Publications by Subir Sarkar


Astrophysical neutrinos and cosmic rays observed by IceCube

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


Astrophysical neutrinos and cosmic rays observed by IceCube

Advances in Space Research Elsevier 62 (2017) 2902-2930

M Ackermann, J Adams, S Sarkar

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


Search for sterile neutrino mixing using three years of IceCube DeepCore data

Physical Review D American Physical Society 95 (2017) 112002-

M Ackermann, J Adams, S Sarkar

We present a search for a light sterile neutrino using three years of atmospheric neutrino data from the DeepCore detector in the energy range of approximately 10-60 GeV. DeepCore is the low-energy subarray of the IceCube Neutrino Observatory. The standard three-neutrino paradigm can be probed by adding an additional light (Δm412∼1 eV2) sterile neutrino. Sterile neutrinos do not interact through the standard weak interaction and, therefore, cannot be directly detected. However, their mixing with the three active neutrino states leaves an imprint on the standard atmospheric neutrino oscillations for energies below 100 GeV. A search for such mixing via muon neutrino disappearance is presented here. The data are found to be consistent with the standard three-neutrino hypothesis. Therefore, we derive limits on the mixing matrix elements at the level of |Uμ4|2 < 0.11 and |Uτ4|2 < 0.15 (90% C.L.) for the sterile neutrino mass splitting Δm412=1.0 eV2.


Extending the Search for Muon Neutrinos Coincident with Gamma-Ray Bursts in IceCube Data

Astrophysical Journal American Astronomical Society 843 (2017) 1-13

M Ackermann, J Adams, S Sarkar

We present an all-sky search for muon neutrinos produced during the prompt γ-ray emission of 1172 gamma-ray bursts (GRBs) with the IceCube Neutrino Observatory. The detection of these neutrinos would constitute evidence for ultra-high-energy cosmic-ray (UHECR) production in GRBs, as interactions between accelerated protons and the prompt γ-ray field would yield charged pions, which decay to neutrinos. A previously reported search for muon neutrino tracks from northern hemisphere GRBs has been extended to include three additional years of IceCube data. A search for such tracks from southern hemisphere GRBs in five years of IceCube data has been introduced to enhance our sensitivity to the highest energy neutrinos. No significant correlation between neutrino events and observed GRBs is seen in the new data. Combining this result with previous muon neutrino track searches and a search for cascade signature events from all neutrino flavors, we obtain new constraints for single-zone fireball models of GRB neutrino and UHECR production.


Search for astrophysical sources of neutrinos using cascade events in IceCube

Astrophysical Journal American Astronomical Society 846 (2017) 1-12

J Adams, M Ackermann, S Sarkar

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


PINGU: a vision for neutrino and particle physics at the South Pole

Journal of Physics G: Nuclear and Particle Physics IOP Publishing 44 (2017) 054006

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

The Precision IceCube Next Generation Upgrade (PINGU) is a proposed lowenergy in-fill extension to the IceCube Neutrino Observatory. With detection technology modeled closely on the successful IceCube example, PINGU will provide a 6 Mton effective mass for neutrino detection with an energy threshold of a few GeV. With an unprecedented sample of over 60 000 atmospheric neutrinos per year in this energy range, PINGU will make highly competitive measurements of neutrino oscillation parameters in an energy range over an order of magnitude higher than long-baseline neutrino beam experiments. PINGU will measure the mixing parameters Θ23 and Δm232, including the octant of Θ23 for a wide range of values, and determine the neutrino mass ordering at 3σ median significance within five years of operation. PINGU's high precision measurement of the rate of nt appearance will provide essential tests of the unitarity of the 3 ×3 PMNS neutrino mixing matrix. PINGU will also improve the sensitivity of searches for low mass dark matter in the Sun, use neutrino tomography to directly probe the composition of the Earth's core, and improve IceCube's sensitivity to neutrinos from Galactic supernovae. Reoptimization of the PINGU design has permitted substantial reduction in both cost and logistical requirements while delivering performance nearly identical to configurations previously studied.


Search for neutrinos from dark matter self-annihilations in the center of the Milky Way with 3 years of IceCube/DeepCore

European Physical Journal C Springer Verlag 77 (2017) 1-11

M Ackermann, J Adams, S Sarkar

We present a search for a neutrino signal from dark matter self-annihilations in the Milky Way using the IceCube Neutrino Observatory (IceCube). In 1005 days of data we found no significant excess of neutrinos over the background of neutrinos produced in atmospheric air showers from cosmic ray interactions. We derive upper limits on the velocity averaged product of the dark matter self-annihilation cross section and the relative velocity of the dark matter particles $\langle\sigma_{\text{A}}v\rangle$. Upper limits are set for dark matter particle candidate masses ranging from 10 GeV up to 1 TeV while considering annihilation through multiple channels. This work sets the most stringent limit on a neutrino signal from dark matter with mass between 10 GeV and 100 GeV, with a limit of $1.18\cdot10^{-23}\text{cm}^3\text{s}^{-1}$ for 100 GeV dark matter particles self-annihilating via $\tau^+\tau^-$ to neutrinos (assuming the Navarro-Frenk-White dark matter halo profile).


Atmospheric neutrino results from IceCube-DeepCore and plans for PINGU

Journal of Physics: Conference Series 888 (2017)

D Jason Koskinen

© Published under licence by IOP Publishing Ltd. The IceCube neutrino observatory at the South Pole is the largest operating neutrino detector in the world and spans a wide range of science topics, from astronomy at the PeV-scale to particle physics at the GeV-scale. We present results from the search for a light, O(1) eV 2 , sterile neutrino using the large IceCube array and, separately, using the lower energy extension DeepCore sub-array. Additionally, we review the atmospheric neutrino results and expected sensitivities related to oscillation physics (ν μ disappearance and ν τ appearance) as well as new limits on non-standard interactions. Continuing the success of the IceCube-DeepCore physics program, a proposed next generation in-fill detector with increased sensitivity to neutrinos of O(1) GeV will be covered.


Prospects for Cherenkov Telescope Array observations of the young supernova remnant RX J1713.7−3946

Astrophysical Journal American Astronomical Society 840 (2017) 74

F Acero, R Aloisio, J Amans, A De Franco, G Cotter, S Sarkar, JJ Watson, E Et al.

We perform simulations for future Cherenkov Telescope Array (CTA) observations of RX J1713.7−3946, a young supernova remnant (SNR) and one of the brightest sources ever discovered in very high energy (VHE) gamma rays. Special attention is paid to exploring possible spatial (anti)correlations of gamma rays with emission at other wavelengths, in particular X-rays and CO/H i emission. We present a series of simulated images of RX J1713.7−3946 for CTA based on a set of observationally motivated models for the gamma-ray emission. In these models, VHE gamma rays produced by high-energy electrons are assumed to trace the nonthermal X-ray emission observed by XMM-Newton, whereas those originating from relativistic protons delineate the local gas distributions. The local atomic and molecular gas distributions are deduced by the NANTEN team from CO and H i observations. Our primary goal is to show how one can distinguish the emission mechanism(s) of the gamma rays (i.e., hadronic versus leptonic, or a mixture of the two) through information provided by their spatial distribution, spectra, and time variation. This work is the first attempt to quantitatively evaluate the capabilities of CTA to achieve various proposed scientific goals by observing this important cosmic particle accelerator.


High redshift radio galaxies and divergence from the CMB dipole

Monthly Notices of the Royal Astronomical Society Oxford University Press 471 (2017) 1045-1055

J Colin, R Mohayaee, M Rameez, S Sarkar

Previous studies have found our velocity in the rest frame of radio galaxies at high redshift to be much larger than that inferred from the dipole anisotropy of the cosmic microwave background. We construct a full sky catalogue, NVSUMSS, by merging the NRAO VLA Sky Survey and the Sydney University Molonglo Sky Survey catalogues and removing local sources by various means including cross-correlating with the 2MASS Redshift Survey catalogue. We take into account both aberration and Doppler boost to deduce our velocity from the hemispheric number count asymmetry, as well as via a three-dimensional linear estimator. Both its magnitude and direction depend on cuts made to the catalogue, e.g. on the lowest source flux; however these effects are small. From the hemispheric number count asymmetry we obtain a velocity of 1729 ± 187 km s−1, i.e. about four times larger than that obtained from the cosmic microwave background dipole, but close in direction, towards RA=149° ± 2°, Dec. = −17° ± 12°. With the three-dimensional estimator, the derived velocity is 1355 ± 174 km s−1 towards RA = 141° ± 11°, Dec. = −9° ± 10°. We assess the statistical significance of these results by comparison with catalogues of random distributions, finding it to be 2.81σ (99.75 per cent confidence).


Search for high-energy neutrinos from gravitational wave event GW151226 and candidate LVT151012 with ANTARES and IceCube

Physical Review D American Physcial Society 96 (2017) 022005-

A Albert, M Anghinolfi, M André, S Sarkar

The Advanced LIGO observatories detected gravitational waves from two binary black hole mergers during their first observation run (O1). We present a high-energy neutrino follow-up search for the second gravitational wave event, GW151226, as well as for gravitational wave candidate LVT151012. We find 2 and 4 neutrino candidates detected by IceCube, and 1 and 0 detected by ANTARES, within $\pm500$ s around the respective gravitational wave signals, consistent with the expected background rate. None of these neutrino candidates are found to be directionally coincident with GW151226 or LVT151012. We use non-detection to constrain isotropic-equivalent high-energy neutrino emission from GW151226 adopting the GW event's 3D localization, to less than $2\times 10^{51}-2\times10^{54}$ erg.


The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part I: Searches for the Sources of Astrophysical Neutrinos

35th International Cosmic Ray Conference 2017(ICRC2017) International School for Advanced Studies (2017)

M Ackermann, J Adams, S Sarkar

<p>Papers on the searches for the sources of astrophysical neutrinos, submitted to the 35th International Cosmic Ray Conference (ICRC 2017, Busan, South Korea) by the IceCube Collaboration</p> <p>Contents</p> <p>1 - Searching for VHE gamma-ray emission associated with IceCube astrophysical neutrinos using FACT, H.E.S.S., MAGIC, and VERITAS</p> <p>2 - Search for point-like sources in the astrophysical muon neutrino flux with IceCube</p> <p>3 - Search for weak neutrino point sources using angular auto-correlation analyses in IceCube</p> <p>4 - All-sky search for correlations in the arrival directions of astrophysical neutrino candidates and ultrahigh-energy cosmic rays</p> <p>5 - Results of IceCube searches for neutrinos from blazars using seven years of through-going muon data</p> <p>6 - IceCube Search for Neutrinos from 1ES 1959+650: Completing the Picture</p> <p>7 - Using all-flavor and all-sky event selections by IceCube to search for neutrino emission from the Galactic plane</p> <p>8 - Constraints on diffuse neutrino emission from the Galactic Plane with 7 years of IceCube data</p> <p>9 - Search for extended sources of neutrino emission with 7 years of IceCube data</p> <p>10 - Search for a cumulative neutrino signal from blazar flares using IceCube data</p> <p>11 - Investigation of Obscured Flat Spectrum Radio AGN with the IceCube Neutrino Observatory</p> <p>12 - Realtime neutrino alerts and follow-up in IceCube</p> <p>13 - Search for High-Energy Neutrino Emission from Fast Radio Bursts</p> <p>14 - IceCube as a Neutrino Follow-up Observatory for Astronomical Transients</p>


The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part II: Properties of the Atmospheric and Astrophysical Neutrino Flux

35th International Cosmic Ray Conference 2017(ICRC2017) International School for Advanced Studies (2017)

S Sarkar, M Ackermann, J Adams

<p>Papers on the properties of the atmospheric and astrophysical neutrino flux submitted to the 35th International Cosmic Ray Conference (ICRC 2017, Busan, South Korea) by the IceCube Collaboration</p> <p>Contents:</p> <p>1 - Search for Astrophysical Tau Neutrinos in Six Years of High-Energy Starting Events in IceCube</p> <p>2 - Multi-flavour PeV neutrino search with IceCube</p> <p>3 - High Energy Astrophysical Neutrino Flux Measurement Using Neutrinoinduced Cascades Observed in 4 Years of IceCube Data</p> <p>4 - A Measurement of the Diffuse Astrophysical Muon Neutrino Flux Using Eight Years of IceCube Data</p> <p>5 - Characterizing the Flux of Atmospheric Neutrinos with IceCube-DeepCore</p> <p>6 - Measurement of High Energy Neutrino – Nucleon Cross Section and Astrophysical Neutrino Flux Anisotropy Study of Cascade Channel with IceCube</p> <p>7 - Observation of Astrophysical Neutrinos in Six Years of IceCube Data</p> <p>8 - All-flavor Multi-Channel Analysis of the Astrophysical Neutrino Spectrum with IceCube</p> <p>9 - Differential limit on an EHE neutrino flux component in the presence of astrophysical background from nine years of IceCube data</p> <p>10 - Improving Future High-Energy Tau Neutrino Searches in IceCube</p> <p>11 - Search for Astrophysical Tau Neutrinos with the IceCube Waveforms</p>


The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part III: Cosmic Rays

35th International Cosmic Ray Conference 2017(ICRC2017) International School for Advanced Studies (2017)

Aartsen, M Ackermann, J Adams, S Sarkar


The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part IV: Searches for Beyond the Standard Model Physics

35th International Cosmic Ray Conference 2017(ICRC2017) International School for Advanced Studies (2017)

S Sarkar, M Ackermann, J Adams

<p>Papers on searches for beyond the standard model physics, submitted to the 35th International Cosmic Ray Conference (ICRC 2017, Busan, South Korea) by the IceCube Collaboration</p> <p>Contents:</p> <p>1 - Delayed light emission to distinguish astrophysical neutrino flavors in IceCube</p> <p>2 - Search for Signatures of Heavy Decaying Dark Matter with IceCube</p> <p>3 - Latest results and sensitivities for solar dark matter searches with IceCube</p> <p>4 - Searches for annihilating dark matter in the Milky Way halo with IceCube</p> <p>5 - Searches for Dark Matter in the center of the Earth with the IceCube detector</p> <p>6 - Measurement of water luminescence – a new detection method for neutrino telescopes</p> <p>7 - Combined Search for Neutrinos from Dark Matter Annihilation in the Galactic Center using IceCube and ANTARES</p>


The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part V: Solar flares, Supernovae, Event reconstruction, Education &amp; Outreach

35th International Cosmic Ray Conference (ICRC 2017) Proceedings of Science (2017)

Aartsen, M Ackermann, J Adams, S Sarkar

<p>Contents:</p> <p>1 Search for GeV neutrinos associated with solar flares with IceCube</p> <p>2 Estimating the Sensitivity of IceCube to Signatures of Axion Production in a Galactic Supernova</p> <p>3 Searching for Arbitrary Low-Energy Neutrino Transients with IceCube</p> <p>4 Deep Learning in Physics exemplified by the Reconstruction of Muon-Neutrino Events in IceCube</p> <p>5 Connecting Beyond the Research Community: IceCube Education, Outreach, and Communication Efforts</p>


The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part VI: IceCube-Gen2, the Next Generation Neutrino Observatory

35th International Cosmic Ray Conference (ICRC 2017) Proceedings of Science (2017)

M Ackermann, J Adams, S Sarkar

<p>Contents:</p> <p>1 IceCube-Gen2: the next-generation neutrino observatory for the South Pole</p> <p>2 IceAct: Imaging Air Cherenkov Telescopes with SiPMs at the South Pole for IceCube-Gen2</p> <p>3 Overview and performance of the D-Egg optical sensor for IceCube-Gen2</p> <p>4 Muon track reconstruction and veto performance with D-Egg sensor for IceCube-Gen2</p> <p>5 In-ice self-veto techniques for IceCube-Gen2</p> <p>6 A camera system for IceCube-Gen21</p> <p>7 The mDOM – A multi-PMT Digital Optical Module for the IceCube-Gen2 neutrino telescope</p> <p>8 The IceTop Scintillator Upgrade</p> <p>9 Overview and Performance of the Wavelength-shifting Optical Module (WOM)</p> <p>10 The Precision Optical CAlibration Module for IceCube-Gen2: First Prototype</p>


The IceCube realtime alert system

Astroparticle Physics Elsevier 92 (2017) 30-41

M Ackermann, J Adams, S Sarkar

Although high-energy astrophysical neutrinos were discovered in 2013, their origin is still unknown. Aiming for the identification of an electromagnetic counterpart of a rapidly fading source, we have implemented a realtime analysis framework for the IceCube neutrino observatory. Several analyses selecting neutrinos of astrophysical origin are now operating in realtime at the detector site in Antarctica and are producing alerts for the community to enable rapid follow-up observations. The goal of these observations is to locate the astrophysical objects responsible for these neutrino signals. This paper highlights the infrastructure in place both at the South Pole site and at IceCube facilities in the north that have enabled this fast follow-up program to be implemented. Additionally, this paper presents the first realtime analyses to be activated within this framework, highlights their sensitivities to astrophysical neutrinos and background event rates, and presents an outlook for future discoveries.


All-sky search for correlations in the arrival directions of astrophysical neutrino candidates and ultrahigh-energy cosmic rays

35th International Cosmic Ray Conference (ICRC 2017) Proceedings of Science (2017)

S Sarkar

High-energy neutrinos, being neutral and weakly interacting particles, are powerful probes of the sites of production and acceleration of cosmic rays. The challenging discovery of cosmic neutrinos by the IceCube Collaboration has moved the field closer to realizing the potential of neutrino astronomy. Meanwhile, ground-based cosmic ray detectors like the Pierre Auger Observatory and the Telescope Array have reached an unprecedented accuracy in the determination of the features of the cosmic rays at the highest energies. We report on a collaborative effort between IceCube, the Pierre Auger Observatory and Telescope Array to identify directional correlations between the arrival directions of the highest-energy cosmic rays from both hemispheres and of the most probable cosmic neutrino events detected by IceCube. We describe the updated results of two independent searches using seven years of IceCube neutrino data and the most energetic cosmicray events detected by the Pierre Auger Observatory and the Telescope Array. The directional correlation found between UHECRs and neutrinos is reported with a significance of ~ 2σ.


Combined Analysis of Cosmic-Ray Anisotropy with IceCube and HAWC

35th International Cosmic Ray Conference 2017(ICRC2017) International School for Advanced Studies (2017)

S Sarkar

During the past two decades, experiments in both the northern and southern hemispheres have observed a small but measurable energy-dependent sidereal anisotropy in the arrival direction distribution of Galactic cosmic rays with relative intensities at the level of one per mille. Individually, these measurements are restricted by limited sky coverage, and so the power spectrum of the anisotropy obtained from any one measurement displays a systematic correlation between different multipole modes $C_\ell$. We present the results of a joint analysis of the anisotropy on all angular scales using cosmic-ray data collected during 336 days of operation of the High-Altitude Water Cherenkov (HAWC) Observatory (located at 19$^\circ$ N) and 5 years of data taking from the IceCube Neutrino Observatory (located at 90$^\circ$ S) The results include a combined sky map and an all-sky power spectrum in the overlapping energy range of the two experiments at around 10 TeV. We describe the methods used to combine the IceCube and HAWC data, address the individual detector systematics, and study the region of overlapping field of view between the two observatories.

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