Publications by Subir Sarkar


Search for annihilating dark matter in the Sun with 3 years of IceCube data: IceCube Collaboration

European Physical Journal C 77 (2017)

MG Aartsen, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Ahrens, D Altmann, K Andeen, T Anderson, I Ansseau, G Anton, M Archinger, C Argüelles, J Auffenberg, S Axani, X Bai, SW Barwick, V Baum, R Bay, JJ Beatty, J BeckerTjus, KH Becker, S BenZvi, D Berley, E Bernardini, A Bernhard, DZ Besson, G Binder, D Bindig, M Bissok, E Blaufuss, S Blot, C Bohm, M Börner, F Bos, D Bose, S Böser, O Botner, J Braun, L Brayeur, HP Bretz, S Bron, A Burgman, T Carver, M Casier, E Cheung, D Chirkin, A Christov, K Clark, L Classen, S Coenders, GH Collin, JM Conrad, DF Cowen, R Cross, M Day, JPAM de André, C De Clercq, E delPinoRosendo, H Dembinski, S De Ridder, P Desiati, KD de Vries, G de Wasseige, M de With, T DeYoung, JC Díaz-Vélez, V di Lorenzo, H Dujmovic, JP Dumm, M Dunkman, B Eberhardt, T Ehrhardt, B Eichmann, P Eller, S Euler, PA Evenson, S Fahey, AR Fazely, J Feintzeig, J Felde, K Filimonov, C Finley, S Flis, CC Fösig, A Franckowiak, E Friedman, T Fuchs, TK Gaisser, J Gallagher, L Gerhardt, K Ghorbani, W Giang, L Gladstone, T Glauch, T Glüsenkamp

© 2017, The Author(s). We present results from an analysis looking for dark matter annihilation in the Sun with the IceCube neutrino telescope. Gravitationally trapped dark matter in the Sun’s core can annihilate into Standard Model particles making the Sun a source of GeV neutrinos. IceCube is able to detect neutrinos with energies > 100 GeV while its low-energy infill array DeepCore extends this to > 10 GeV. This analysis uses data gathered in the austral winters between May 2011 and May 2014, corresponding to 532 days of livetime when the Sun, being below the horizon, is a source of up-going neutrino events, easiest to discriminate against the dominant background of atmospheric muons. The sensitivity is a factor of two to four better than previous searches due to additional statistics and improved analysis methods involving better background rejection and reconstructions. The resultant upper limits on the spin-dependent dark matter-proton scattering cross section reach down to 1.46 × 10 - 5  pb for a dark matter particle of mass 500 GeV annihilating exclusively into τ + τ - particles. These are currently the most stringent limits on the spin-dependent dark matter-proton scattering cross section for WIMP masses above 50 GeV.


The IceCube Neutrino Observatory: instrumentation and online systems

Journal of Instrumentation IOP Publishing 12 (2017) P03012

MG Aartsen, M Ackermann, J Adams, S Sarkar, E Et al.

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.


First search for dark matter annihilations in the Earth with the IceCube detector

European Physical Journal C Springer Berlin Heidelberg 2017 (2017) 82

K Abraham, M Ackermann, S Sarkar, MG Aartsen, E Et al.

We present the results of the first IceCube search for dark matter annihilation in the center of the Earth. Weakly interacting massive particles (WIMPs), candidates for dark matter, can scatter off nuclei inside the Earth and fall below its escape velocity. Over time the captured WIMPs will be accumulated and may eventually self-annihilate. Among the annihilation products only neutrinos can escape from the center of the Earth. Large-scale neutrino telescopes, such as the cubic kilometer IceCube Neutrino Observatory located at the South Pole, can be used to search for such neutrino fluxes. Data from 327 days of detector livetime during 2011/2012 were analyzed. No excess beyond the expected background from atmospheric neutrinos was detected. The derived upper limits on the annihilation rate of WIMPs in the Earth and the resulting muon flux are an order of magnitude stronger than the limits of the last analysis performed with data from IceCube’s predecessor AMANDA. The limits can be translated in terms of a spin-independent WIMP–nucleon cross section. For a WIMP mass of 50 GeV this analysis results in the most restrictive limits achieved with IceCube data.


All-sky search for time-integrated neutrino emission from astrophysical sources with 7 yr of IceCube data

Astrophysical Journal Institute of Physics 835 (2017) 151-

M Ackermann, S Sarkar, K Abraham

Since the recent detection of an astrophysical flux of high-energy neutrinos, the question of its origin has not yet fully been answered. Much of what is known about this flux comes from a small event sample of high neutrino purity, good energy resolution, but large angular uncertainties. In searches for point-like sources, on the other hand, the best performance is given by using large statistics and good angular reconstructions. Track-like muon events produced in neutrino interactions satisfy these requirements. We present here the results of searches for point-like sources with neutrinos using data acquired by the IceCube detector over 7 yr from 2008 to 2015. The discovery potential of the analysis in the northern sky is now significantly below E 2 v doφ/dE v = 10 -12 TeV cm -2 s -1 , on average 38% lower than the sensitivity of the previously published analysis of 4 yr exposure. No significant clustering of neutrinos above background expectation was observed, and implications for prominent neutrino source candidates are discussed.


The contribution of Fermi-2LAC blazars to diffuse TeV-PeV neutrino flux

Astrophysical Journal Institute of Physics 835 (2017) 45-

MG Aartsen, K Abraham, M Ackermann, S Sarkar

The recent discovery of a diffuse cosmic neutrino flux extending up to PeV energies raises the question of which astrophysical sources generate this signal. Blazars are one class of extragalactic sources which may produce such high-energy neutrinos. We present a likelihood analysis searching for cumulative neutrino emission from blazars in the 2nd Fermi-LAT AGN catalog (2LAC) using IceCube neutrino data set 2009-12, which was optimized for the detection of individual sources. In contrast to those in previous searches with IceCube, the populations investigated contain up to hundreds of sources, the largest one being the entire blazar sample in the 2LAC catalog. No significant excess is observed, and upper limits for the cumulative flux from these populations are obtained. These constrain the maximum contribution of 2LAC blazars to the observed astrophysical neutrino flux to 27% or less between around 10 TeV and 2 PeV, assuming the equipartition of flavors on Earth and a single power-law spectrum with a spectral index of -2.5. We can still exclude the fact that 2LAC blazars (and their subpopulations) emit more than 50% of the observed neutrinos up to a spectral index as hard as -2.2 in the same energy range. Our result takes into account the fact that the neutrino source count distribution is unknown, and it does not assume strict proportionality of the neutrino flux to the measured 2LAC γ-ray signal for each source. Additionally, we constrain recent models for neutrino emission by blazars.


Frames of most uniform Hubble flow

Journal of Cosmology and Astroparticle Physics IOP Publishing 2016 (2016) 1-17

D Kraljic, S Sarkar

It has been observed [1,2] that the locally measured Hubble parameter converges quickest to the background value and the dipole structure of the velocity field is smallest in the reference frame of the Local Group of galaxies. We study the statistical properties of Lorentz boosts with respect to the Cosmic Microwave Background frame which make the Hubble flow look most uniform around a particular observer. We use a very large N-Body simulation to extract the dependence of the boost velocities on the local environment such as underdensities, overdensities, and bulk flows. We find that the observation [1,2] is not unexpected if we are located in an underdensity, which is indeed the case for our position in the universe. The amplitude of the measured boost velocity for our location is consistent with the expectation in the standard cosmology.


First combined search for neutrino point-sources in the Southern Hemisphere with the ANTARES and IceCube neutrino telescopes

Astrophysical Journal IOP Publishing 823 (2016)

S Adrián-Martínez, A Albert, M André, S Sarkar

We present the results of searches for point-like sources of neutrinos based on the first combined analysis of data from both the ANTARES and IceCube neutrino telescopes. The combination of both detectors, which differ in size and location, forms a window in the southern sky where the sensitivity to point sources improves by up to a factor of 2 compared with individual analyses. Using data recorded by ANTARES from 2007 to 2012, and by IceCube from 2008 to 2011, we search for sources of neutrino emission both across the southern sky and from a preselected list of candidate objects. No significant excess over background has been found in these searches, and flux upper limits for the candidate sources are presented for E -2.5 and E -2 power-law spectra with different energy cut-offs.


Search for annihilating dark matter in the Sun with 3 years of IceCube data

European Physical Journal C: Particles and Fields Springer Berlin Heidelberg 2017 (2017) 146

S Sarkar, MG Aartsen, M Ackermann, E Et al., J Adams

We present results from an analysis looking for dark matter annihilation in the Sun with the IceCube neutrino telescope. Gravitationally trapped dark matter in the Sun's core can annihilate into Standard Model particles making the Sun a source of GeV neutrinos. IceCube is able to detect neutrinos with energies >100 GeV while its low-energy infill array DeepCore extends this to >10 GeV. This analysis uses data gathered in the austral winters between May 2011 and May 2014, corresponding to 532 days of livetime when the Sun, being below the horizon, is a source of up-going neutrino events, easiest to discriminate against the dominant background of atmospheric muons. The sensitivity is a factor of two to four better than previous searches due to additional statistics and improved analysis methods involving better background rejection and reconstructions. The resultant upper limits on the spin-dependent dark matter-proton scattering cross section reach down to $1.46\times10^{-5}$ pb for a dark matter particle of mass 500 GeV annihilating exclusively into $\tau^{+}\tau^{-}$ particles. These are currently the most stringent limits on the spin-dependent dark matter-proton scattering cross section for WIMP masses above 50 GeV.


All-flavour search for neutrinos from dark matter annihilations in the Milky Way with IceCube/DeepCore

European Physical Journal C Springer 76 (2016) 531-

S Sarkar

<p>We present the first IceCube search for a signal of dark matter annihilations in the Milky Way using all-flavour neutrino-induced particle cascades. The analysis focuses on the DeepCore sub-detector of IceCube, and uses the surrounding IceCube strings as a veto region in order to select starting events in the DeepCore volume. We use 329 live-days of data from IceCube operating in its 86-string configuration during 2011–2012. No neutrino excess is found, the final result being compatible with the background-only hypothesis. From this null result, we derive upper limits on the velocity-averaged self-annihilation cross-section,&lt;σAv&gt;, for dark matter candidate masses ranging from 30 GeV up to 10 TeV, assuming both a cuspy and a flat-cored dark matter halo profile. For dark matter masses between 200 GeV and 10 TeV, the results improve on all previous IceCube results on &lt;σAv&gt;, reaching a level of 10^−23 cm3 s^−1 , depending on the annihilation channel assumed, for a cusped NFW profile. The analysis demonstrates that all-flavour searches are competitive with muon channel searches despite the intrinsically worse angular resolution of cascades compared to muon tracks in IceCube.</p>


Searches for sterile neutrinos with the IceCube detector

Physical Review Letters American Physical Society 117 (2016) 071801

JA Aguilar, D Altmann, K Andeen, I Ansseau, G Anton, C Argüelles, TC Arlen, J Auffenberg, S Axani, X Bai, SW Barwick, V Baum, R Bay, JJ Beatty, KH Becker, J Becker Tjus, S BenZvi, P Berghaus, D Berley, E Bernardini, A Bernhard, DZ Besson, G Binder, D Bindig, E Blaufuss

The IceCube neutrino telescope at the South Pole has measured the atmospheric muon neutrino spectrum as a function of zenith angle and energy in the approximate 320 GeV to 20 TeV range, to search for the oscillation signatures of light sterile neutrinos. No evidence for anomalous ν_{μ} or ν[over ¯]_{μ} disappearance is observed in either of two independently developed analyses, each using one year of atmospheric neutrino data. New exclusion limits are placed on the parameter space of the 3+1 model, in which muon antineutrinos experience a strong Mikheyev-Smirnov-Wolfenstein-resonant oscillation. The exclusion limits extend to sin^{2}2θ_{24}≤0.02 at Δm^{2}∼0.3  eV^{2} at the 90% confidence level. The allowed region from global analysis of appearance experiments, including LSND and MiniBooNE, is excluded at approximately the 99% confidence level for the global best-fit value of |U_{e4}|^{2}.


An all-sky search for three flavors of neutrinos from gamma-ray bursts with the IceCube Neutrino Observatory

Astrophysical Journal Institute of Physics 824 (2016) 115-

S Sarkar

We present the results and methodology of a search for neutrinos produced in the decay of charged pions created in interactions between protons and gamma-rays during the prompt emission of 807 gamma-ray bursts (GRBs) over the entire sky. This three-year search is the first in IceCube for shower-like Cherenkov light patterns from electron, muon, and tau neutrinos correlated with GRBs. We detect five low-significance events correlated with five GRBs. These events are consistent with the background expectation from atmospheric muons and neutrinos. The results of this search in combination with those of IceCube’s four years of searches for track-like Cherenkov light patterns from muon neutrinos correlated with Northern-Hemisphere GRBs produce limits that tightly constrain current models of neutrino and ultra high energy cosmic ray production in GRB fireballs.


Lowering IceCube's energy threshold for point source searches in the southern sky

Astrophysical Journal Letters Institute of Physics 824 (2016) L28-

S Sarkar

Observation of a point source of astrophysical neutrinos would be a“smoking gun”signature of a cosmic-ray accelerator. While IceCube has recently discovered a diffuseflux of astrophysical neutrinos, no localized point source has been observed. Previous IceCube searches for point sources in the southern sky were restricted by either an energy threshold above a few hundred TeV or poor neutrino angular resolution. Here we present a search for southern sky point sources with greatly improved sensitivities to neutrinos with energies below 100 TeV. By selecting charged-currentνμinteracting inside the detector, we reduce the atmospheric background while retaining efficiency for astrophysical neutrino-induced events reconstructed with sub-degree angular resolution. The new event sample covers three years of detector data and leads to a factor of 10 improvement in sensitivity to point sources emitting below 100 TeV in the southern sky. No statistically significant evidence of point sources was found, and upper limits are set on neutrino emission from individual sources. A posteriori analysis of the highestenergy(∼100 TeV) starting event in the sample found that this event alone represents a 2.8σdeviation from the hypothesis that the data consists only of atmospheric background.


REVIEW OF PARTICLE PHYSICS Particle Data Group

40 (2016) UNSP 100001

C Patrignani, K Agashe, G Aielli, C Amsler, M Antonelli, DM Asner, H Baer, S Banerjee, RM Barnett, T Basaglia, CW Bauer, JJ Beatty, VI Belousov, J Beringer, S Bethke, H Bichsel, O Biebel, E Blucher, G Brooijmans, O Buchmueller, V Burkert, MA Bychkov, RN Cahn, M Carena, A Ceccucci, A Cerri, D Chakraborty, M-C Chen, RS Chivukula, K Copic, G Cowan, O Dahl, G D'Ambrosio, T Damour, D de Florian, A de Gouvea, T DeGrand, P de Jong, G Dissertori, BA Dobrescu, M D'Onofrio, M Doser, M Drees, HK Dreiner, DA Dwyer, P Eerola, S Eidelman, J Ellis, J Erler, VV Ezhela, W Fetscher, BD Fields, B Foster, A Freitas, H Gallagher, L Garren, H-J Gerber, G Gerbier, T Gershon, T Gherghetta, AA Godizov, M Goodman, C Grab, AV Gritsan, C Grojean, DE Groom, M Gruenewald, A Gurtu, T Gutsche, HE Haber, K Hagiwara, C Hanhart, S Hashimoto, Y Hayato, KG Hayes, A Hebecker, B Heltsley, JJ Hernandez-Rey, K Hikasa, J Hisano, A Hoecker, J Holder, A Holtkamp, J Huston, T Hyodo, K Irwin, JD Jackson, KF Johnson, M Kado, M Karliner, UF Katz, SR Klein, E Klempt, RV Kowalewski, F Krauss, M Kreps, B Krusche, YV Kuyanov, Y Kwon, O Lahav, J Laiho, P Langacker, A Liddle, Z Ligeti, C-J Lin, C Lippmann, TM Liss, L Littenberg, KS Lugovsky, SB Lugovsky, A Lusiani, Y Makida, F Maltoni, T Mannel, AV Manohar, WJ Marciano, AD Martin, A Masoni, J Matthews, U-G Meissner, D Milstead, RE Mitchell, P Molaro, K Monig, F Moortgat, MJ Mortonson, H Murayama, K Nakamura, M Narain, P Nason, S Navas, M Neubert, P Nevski, Y Nir, KA Olive, SP Griso, J Parsons, JA Peacock, M Pennington, ST Petcov, VA Petrov, A Piepke, A Pomarol, A Quadt, S Raby, J Rademacker, G Raffelt, BN Ratcliff, P Richardson, A Ringwald, S Roesler, S Rolli, A Romaniouk, LJ Rosenberg, JL Rosner, G Rybka, RA Ryutin, CT Sachrajda, Y Sakai, GP Salam, S Sarkar, F Sauli, O Schneider, K Scholberg, AJ Schwartz, D Scott, V Sharma, SR Sharpe, T Shutt, M Silari, T Sjostrand, P Skands, T Skwarnicki, JG Smith, GF Smoot, S Spanier, H Spieler, C Spiering, A Stahl, SL Stone, Y Sumino, T Sumiyoshi, MJ Syphers, F Takahashi, M Tanabashi, K Terashi, J Terning, RS Thorne, L Tiator, M Titov, NP Tkachenko, NA Tornqvist, D Tovey, G Valencia, R Van de Water, N Varelas, G Venanzoni, MG Vincter, P Vogel, A Vogt, SP Wakely, W Walkowiak, CW Walter, D Wands, DR Ward, MO Wascko, G Weiglein, DH Weinberg, EJ Weinberg, M White, LR Wiencke, S Wiliocq, CC Wohl, L Wolfenstein, J Womersley, CL Woody, RL Workman, W-M Yao, GP Zeller, OV Zenin, R-Y Zhu, F Zimmermann, PA Zyla, J Anderson, G Harper, VS Lugovsky, P Schaffner, PD Grp


High-energy neutrino follow-up search of gravitational wave event GW150914 with ANTARES and IceCube

Physical Review D American Physical Society 93 (2016) 122010-

S Sarkar

We present the high-energy-neutrino follow-up observations of the first gravitational wave transient GW150914 observed by the Advanced LIGO detectors on September 14, 2015. We search for coincident neutrino candidates within the data recorded by the IceCube and ANTARES neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves. We find no neutrino candidates in both temporal and spatial coincidence with the gravitational wave event. Within ±500 s of the gravitational wave event, the number of neutrino candidates detected by IceCube and ANTARES were three and zero, respectively. This is consistent with the expected atmospheric background, and none of the neutrino candidates were directionally coincident with GW150914. We use this nondetection to constrain neutrino emission from the gravitational-wave event.


Very high-energy gamma-ray follow-up program using neutrino triggers from IceCube

Journal of Instrumentation Institute of Physics 11 (2016) P11009-

MG Aartsen, K Abraham, M Ackermann, S Sarkar

We describe and report the status of a neutrino-triggered program in IceCube that generates real-time alerts for gamma-ray follow-up observations by atmospheric-Cherenkov telescopes (MAGIC and VERITAS). While IceCube is capable of monitoring the whole sky continuously, high-energy gamma-ray telescopes have restricted fields of view and in general are unlikely to be observing a potential neutrino-flaring source at the time such neutrinos are recorded. The use of neutrino-triggered alerts thus aims at increasing the availability of simultaneous multi-messenger data during potential neutrino flaring activity, which can increase the discovery potential and constrain the phenomenological interpretation of the high-energy emission of selected source classes (e.g. blazars). The requirements of a fast and stable online analysis of potential neutrino signals and its operation are presented, along with first results of the program operating between 14 March 2012 and 31 December 2015.


Anisotropy in cosmic-ray arrival directions in the Southern Hemisphere based on six years of data from the IceCube Detector

Astrophysical Journal Institute of Physics 826 (2016) 220-

S Sarkar

The IceCube Neutrino Observatory accumulated a total of 318 billion cosmic-ray-induced muon events between 2009 May and 2015 May. This data set was used for a detailed analysis of the sidereal anisotropy in the arrival directions of cosmic rays in the TeV to PeV energy range. The observed global sidereal anisotropy features large regions of relative excess and deficit, with amplitudes of the order of 10^−3 up to about 100 TeV. A decomposition of the arrival direction distribution into spherical harmonics shows that most of the power is contained in the lowmultipole(ℓ„4)moments. However, higher multipole components are found to be statistically significant down to an angular scale of less than 10°, approaching the angular resolution of the detector. Above 100 TeV, a change in the morphology of the arrival direction distribution is observed, and the anisotropy is characterized by a wide relative deficit whose amplitude increases with primary energy up to at least 5 PeV, the highest energies currently accessible to IceCube. No time dependence of the large- and small-scale structures is observed in the period of six years covered by this analysis. The high-statistics data set reveals more details of the properties of the anisotropy and is potentially able to shed light on the various physical processes that are responsible for the complex angular structure and energy evolution.


Marginal evidence for cosmic acceleration from Type Ia supernovae

Scientific Reports Nature Publishing Group (2016)

JT Nielsen, A Guffanti, S Sarkar

The ‘standard’ model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present — as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these ‘standardisable candles’ indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.


Search for sources of High-Energy neutrons with four years of data from the Icetop Detector

Astrophysical Journal Institute of Physics 830 (2016) 129-129

S Sarkar

IceTop is an air-shower array located on the Antarctic ice sheet at the geographic South Pole. IceTop can detect an astrophysicalflux of neutrons from Galactic sources as an excess of cosmic-ray air showers arriving from the source direction. Neutrons are undeflected by the Galactic magneticfield and can typically travel 10(E/PeV)pc before decay. Two searches are performed using 4 yr of the IceTop data set to look for a statistically significant excess of events with energies above 10 PeV(10^16 eV) arriving within a small solid angle. The all-sky search method covers from−90°to approximately −50°in declination. No significant excess is found. A targeted search is also performed, looking for significant correlation with candidate sources in different target sets. This search uses a higher-energy cut(100 PeV) since most target objects lie beyond 1 kpc. The target sets include pulsars with confirmed TeV energy photon fluxes and high-mass X-ray binaries. No significant correlation is found for any target set. Flux upper limits are determined for both searches, which can constrain Galactic neutron sources and production scenarios.


Constraints on ultrahigh-energy cosmic-ray sources from a search for neutrinos above 10 PeV with IceCube

Physical Review Letters American Physical Society 117 (2016) 241101

MG Aartsen, K Abraham, M Ackermann, S Sarkar, E Et al.

We report constraints on the sources of ultrahigh-energy cosmic rays (UHECRs) above 109 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 5×105 GeV to above 1011 GeV. Two neutrino-induced events with an estimated deposited energy of (2.6±0.3)×106 GeV, the highest neutrino energy observed so far, and (7.7±2.0)×105 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 &gt;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.


Observation and Characterization of a Cosmic Muon Neutrino Flux from the Northern Hemisphere using six years of IceCube data

Astrophysical Journal IOP Publishing 833 (2016) 3-

MG Aartsen, M Ackermann, K Abraham, S Sarkar

The IceCube Collaboration has previously discovered a high-energy astrophysical neutrino flux using neutrino events with interaction vertices contained within the instrumented volume of the IceCube detector. We present a complementary measurement using charged current muon neutrino events where the interaction vertex can be outside this volume. As a consequence of the large muon range the effective area is significantly larger but the field of view is restricted to the Northern Hemisphere. IceCube data from 2009 through 2015 have been analyzed using a likelihood approach based on the reconstructed muon energy and zenith angle. At the highest neutrino energies between 194 TeV and 7.8 PeV a significant astrophysical contribution is observed, excluding a purely atmospheric origin of these events at 5.6s significance. The data are well described by an isotropic, unbroken power-law flux with a normalization at 100 TeV neutrino energy of (0.90 -0.27+0.30) × 10-18 Gev-1 cm-2 s-1 sr-1and a hard spectral index of γ = 2.13 ± 0.13. The observed spectrum is harder in comparison to previous IceCube analyses with lower energy thresholds which may indicate a break in the astrophysical neutrino spectrum of unknown origin. The highest-energy event observed has a reconstructed muon energy of (4.5 ± 1.2) PeV which implies a probability of less than 0.005% for this event to be of atmospheric origin. Analyzing the arrival directions of all events with reconstructed muon energies above 200 TeV no correlation with known γ-ray sources was found. Using the high statistics of atmospheric neutrinos we report the current best constraints on a prompt atmospheric muon neutrino flux originating from charmed meson decays which is below 1.06 in units of the flux normalization of the model in Enberg et al.

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