0, 1]. 2 We compare the results with the latest diffuse IceCube data (6 year cascade [9] and 10 year of starting tracks [11]) and the expected KM3NeT differential sensitivity after 10 years of full operation considering all-sky shower events [48]. We notice that if we assume that all the Seyfert galaxies have the same relation between L ν and L X as imposed by the four sources used in the statistical analysis, we overproduce the diffuse neutrino flux at ∼ 1−10 TeV. ...
The IceCube Collaboration has recently reported compelling evidence of high-energy neutrino emission from NGC 1068, and also mild excesses for NGC 4151 and CGCG420-015, local Seyfert galaxies. This has increased the interest along neutrino emission from hot-corona surrounding the super massive black holes of Seyfert Galaxies. In this paper, we revisit phenomenological constraints on the neutrino emission from hot-coronae of Seyfert galaxies, using an assumption of sub-equipartition between cosmic-rays and magnetic energy densities. We show that not only these sources are consistent with such an assumption but also that the data point towards low values for the ratio between thermal and magnetic pressure, the so called beta plasma parameters inside Seyfert galaxies. We exploit this finding to constrain the Seyfert diffuse neutrino flux and we obtain that, in order not to overproduce neutrinos, not all the sources can be efficient neutrino emitters. In our approach (along with previous findings), Seyfert galaxies provide a negligible contribution to the diffuse neutrino spectrum above ∼ 100 TeV, allowing space for other astrophysical sources. However, future data from high-energy neutrino telescopes will be crucial to shed more light onto the contribution of this source class to the cosmic neutrino background.
... One of the main goals of the IceCube observatory is the observation and description of ultrahigh-energy astrophysical neutrinos. Since the beginning of its operation, the associated neutrino flux have been constrained in different analyses, considering new data and distinct topologies [1][2][3][4][5]. These analyses are strongly motivated by our lack of knowledge about the origin and propagation of neutrinos, and by the fact that they can even be a powerful tool for searching for beyond the standard model physics [6]. ...
... In order to quantify the importance of tracks coming from subdominant channels, in Fig. 5 (left panel) we present the number of tracks events expected in the IceCube Observatory considering a period of 7.5 years, as a function of the electromagnetic equivalent deposited energy. In this calculation, we have assumed γ ¼ 2.38 and Φ 0 ¼ 1.51 in the astrophysical neutrino flux, which is the best fit for the last analysis of tracks in the IceCube Observatory [3]. The number of events is dominated by muon neutrino interactions, except at the peak of the Glashow resonance. ...
Tracks events at the IceCube Observatory are characterized by an energetic muon crossing several kilometers before decaying. Such muons are dominantly produced in charged current (CC) muon neutrino-hadron interactions. However, muons are also produced through W -boson production and in the decay of tau leptons and heavy mesons created in neutral and charged current interactions induced by all neutrino flavors. In this paper, we investigate the contribution of these subleading channels to events characterized as tracks at the IceCube. Our results indicate that these channels correspond to a non-negligible fraction of the high-energy starting events track events. In addition, we show that its contributions are concentrated in muons that are less energetic than those arising from muonic neutrino CC interactions for the same visible energies of the process. Finally, we investigate the impact of these additional channels on the description of the astrophysical neutrino flux, and we find that the inclusion of these subleading processes are important in determining the parameters of the astrophysical neutrino flux.
Published by the American Physical Society 2024
... IceCube has been continuously operating for 13 years and found the celestial neutrinos generally follow simple power laws with several selection criteria, equally distributed among flavors, as seen in Figure 1. The measurements with various analyses, such as high energy starting events [10], through-going muon tracks [11], cascades [12] and enhanced starting track event selection [13] are shown. These independent measurements are compatible with each other with spectral index ∼ 2.4 -2.9. ...
... Moreover, neutrino observation can provide hints on fundamental physics like dark matter and neutrino physics and investigate the fields of marine and Earth sciences. [11,13,28,29]. The blue band shows the best-fitting result of the π 0 model for Galactic plane [17]. ...
Over the past ten years, several breakthroughs have been made in multi-messenger astronomy. Thanks to the IceCube Neutrino Observatory, the detection of astrophysical neutrinos was proved to be practical. However, no source has been significantly identified due to the lack of statistics and uncovered field of view. The next generation of high-energy neutrino telescope is in high demand. We propose the NEutrino Observatory in the Nanhai (NEON), located in the South China Sea to be complementary for the global neutrino detectors. This proposal describes the design and layout of the array and reports on comprehensive simulations conducted to assess its performance. The NEON project, with a volume of 10 km$^3$, achieves an angular resolution of 0.1$^\circ$ at 100 TeV. With 10 years of operation, the project's 5$\sigma$ sensitivity is estimated as $E^2\Phi \sim 2 \times 10^{-10}$ GeV cm$^{-2}$ s$^{-1}$ for a source spectrum index of -2. We found that the variation in depth from 1700 to 3500 meters does not significantly influence the sensitivity to steady sources.
... In 2013, the IceCube neutrino telescope-the largest in operation-discovered TeV-PeV astrophysical neutrinos [55,56]. Today, IceCube continues to observe them regularly [57][58][59], and neutrino telescopes Baikal-GVD [60,61] and KM3NeT [62], currently under construction, (and ANTARES [63], now decommissioned) have reported hints of detection. Still, the origin of the bulk of high-energy astrophysical neutrinos detected remains unknown [64]. ...
Time-of-flight measurements of high-energy astrophysical neutrinos can be used to probe Lorentz invariance, a pillar of modern physics. If Lorentz-invariance violation (LIV) occurs, it could cause neutrinos to slow down, with the delay scaling linearly or quadratically with their energy. We introduce non-parametric statistical methods designed to detect LIV-induced distortions in the temporal structure of a high-energy neutrino flare as it travels to Earth from a distant astrophysical source, independently of the intrinsic timing properties of the source. Our approach, illustrated using the 2014/2015 TeV-PeV neutrino flare from the blazar TXS 0506+056 detected by IceCube, finds that the LIV energy scale must exceed 10^{14} GeV (linear) or 10^9 GeV (quadratic). Our methods provide a robust means to investigate LIV by focusing solely on a neutrino flare, without relying on electromagnetic counterparts, and account for realistic energy and directional uncertainties. For completeness, we compare our limits inferred from TXS 0506+056 to the sensitivity inferred from multi-messenger detection of tentative coincidences between neutrinos and electromagnetic emission from active galactic nuclei and tidal disruption events.
... From Fig. 8 of Ref. [51], the corresponding coupling ruled out by the IceCube (Refs. [52][53][54][55]) is above 0.6 − 1.0 in 0.5−1 TeV leptoquark mass range, while the low-energy experiments ruled out the couplings above 0.2 for M U 1 = 500 GeV and above 0.8 for M U 1 = 2500 GeV. Note that the strongest bound from the LHC search is due to pair production, which is independent of the Yukawa couplings as long as they are not extremely small as the current search is based on prompt decays of the leptoquarks [56,57]. ...
In this study, we calculate the sensitivity reach on the vector leptoquark (LQ) $U_1$ at the experiments proposed in Forward Physics Facility (FPF), including FASER$\nu$, FASER$\nu2$, FLArE (10 tons), and FLArE (100 tons) using the neutrino-nucleon scattering ($\nu N \rightarrow \nu N'$ and $\nu N \rightarrow l N'$). We cover a wide mass range of $10^{-3}$ GeV $\leq M_{LQ}\leq 10^4$ GeV. The new result shows that the FLArE (100 tons) offers the best sensitivity to the LQ model. The sensitivity curves for all the experiments follow a similar pattern with weakened sensitivities with the increment of the LQ mass. We combine the sensitivities obtained from the neutral- and charged-current interactions of the neutrinos.
... In particular, with the 10-year data-set of through-going (TG) events, a best-fit is found for the spectral index of TG = 2.28 [1], as expected from an extragalactic astrophysical origin. Differently, the 7.5-year High-Energy Starting Events (HESE) data-set, which observes the full sky, prefers a steeper spectrum with a best-fit index of HESE = 2.89 [2]. We test the hypothesis of an isotropic astrophysical flux with the TG best-fit and an additional DM signal, against an isotropic astrophysical flux only from the HESE best-fit. ...
Low fluxes of astrophysical neutrinos at TeV energies, and the overwhelming background of atmospheric neutrinos below that, render the current paradigm of neutrino astronomy a severely statistics-limited one. While many hints have emerged, all the evidence gathered by IceCube and ANTARES, over the course of almost a decade and a half of operation, has fallen short of providing any conclusive answer to the puzzle of the origin of high-energy cosmic rays and neutrinos. The advancement of the field is thus closely associated with not only the neutrino observatories coming online in the next few years, but also on the coordinated efforts of the EM, GW and cosmic ray communities to develop dedicated channels and infrastructure that allow for the swift and comprehensive multi-messenger follow-up of relevant events detected in any of these sectors. This paper highlights the strides that have been already taken in that direction and the fruits that they have borne, as well as the challenges that lie ahead.
We describe a new data sample of IceCube DeepCore and report on the latest measurement of atmospheric neutrino oscillations obtained with data recorded between 2011–2019. The sample includes significant improvements in data calibration, detector simulation, and data processing, and the analysis benefits from a sophisticated treatment of systematic uncertainties, with significantly greater level of detail since our last study. By measuring the relative fluxes of neutrino flavors as a function of their reconstructed energies and arrival directions we constrain the atmospheric neutrino mixing parameters to be sin2θ23=0.51±0.05 and Δm322=2.41±0.07×10−3 eV2, assuming a normal mass ordering. The errors include both statistical and systematic uncertainties. The resulting 40% reduction in the error of both parameters with respect to our previous result makes this the most precise measurement of oscillation parameters using atmospheric neutrinos. Our results are also compatible and complementary to those obtained using neutrino beams from accelerators, which are obtained at lower neutrino energies and are subject to different sources of uncertainties.
The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 2,143 new measurements from 709 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Among the 120 reviews are many that are new or heavily revised, including a new review on Machine Learning, and one on Spectroscopy of Light Meson Resonances.
The Review is divided into two volumes. Volume 1 includes the Summary Tables and 97 review articles. Volume 2 consists of the Particle Listings and contains also 23 reviews that address specific aspects of the data presented in the Listings.
The complete Review (both volumes) is published online on the website of the Particle Data Group (pdg.lbl.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print, as a web version optimized for use on phones, and as an Android app.
The diffuse flux of cosmic neutrinos has been measured by the IceCube Observatory from TeV to PeV energies. We show that an improved characterization of this flux at lower energies, TeV and sub-TeV, reveals important information on the nature of the astrophysical neutrino sources in a model-independent way. Most significantly, it could confirm the present indications that neutrinos originate in cosmic environments that are optically thick to GeV–TeV γ -rays. This conclusion will become inevitable if an uninterrupted or even steeper neutrino power law is observed in the TeV region. In such γ -ray-obscured sources, the γ -rays that inevitably accompany cosmic neutrinos will cascade down to MeV–GeV energies. The requirement that the cascaded γ -ray flux accompanying cosmic neutrinos should not exceed the observed diffuse γ -ray background puts constraints on the peak energy and density of the radiation fields in the sources. Our calculations inspired by the existing data suggest that a fraction of the observed diffuse MeV–GeV γ -ray background may be contributed by neutrino sources with intense radiation fields that obscure the high-energy γ -ray emission accompanying the neutrinos.
We present a measurement of the high-energy astrophysical muon–neutrino flux with the IceCube Neutrino Observatory. The measurement uses a high-purity selection of 650k neutrino-induced muon tracks from the northern celestial hemisphere, corresponding to 9.5 yr of experimental data. With respect to previous publications, the measurement is improved by the increased size of the event sample and the extended model testing beyond simple power-law hypotheses. An updated treatment of systematic uncertainties and atmospheric background fluxes has been implemented based on recent models. The best-fit single power-law parameterization for the astrophysical energy spectrum results in a normalization of ϕ@100TeVνμ+ν¯μ=1.44−0.26+0.25×10−18GeV−1cm−2s−1sr−1 and a spectral index γSPL=2.37−0.09+0.09 constrained in the energy range from 15 TeV to 5 PeV. The model tests include a single power law with a spectral cutoff at high energies, a log-parabola model, several source-class-specific flux predictions from the literature, and a model-independent spectral unfolding. The data are consistent with a single power-law hypothesis, however, spectra with softening above one PeV are statistically more favorable at a two-sigma level.
The Glashow resonance describes the resonant formation of a W− boson during the interaction of a high-energy electron antineutrino with an electron1, peaking at an antineutrino energy of 6.3 petaelectronvolts (PeV) in the rest frame of the electron. Whereas this energy scale is out of reach for currently operating and future planned particle accelerators, natural astrophysical phenomena are expected to produce antineutrinos with energies beyond the PeV scale. Here we report the detection by the IceCube neutrino observatory of a cascade of high-energy particles (a particle shower) consistent with being created at the Glashow resonance. A shower with an energy of 6.05 ± 0.72 PeV (determined from Cherenkov radiation in the Antarctic Ice Sheet) was measured. Features consistent with the production of secondary muons in the particle shower indicate the hadronic decay of a resonant W− boson, confirm that the source is astrophysical and provide improved directional localization. The evidence of the Glashow resonance suggests the presence of electron antineutrinos in the astrophysical flux, while also providing further validation of the standard model of particle physics. Its unique signature indicates a method of distinguishing neutrinos from antineutrinos, thus providing a way to identify astronomical accelerators that produce neutrinos via hadronuclear or photohadronic interactions, with or without strong magnetic fields. As such, knowledge of both the flavour (that is, electron, muon or tau neutrinos) and charge (neutrino or antineutrino) will facilitate the advancement of neutrino astronomy. A particle shower detected by the IceCube Neutrino Observatory at the very high energy of the Glashow resonance demonstrates its potential for the study of high-energy particle physics and astrophysics.
Despite the spectacular discovery of an astrophysical neutrino flux by IceCube in 2013, its origin remains a mystery. Whatever its sources, we expect the neutrino flux to be accompanied by a comparable gamma-ray flux. These photons should be degraded in energy by electromagnetic cascades and contribute to the diffuse GeV-TeV flux precisely measured by the Fermi-LAT. Population studies have also permitted to identify the main classes of contributors to this flux, which at the same time have not been associated with major neutrino sources in cross-correlation studies. These considerations allow one to set constraints on the origin and spectrum of the IceCube flux, in particular its low-energy part. We find that, even accounting for known systematic errors, the Fermi-LAT data exclude to at least 95% C.L. any extragalactic transparent source class, irrespective of its redshift evolution, if the neutrino spectrum extends to the TeV scale or below. If the neutrino spectrum has an abrupt cutoff at ∼10 TeV, barely compatible with current observations, the tension can be reduced, but this way out requires a significant modification to the current understanding of the origin of the diffuse extragalactic gamma-ray flux at GeV energies. In contrast, these considerations do not apply if a sizable fraction of IceCube data originates within the Galactic halo (a scenario however typically in tension with other constraints) or from a yet unidentified class of "opaque"extragalactic emitters, which do not let the high-energy gamma rays get out.
We present constraints on an astrophysical population of neutrino sources imposed by recent data from the IceCube neutrino observatory. By using the IceCube point source search method to model the detection of sources, our detection criterion is more sensitive than using the observation of high-energy neutrino multiplets for source identification. We frame the problem as a Bayesian hierarchical model to connect the high-level population parameters to the IceCube data, allowing us to consistently account for all relevant sources of uncertainty in our model assumptions. Our results show that sources with a local density of n0≳10−7 Mpc−3 and luminosity L≲1043 erg s−1 are the most likely candidates, but that populations of rare sources with n0≃10−9 Mpc−3 and L≃1045 erg s−1 can still be consistent with the IceCube observations. We demonstrate that these conclusions are strongly dependent on the source evolution considered, for which we consider a wide range of models. In doing so, we present realistic, model-independent constraints on the population parameters that reflect our current state of knowledge from astrophysical neutrino observations. We also use our framework to investigate constraints in the case of possible source detections and future instrument upgrades. Our approach is flexible and can be used to model specific source cases and extended to include multimessenger information.
Muons and neutrinos from cosmic ray interactions in the atmosphere originate from decays of mesons in air-showers. sibyll-2.3c aims to give a precise description of hadronic interactions in the relevant phase space for conventional and prompt leptons in light of new accelerator data, including that from the LHC. sibyll is designed primarily as an event generator for use in simulation of extensive air showers. Because it has been tuned for forward physics as well as the central region, it can also be used to calculate inclusive fluxes. The purpose of this paper is to describe the use of sibyll-2.3c for calculation of fluxes of atmospheric leptons.
Low-background searches for astrophysical neutrino sources anywhere in the sky can be performed using cascade events induced by neutrinos of all flavors interacting in IceCube with energies as low as similar to 1 TeV. Previously we showed that, even with just two years of data, the resulting sensitivity to sources in the southern sky is competitive with IceCube and ANTARES analyses using muon tracks induced by charge current muon neutrino interactions-especially if the neutrino emission follows a soft energy spectrum or originates from an extended angular region. Here, we extend that work by adding five more years of data, significantly improving the cascade angular resolution, and including tests for point-like or diffuse Galactic emission to which this data set is particularly well suited. For many of the signal candidates considered, this analysis is the most sensitive of any experiment to date. No significant clustering was observed, and thus many of the resulting constraints are the most stringent to date. In this paper we will describe the improvements introduced in this analysis and discuss our results in the context of other recent work in neutrino astronomy.
Inelasticity, the fraction of a neutrino’s energy transferred to hadrons, is a quantity of interest in the study of astrophysical and atmospheric neutrino interactions at multi-TeV energies with IceCube. In this work, a sample of contained neutrino interactions in IceCube is obtained from five years of data and classified as 2650 tracks and 965 cascades. Tracks arise predominantly from charged-current νμ interactions, and we demonstrate that we can reconstruct their energy and inelasticity. The inelasticity distribution is found to be consistent with the calculation of Cooper-Sarkar et al. across the energy range from ∼1 to ∼100 TeV. Along with cascades from neutrinos of all flavors, we also perform a fit over the energy, zenith angle, and inelasticity distribution to characterize the flux of astrophysical and atmospheric neutrinos. The energy spectrum of diffuse astrophysical neutrinos is described well by a power law in both track and cascade samples, and a best-fit index γ=2.62±0.07 is found in the energy range from 3.5 TeV to 2.6 PeV. Limits are set on the astrophysical flavor composition and are compatible with a ratio of (13∶13∶13)⊕. Exploiting the distinct inelasticity distribution of νμ and ν¯μ interactions, the atmospheric νμ to ν¯μ flux ratio in the energy range from 770 GeV to 21 TeV is found to be 0.77−0.25+0.44 times the calculation by Honda et al. Lastly, the inelasticity distribution is also sensitive to neutrino charged-current charm production. The data are consistent with a leading-order calculation, with zero charm production excluded at 91% confidence level. Future analyses of inelasticity distributions may probe new physics that affects neutrino interactions both in and beyond the Standard Model.
The existence of diffuse Galactic neutrino production is expected from cosmic-ray interactions with Galactic gas and radiation fields. Thus, neutrinos are a unique messenger offering the opportunity to test the products of Galactic cosmic-ray interactions up to energies of hundreds of TeV. Here we present a search for this production using ten years of Astronomy with a Neutrino Telescope and Abyss environmental RESearch (ANTARES) track and shower data, as well as seven years of IceCube track data. The data are combined into a joint likelihood test for neutrino emission according to the KRAg model assuming a 5 PeV per nucleon Galactic cosmic-ray cutoff. No significant excess is found. As a consequence, the limits presented in this Letter start constraining the model parameter space for Galactic cosmic-ray production and transport. © 2018. The American Astronomical Society. All rights reserved.
The atmospheric neutrino passing fraction, or self-veto, is defined as the probability for an atmospheric neutrino not to be accompanied by a detectable muon from the same cosmic-ray air shower. Building upon previous work, we propose a redefinition of the passing fractions by unifying the treatment for muon and electron neutrinos. Several approximations have also been removed. This enables performing detailed estimations of the uncertainties in the passing fractions from several inputs: muon losses, cosmic-ray spectrum, hadronic-interaction models and atmosphere-density profiles. We also study the passing fractions under variations of the detector configuration: depth, surrounding medium and muon veto trigger probability. The calculation exhibits excellent agreement with passing fractions obtained from Monte Carlo simulations. Finally, we provide a general software framework to implement this veto technique for all large-scale neutrino observatories.
Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of e290 tera–electronvolts. Its arrival direction was consistent with the location of a known g-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to g-rays. These observations characterize the
variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy g-rays. This observation of a neutrino in spatial coincidence with a g-ray–emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.
Neutrino emission from a flaring blazar
Neutrinos interact only very weakly with matter, but giant detectors have succeeded in detecting small numbers of astrophysical neutrinos. Aside from a diffuse background, only two individual sources have been identified: the Sun and a nearby supernova in 1987. A multiteam collaboration detected a high-energy neutrino event whose arrival direction was consistent with a known blazar—a type of quasar with a relativistic jet oriented directly along our line of sight. The blazar, TXS 0506+056, was found to be undergoing a gamma-ray flare, prompting an extensive multiwavelength campaign. Motivated by this discovery, the IceCube collaboration examined lower-energy neutrinos detected over the previous several years, finding an excess emission at the location of the blazar. Thus, blazars are a source of astrophysical neutrinos.
Science , this issue p. 147 , p. eaat1378
The origins of high-energy astrophysical neutrinos remain a mystery despite extensive searches for their sources. We present constraints from seven years of IceCube Neutrino Observatory muon data on the neutrino flux coming from the Galactic plane. This flux is expected from cosmic-ray interactions with the interstellar medium or near localized sources. Two methods were developed to test for a spatially extended flux from the entire plane, both of which are maximum likelihood fits but with different signal and background modeling techniques. We consider three templates for Galactic neutrino emission based primarily on gamma-ray observations and models that cover a wide range of possibilities. Based on these templates and in the benchmark case of an unbroken power-law energy spectrum, we set 90% confidence level upper limits, constraining the possible Galactic contribution to the diffuse neutrino flux to be relatively small, less than 14% of the flux reported in Aartsen et al. above 1 TeV. A stacking method is also used to test catalogs of known high-energy Galactic gamma-ray sources.
The IceCube Neutrino Observatory is a cubic-kilometer-scale high-energy neutrino detector built into the ice at the South Pole. Construction of IceCube, the largest neutrino detector built to date, was completed in 2011 and enabled the discovery of high-energy astrophysical neutrinos. We describe here the design, production, and calibration of the IceCube digital optical module (DOM), the cable systems, computing hardware, and our methodology for drilling and deployment. We also describe the online triggering and data filtering systems that select candidate neutrino and cosmic ray events for analysis. Due to a rigorous pre-deployment protocol, 98.4% of the DOMs in the deep ice are operating and collecting data. IceCube routinely achieves a detector uptime of 99% by emphasizing software stability and monitoring. Detector operations have been stable since construction was completed, and the detector is expected to operate at least until the end of the next decade.
We evaluate the prompt atmospheric neutrino flux at high energies using three different frameworks for calculating the heavy quark production cross section in QCD: NLO perturbative QCD, $k_T$ factorization including low-$x$ resummation, and the dipole model including parton saturation. We use QCD parameters, the value for the charm quark mass and the range for the factorization and renormalization scales that provide the best description of the total charm cross section measured at fixed target experiments, at RHIC and at LHC. Using these parameters we calculate differential cross sections for charm and bottom production and compare with the latest data on forward charm meson production from LHCb at $7$ TeV and at $13$ TeV, finding good agreement with the data. In addition, we investigate the role of nuclear shadowing by including nuclear parton distribution functions (PDF) for the target air nucleus using two different nuclear PDF schemes. Depending on the scheme used, we find the reduction of the flux due to nuclear effects varies from $10\%$ to $35 \%$ at the highest energies. Finally, we compare our results with the IceCube limit on the prompt neutrino flux, which is already providing valuable information about some of the QCD models.
The Fermi Large Area Telescope (LAT) Collaboration has recently released a catalog of 360 sources detected above 50 GeV (2FHL). This catalog was obtained using 80 months of data re-processed with Pass 8, the newest event-level analysis, which significantly improves the acceptance and angular resolution of the instrument. Most of the 2FHL sources at high Galactic latitude are blazars. Using detailed Monte Carlo simulations, we measure, for the first time, the source count distribution, dN/dS, of extragalactic γ-ray sources at E>50 GeV and find that it is compatible with a Euclidean distribution down to the lowest measured source flux in the 2FHL (∼8×10-12 ph cm-2 s-1). We employ a one-point photon fluctuation analysis to constrain the behavior of dN/dS below the source detection threshold. Overall, the source count distribution is constrained over three decades in flux and found compatible with a broken power law with a break flux, Sb, in the range [8×10-12,1.5×10-11] ph cm-2 s-1 and power-law indices below and above the break of α2[1.60,1.75] and α1=2.49±0.12, respectively. Integration of dN/dS shows that point sources account for at least 86-14+16% of the total extragalactic γ-ray background. The simple form of the derived source count distribution is consistent with a single population (i.e., blazars) dominating the source counts to the minimum flux explored by this analysis. We estimate the density of sources detectable in blind surveys that will be performed in the coming years by the Cherenkov Telescope Array.
The IceCube Neutrino Observatory consists of 5160 digital optical modules (DOMs) in a cubic kilometer of deep ice below the South Pole. The DOMs record the Cherenkov light from charged particles interacting in the ice. A good understanding of the optical properties of the ice is crucial to the quality of the event reconstruction. While the optical properties of the undisturbed ice are well understood, the properties of the refrozen drill holes still pose a challenge. A new data-acquisition and analysis approach using light originating from LEDs within one DOM detected by the photomultiplier of the same DOM will be described. This method allows us to explore the scattering length in the immediate vicinity of the considered DOMs.
The latest IceCube data suggest that the all-flavor cosmic neutrino flux may
be as large as 10^-7 GeV/cm^2/s/sr around 30 TeV. We show that, if
astrophysical sources of the TeV-PeV neutrinos are transparent to gamma rays
with respect to two-photon annihilation, a large fraction of the isotropic
diffuse gamma-ray background should originate from hadronic emission of such
sources, independently of the production mechanism. Strong tensions with the
diffuse gamma-ray data are unavoidable especially in hadronuclear scenarios. We
further show that, if the IceCube neutrinos have a photohadronic origin, the
sources are expected to be opaque to 1-100 GeV gamma rays. With these general
multimessenger arguments, we find that the latest data may indicate a
population of CR accelerators hidden in GeV-TeV gamma rays. Searches for x-ray
and MeV gamma-ray counterparts are encouraged, and TeV-PeV neutrinos themselves
will serve as special probes of dense source environments.
Muons produced in atmospheric cosmic ray showers account for the by far dominant part of the event yield in large-volume underground particle detectors. The IceCube detector, with an instrumented volume of about a cubic kilometer, has the potential to conduct unique investigations on atmospheric muons by exploiting the large collection area and the possibility to track particles over a long distance. Through detailed reconstruction of energy deposition along the tracks, the characteristics of muon bundles can be quantified, and individual particles of exceptionally high energy identified. The data can then be used to constrain the cosmic ray primary flux and the contribution to atmospheric lepton fluxes from prompt decays of short-lived hadrons.
In this paper, techniques for the extraction of physical measurements from atmospheric muon events are described and first results are presented. The multiplicity spectrum of TeV muons in cosmic ray air showers for primaries in the energy range from the knee to the ankle is derived and found to be consistent with recent results from surface detectors. The single-muon energy spectrum is determined up to PeV energies and shows a clear indication for the emergence of a distinct spectral component from prompt decays of short-lived hadrons. The magnitude of the prompt flux, which should include a substantial contribution from light vector meson di-muon decays, is consistent with current theoretical predictions.
The variety of measurements and high event statistics can also be exploited for the evaluation of systematic effects. In the course of this study, internal inconsistencies were found which indicate the presence of an unexplained effect outside the range of detector systematics. The underlying cause could be related to the hadronic interaction models used to describe muon production in air showers.
High-energy neutrinos and photons are complementary messengers, probing violent astrophysical processes and structural evolution of the Universe. X-ray and neutrino observations jointly constrain conditions in active galactic nuclei (AGN) jets: their baryonic and leptonic contents, and particle production efficiency. Testing two standard neutrino production models for local source Cen A, we calculate the high-energy neutrino spectra of single AGN sources and derive the flux of high-energy neutrinos expected for the current epoch. Assuming that accretion determines both X-rays and particle creation, our parametric scaling relations predict neutrino yield in various AGN classes. We derive redshift-dependent number densities of each class, from Chandra and Swift/BAT X-ray luminosity functions. We integrate the neutrino spectrum expected from the cumulative history of AGN (correcting for cosmological and source effects, e.g. jet orientation and beaming). Both emission scenarios yield neutrino fluxes well above limits set by IceCube (by ∼4–10⁶× at 1 PeV, depending on the assumed jet models for neutrino production). This implies that: (i) Cen A might not be a typical neutrino source as commonly assumed; (ii) both neutrino production models overestimate the efficiency; (iii) neutrino luminosity scales with accretion power differently among AGN classes and hence does not follow X-ray luminosity universally; (iv) some AGN are neutrino-quiet (e.g. below a power threshold for neutrino production); (v) neutrino and X-ray emission have different duty cycles (e.g. jets alternate between baryonic and leptonic flows); or (vi) some combination of the above.
In this Letter we propose a novel interpretation of the anomalous TeV
gamma-ray diffuse emission observed by Milagro in the inner Galactic plane
consistent with the signal reported by H.E.S.S. in the Galactic ridge;
remarkably, our picture also accounts for a relevant portion of the neutrino
flux measured by IceCube. Our scenario is based on a recently proposed
phenomenological model characterized by radially-dependent cosmic-ray (CR)
transport properties. Designed to reproduce both Fermi-LAT gamma-ray data and
local CR observables, this model offers for the first time a self-consistent
picture of both the GeV and the TeV sky.
An efficient method for calculating inclusive conventional and prompt
atmospheric leptons fluxes is presented. The coupled cascade equations are
solved numerically by formulating them as matrix equation. The presented
approach is very flexible and allows the use of different hadronic interaction
models, realistic parametrizations of the primary cosmic-ray flux and the
Earth's atmosphere, and a detailed treatment of particle interactions and
decays. The power of the developed method is illustrated by calculating lepton
flux predictions for a number of different scenarios.
The γ-ray sky can be decomposed into individually detected sources, diffuse emis- sion attributed to the interactions of Galactic cosmic rays with gas and radiation fields, and a residual all-sky emission component commonly called the isotropic diffuse γ-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are approximately isotropic. The first IGRB measurement with the Large Area Tele- scope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey data and considered an energy range between 200 MeV and 100 GeV. Im- provements in event selection and characterization of cosmic-ray backgrounds, better understanding of the diffuse Galactic emission, and a longer data accumulation of 50 months, allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB spectrum shows a significant high-energy cutoff feature, and can be well described over nearly four decades in energy by a power law with exponential cutoff having a spectral index of 2.32 ± 0.02 and a break energy of (279±52) GeV using our baseline diffuse Galactic emission model. The total intensity attributed to the IGRB is (7.2 ± 0.6) × 10−6 cm−2 s−1 sr−1 above 100 MeV, with an additional +15%/−30% systematic uncertainty due to the Galactic diffuse foregrounds.
The IceCube Neutrino Observatory was designed primarily to search for
high-energy (TeV--PeV) neutrinos produced in distant astrophysical objects. A
search for $\gtrsim 100$~TeV neutrinos interacting inside the instrumented
volume has recently provided evidence for an isotropic flux of such neutrinos.
At lower energies, IceCube collects large numbers of neutrinos from the weak
decays of mesons in cosmic-ray air showers. Here we present the results of a
search for neutrino interactions inside IceCube's instrumented volume between
1~TeV and 1~PeV in 641 days of data taken from 2010--2012, lowering the energy
threshold for neutrinos from the southern sky below 10 TeV for the first time,
far below the threshold of the previous high-energy analysis. Astrophysical
neutrinos remain the dominant component in the southern sky down to 10 TeV.
From these data we derive new constraints on the diffuse astrophysical neutrino
spectrum, $\Phi_{\nu} = 2.06^{+0.4}_{-0.3} \times 10^{-18}
\left({E_{\nu}}/{10^5 \,\, \rm{GeV}} \right)^{-2.46 \pm 0.12} {\rm {GeV^{-1} \,
cm^{-2} \, sr^{-1} \, s^{-1}} } $, as well as the strongest upper limit yet on
the flux of neutrinos from charmed-meson decay in the atmosphere, 1.52 times
the benchmark theoretical prediction used in previous IceCube results at 90\%
confidence.
A search for high-energy neutrinos interacting within the IceCube detector between 2010 and 2012 provided the first evidence for a high-energy neutrino flux of extraterrestrial origin. Results from an analysis using the same methods with a third year (2012–2013) of data from the complete IceCube detector are consistent with the previously reported astrophysical flux in the 100 TeV–PeV range at the level of 10−8 GeV cm−2 s−1 sr−1 per flavor and reject a purely atmospheric explanation for the combined three-year data at 5.7σ. The data are consistent with expectations for equal fluxes of all three neutrino flavors and with isotropic arrival directions, suggesting either numerous or spatially extended sources. The three-year data set, with a live time of 988 days, contains a total of 37 neutrino candidate events with deposited energies ranging from 30 to 2000 TeV. The 2000-TeV event is the highest-energy neutrino interaction ever observed.
Extraterrestrial Neutrinos
Neutrinos are thought to be produced in astrophysical sources outside our solar system but, up until recently, they had only been observed from one supernova in 1987. Aartsen et al. ( 10.1126/science.1242856 ; see the cover) report data obtained between 2010 and 2012 with the IceCube neutrino detector that reveal the presence of a high-energy neutrino flux containing the most energetic neutrinos ever observed, including 28 events at energies between 30 and 1200 TeV. Although the origin of this flux is unknown, the findings are consistent with expectations for a neutrino population with origins outside the solar system.
Accurate measurement of neutrino energies is essential to many of the scientific goals of large-volume neutrino telescopes. The fundamental observable in such detectors is the Cherenkov light produced by the transit through a medium of charged particles created in neutrino interactions. The amount of light emitted is proportional to the deposited energy, which is approximately equal to the neutrino energy for νe and νμ charged-current interactions and can be used to set a lower bound on neutrino energies and to measure neutrino spectra statistically in other channels. Here we describe methods and performance of reconstructing charged-particle energies and topologies from the observed Cherenkov light yield, including techniques to measure the energies of uncontained muon tracks, achieving average uncertainties in electromagnetic-equivalent deposited energy of ∼15% above 10 TeV.
The origin of high-energy cosmic rays, atomic nuclei that continuously impact Earth's atmosphere, is unknown. Because of deflection by interstellar magnetic fields, cosmic rays produced within the Milky Way arrive at Earth from random directions. However, cosmic rays interact with matter near their sources and during propagation, which produces high-energy neutrinos. We searched for neutrino emission using machine learning techniques applied to 10 years of data from the IceCube Neutrino Observatory. By comparing diffuse emission models to a background-only hypothesis, we identified neutrino emission from the Galactic plane at the 4.5σ level of significance. The signal is consistent with diffuse emission of neutrinos from the Milky Way but could also arise from a population of unresolved point sources.
A supermassive black hole, obscured by cosmic dust, powers the nearby active galaxy NGC 1068. Neutrinos, which rarely interact with matter, could provide information on the galaxy’s active core. We searched for neutrino emission from astrophysical objects using data recorded with the IceCube neutrino detector between 2011 and 2020. The positions of 110 known gamma-ray sources were individually searched for neutrino detections above atmospheric and cosmic backgrounds. We found that NGC 1068 has an excess of 79 − 20 + 22 neutrinos at tera–electron volt energies, with a global significance of 4.2σ, which we interpret as associated with the active galaxy. The flux of high-energy neutrinos that we measured from NGC 1068 is more than an order of magnitude higher than the upper limit on emissions of tera–electron volt gamma rays from this source.
The IceCube Neutrino Observatory has established the existence of a high-energy all-sky neutrino flux of astrophysical origin. This discovery was made using events interacting within a fiducial region of the detector surrounded by an active veto and with reconstructed energy above 60 TeV, commonly known as the high-energy starting event sample (HESE). We revisit the analysis of the HESE sample with an additional 4.5 years of data, newer glacial ice models, and improved systematics treatment. This paper describes the sample in detail, reports on the latest astrophysical neutrino flux measurements, and presents a source search for astrophysical neutrinos. We give the compatibility of these observations with specific isotropic flux models proposed in the literature as well as generic power-law-like scenarios. Assuming νe:νμ:ντ=1:1:1, and an equal flux of neutrinos and antineutrinos, we find that the astrophysical neutrino spectrum is compatible with an unbroken power law, with a preferred spectral index of 2.87−0.19+0.20 for the 68% confidence interval.
A Correction to this paper has been published: https://doi.org/10.1038/s41586-021-03450-1.
Shock interaction has been argued to play a role in powering a range of optical transients, including supernovae, classical novae, stellar mergers, tidal disruption events, and fast blue optical transients. These same shocks can accelerate relativistic ions, generating high-energy neutrino and gamma-ray emission via hadronic pion production. The recent discovery of time-correlated optical and gamma-ray emission in classical novae has revealed the important role of radiative shocks in powering these events, enabling an unprecedented view of the properties of ion acceleration, including its efficiency and energy spectrum, under similar physical conditions to shocks in extragalactic transients. Here we introduce a model for connecting the radiated optical fluence of nonrelativistic transients to their maximal neutrino and gamma-ray fluence. We apply this technique to a wide range of extragalactic transient classes in order to place limits on their contributions to the cosmological high-energy gamma-ray and neutrino backgrounds. Based on a simple model for diffusive shock acceleration at radiative shocks, calibrated to novae, we demonstrate that several of the most luminous transients can accelerate protons up to 1016 eV, sufficient to contribute to the IceCube astrophysical background. Furthermore, several of the considered sources-particularly hydrogen-poor supernovae-may serve as "gamma-ray-hidden"neutrino sources owing to the high gamma-ray opacity of their ejecta, evading constraints imposed by the nonblazar Fermi Large Area Telescope background. However, adopting an ion acceleration efficiency of ∼0.3%-1% motivated by nova observations, we find that currently known classes of nonrelativistic, potentially shock-powered transients contribute at most a few percent of the total IceCube background.
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 ϕastro=1.66−0.27+0.25 at E0=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.
We describe an improved in-situ calibration of the single-photoelectron charge distributions for each of the in-ice Hamamatsu Photonics R7081-02[MOD] photomultiplier tubes in the IceCube Neutrino Observatory. The characterization of the individual PMT charge distributions is important for PMT calibration, data and Monte Carlo simulation agreement, and understanding the effect of hardware differences within the detector. We discuss the single photoelectron identification procedure and how we extract the single-photoelectron charge distribution using a deconvolution of the multiple-photoelectron charge distribution.
This Letter presents the results from pointlike neutrino source searches using ten years of IceCube data collected between April 6, 2008 and July 10, 2018. We evaluate the significance of an astrophysical signal from a pointlike source looking for an excess of clustered neutrino events with energies typically above ∼1 TeV among the background of atmospheric muons and neutrinos. We perform a full-sky scan, a search within a selected source catalog, a catalog population study, and three stacked Galactic catalog searches. The most significant point in the northern hemisphere from scanning the sky is coincident with the Seyfert II galaxy NGC 1068, which was included in the source catalog search. The excess at the coordinates of NGC 1068 is inconsistent with background expectations at the level of 2.9σ after accounting for statistical trials from the entire catalog. The combination of this result along with excesses observed at the coordinates of three other sources, including TXS 0506+056, suggests that, collectively, correlations with sources in the northern catalog are inconsistent with background at 3.3σ significance. The southern catalog is consistent with background. These results, all based on searches for a cumulative neutrino signal integrated over the 10 years of available data, motivate further study of these and similar sources, including time-dependent analyses, multimessenger correlations, and the possibility of stronger evidence with coming upgrades to the detector.
Tree boosting is a highly effective and widely used machine learning method. In this paper, we describe a scalable end-to-end tree boosting system called XGBoost, which is used widely by data scientists to achieve state-of-the-art results on many machine learning challenges. We propose a novel sparsity-aware algorithm for sparse data and weighted quantile sketch for approximate tree learning. More importantly, we provide insights on cache access patterns, data compression and sharding to build a scalable tree boosting system. By combining these insights, XGBoost scales beyond billions of examples using far fewer resources than existing systems.
We consider limits on the local ($z=0$) density ($n_0$) of extragalactic neutrino sources set by the nondetection of steady high-energy neutrino sources producing $\gtrsim30$ TeV muon multiplets in the present IceCube data, taking into account the redshift evolution, luminosity function and neutrino spectrum of the sources. We show that the lower limit depends weakly on source spectra and strongly on redshift evolution. We find $n_0\gtrsim{10}^{-7}~{\rm Mpc}^{-3}$ for standard candle sources evolving rapidly, $n_s\propto{(1+z)}^3$, and $n_0\gtrsim{10}^{-5}~{\rm Mpc}^{-3}$ for nonevolving sources. The corresponding upper limits on their neutrino luminosity are $L_{{\nu_\mu}}^{\rm eff}\lesssim10^{42}~{\rm erg}~{\rm s}^{-1}$ and $L_{{\nu_\mu}}^{\rm eff}\lesssim10^{41}~{\rm erg}~{\rm s}^{-1}$, respectively. Applying these results to a wide range of classes of potential sources, we show that powerful blazar jets associated with active galactic nuclei are unlikely to be the dominant sources. For almost all other steady candidate source classes (including starbursts, radio galaxies, and galaxy clusters and groups), an order of magnitude increase in the detector sensitivity at $\sim0.1-1$ PeV will enable a detection (as point sources) of the few brightest objects. Such an increase, which may be provided by next-generation detectors like IceCube-Gen2 and an upgraded KM3NET, can improve the limit on $n_0$ by more than two orders of magnitude. Future gamma-ray observations (by Fermi, HAWC and CTA) will play a key role in confirming the association of the neutrinos with their sources.
The IceCube neutrino observatory in operation at the South Pole, Antarctica, comprises three distinct components: a large buried array for ultrahigh energy neutrino detection, a surface air shower array, and a new buried component called DeepCore. DeepCore was designed to lower the IceCube neutrino energy threshold by over an order of magnitude, to energies as low as about 10 GeV. DeepCore is situated primarily 2100 m below the surface of the icecap at the South Pole, at the bottom center of the existing IceCube array, and began taking physics data in May 2010. Its location takes advantage of the exceptionally clear ice at those depths and allows it to use the surrounding IceCube detector as a highly efficient active veto against the principal background of downward-going muons produced in cosmic-ray air showers. DeepCore has a module density roughly five times higher than that of the standard IceCube array, and uses photomultiplier tubes with a new photocathode featuring a quantum efficiency about 35% higher than standard IceCube PMTs. Taken together, these features of DeepCore will increase IceCube's sensitivity to neutrinos from WIMP dark matter annihilations, atmospheric neutrino oscillations, galactic supernova neutrinos, and point sources of neutrinos in the northern and southern skies. In this paper we describe the design and initial performance of DeepCore. (c) 2012 Elsevier B.V. All rights reserved.
The hypothesis of an unstable charged boson to mediate muon decay radically affects the cross section for the process $\overline{$\nu${}}+e$\rightarrow${}\overline{$\nu${}}+{$\mu${}}^{$-${}}$ near the energy at which the intermediary may be produced. If the boson is assumed to have $K$-meson mass, the resonance occurs at an incident antineutrino energy of $\sim${}2\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ev. The flux of energetic antineutrinos produced in association with cosmic-ray muons will then produce two muon counts per day per square meter of detector, independently of the depth and the orientation at which the experiment is performed.
The Fisher information matrix summarizes the amount of information in the data relative to the quantities of interest. There are many applications of the information matrix in modeling, systems analysis, and estimation, including confidence region calculation, input design, prediction bounds, and “noninformative” priors for Bayesian analysis. This article reviews some basic principles associated with the information matrix, presents a resampling-based method for computing the information matrix together with some new theory related to efficient implementation, and presents some numerical results. The resampling-based method relies on an efficient technique for estimating the Hessian matrix, introduced as part of the adaptive (“second-order”) form of the simultaneous perturbation stochastic approximation (SPSA) optimization algorithm.
Neutrino telescopes such as IceCube search for an excess of high energy
neutrinos above the steeply falling atmospheric background as one approach to
finding extraterrestrial neutrinos. For samples of events selected to start in
the detector, the atmospheric background can be reduced to the extent that a
neutrino interaction inside the fiducial volume is accompanied by a detectable
muon from the same cosmic-ray cascade in which the neutrino was produced. Here
we provide an approximate calculation of the veto probability as a function of
neutrino energy and zenith angle.
DOI:https://doi.org/10.1103/PhysRevLett.69.2738
The IceCube project has transformed 1 km3 of deep natural Antarctic ice into a Cherenkov detector. Muon neutrinos are detected and their direction is inferred by mapping the light produced by the secondary muon track inside the volume instrumented with photomultipliers. Reconstructing the muon track from the observed light is challenging due to noise, light scattering in the ice medium, and the possibility of simultaneously having multiple muons inside the detector, resulting from the large flux of cosmic ray muons.
This paper describes work on two problems: (1) the track reconstruction problem, in which, given a set of observations, the goal is to recover the track of a muon; and (2) the coincident event problem, which is to determine how many muons are active in the detector during a time window. Rather than solving these problems by developing more complex physical models that are applied at later stages of the analysis, our approach is to augment the detector's early reconstruction with data filters and robust statistical techniques. These can be implemented at the level of on-line reconstruction and, therefore, improve all subsequent reconstructions. Using the metric of median angular resolution, a standard metric for track reconstruction, we improve the accuracy in the initial reconstruction direction by 13%. We also present improvements in measuring the number of muons in coincident events: we can accurately determine the number of muons 98% of the time.
Random forests are a combination of tree predictors such that each tree depends on the values of a random vector sampled independently and with the same distribution for all trees in the forest. The generalization error for forests converges a.s. to a limit as the number of trees in the forest becomes large. The generalization error of a forest of tree classifiers depends on the strength of the individual trees in the forest and the correlation between them. Using a random selection of features to split each node yields error rates that compare favorably to Adaboost (Y. Freund & R. Schapire, Machine Learning: Proceedings of the Thirteenth International conference, ***, 148–156), but are more robust with respect to noise. Internal estimates monitor error, strength, and correlation and these are used to show the response to increasing the number of features used in the splitting. Internal estimates are also used to measure variable importance. These ideas are also applicable to regression.