Publications by Subir Sarkar


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.


Neutrino oscillation studies with IceCube-DeepCore

Nuclear Physics B Elsevier 908 (2016) 161–177-

MG Aartsen, K Abraham, M Ackermann, S Sarkar

IceCube, a gigaton-scale neutrino detector located at the South Pole, was primarily designed to search for astrophysical neutrinos with energies of PeV and higher. This goal has been achieved with the detection of the highest energy neutrinos to date. At the other end of the energy spectrum, the DeepCore extension lowers the energy threshold of the detector to approximately 10 GeV and opens the door for oscillation studies using atmospheric neutrinos. An analysis of the disappearance of these neutrinos has been completed, with the results produced being complementary with dedicated oscillation experiments. Following a review of the detector principle and performance, the method used to make these calculations, as well as the results, is detailed. Finally, the future prospects of IceCube-DeepCore and the next generation of neutrino experiments at the South Pole (IceCube-Gen2, specifically the PINGU sub-detector) are briefly discussed.


Footprints of Loop I on cosmic microwave background maps

Journal of Cosmology and Astroparticle Physics IOP Publishing 2016 (2016) 023

S Hausegger, H Liu, P Mertsch, S Sarkar

Cosmology has made enormous progress through studies of the cosmic microwave background, however the subtle signals being now sought such as B-mode polarisation due to primordial gravitational waves are increasingly hard to disentangle from residual Galactic foregrounds in the derived CMB maps. We revisit our finding that on large angular scales there are traces of the nearby old supernova remnant Loop I in the WMAP 9-year map of the CMB and confirm this with the new SMICA map from the Planck satellite.


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, J Becker Tjus, KH Becker, 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.


The prompt atmospheric neutrino flux in the light of LHCb

JHEP Springer Berlin Heidelberg 02 (2016) 130-

R Gauld, J Rojo, L Rottoli, S Sarkar, J Talbert

The recent observation of very high energy cosmic neutrinos by IceCube heralds the beginning of neutrino astronomy. At these energies, the dominant background to the astrophysical signal is the flux of `prompt' neutrinos, arising from the decay of charmed mesons produced by cosmic ray collisions in the atmosphere. In this work we provide predictions for the prompt atmospheric neutrino flux in the framework of perturbative QCD, using state-of-the-art Monte Carlo event generators. Our calculation includes the constraints set by charm production measurements from the LHCb experiment at 7 TeV, and has been recently validated with the corresponding 13 TeV data. Our results for the prompt flux are a factor of about 2 below the previous benchmark calculation, in general agreement with two other recent estimates, and with an improved estimate of the uncertainty. This alleviates the existing tension between the theoretical prediction and IceCube limits, and suggests that a direct direction of the prompt flux is imminent.


Search for astrophysical tau neutrinos in three years of IceCube data

Physical Review D American Physical Society 93 (2016)

MG Aartsen, K Abraham, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Ahrens, D Altmann, T Anderson, I Ansseau, M Archinger, C Arguelles, TC Arlen, J Auffenberg, X Bai, SW Barwick, V Baum, R Bay, JJ Beatty, JB Tjus, K-H Becker, E Beiser, S BenZvi, P Berghaus

The IceCube Neutrino Observatory has observed a diffuse flux of TeV-PeV astrophysical neutrinos at 5.7σ significance from an all-flavor search. The direct detection of tau neutrinos in this flux has yet to occur. Tau neutrinos become distinguishable from other flavors in IceCube at energies above a few hundred TeV, when the cascade from the tau neutrino charged current interaction becomes resolvable from the cascade from the tau lepton decay. This paper presents results from the first dedicated search for tau neutrinos with energies between 214 TeV and 72 PeV in the full IceCube detector. The analysis searches for IceCube optical sensors that observe two separate pulses in a single event - one from the tau neutrino interaction and a second from the tau decay. No candidate events were observed in three years of IceCube data. For the first time, a differential upper limit on astrophysical tau neutrinos is derived around the PeV energy region, which is nearly 3 orders of magnitude lower in energy than previous limits from dedicated tau neutrino searches.


The search for transient astrophysical neutrino emission with icecube-deepcore

Astrophysical Journal IOP Publishing 816 (2016) 75-75

JB Tjus, K-H Becker, E Beiser, S Benzvi, P Berghaus, D Berley, E Bernardini, A Bernhard, DZ Besson, G Binder, D Bindig, M Bissoki, E Blaufuss, J Blumenthal, DJ Boersma, C Bohm, M Boerner, F Bos, D Bose, S Boeser, O Botner, J Braun, L Brayeur, C De Clercq, M Zoll

We present the results of a search for astrophysical sources of brief transient neutrino emission using IceCube and DeepCore data acquired between 2012 May 15 and 2013 April 30. While the search methods employed in this analysis are similar to those used in previous IceCube point source searches, the data set being examined consists of a sample of predominantly sub-TeV muon-neutrinos from the Northern Sky (−5$^\circ \lt \delta \lt 90^\circ $) obtained through a novel event selection method. This search represents a first attempt by IceCube to identify astrophysical neutrino sources in this relatively unexplored energy range. The reconstructed direction and time of arrival of neutrino events are used to search for any significant self-correlation in the data set. The data revealed no significant source of transient neutrino emission. This result has been used to construct limits at timescales ranging from roughly 1 s to 10 days for generic soft-spectra transients. We also present limits on a specific model of neutrino emission from soft jets in core-collapse supernovae.


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.


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.


Search for correlations between the arrival directions of IceCube neutrino events and ultrahigh-energy cosmic rays detected by the Pierre Auger Observatory and the Telescope Array

Journal of Cosmology and Astroparticle Physics IOP Publishing 2016 (2016) 037-037

J Felde, K Filimonov, C Finley, T Fischer-Wasels, S Flis, C-C Foesig, T Fuchs, TK Gaisser, R Gaior, J Gallagher, L Gerhardt, K Ghorbani, D Gier, L Gladstone, M Glagla, T Gluesenkamp, A Goldschmidt, G Golup, JG Gonzalez, D Gora, D Grant, Z Griffith, A Gross, C Ha, C Haack

This paper presents the results of different searches for correlations between very high-energy neutrino candidates detected by IceCube and the highest-energy cosmic rays measured by the Pierre Auger Observatory and the Telescope Array. We first consider samples of cascade neutrino events and of high-energy neutrino-induced muon tracks, which provided evidence for a neutrino flux of astrophysical origin, and study their cross-correlation with the ultrahigh-energy cosmic ray (UHECR) samples as a function of angular separation. We also study their possible directional correlations using a likelihood method stacking the neutrino arrival directions and adopting different assumptions on the size of the UHECR magnetic deflections. Finally, we perform another likelihood analysis stacking the UHECR directions and using a sample of through-going muon tracks optimized for neutrino point-source searches with sub-degree angular resolution. No indications of correlations at discovery level are obtained for any of the searches performed. The smallest of the p-values comes from the search for correlation between UHECRs with IceCube high-energy cascades, a result that should continue to be monitored.


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.


Improved limits on dark matter annihilation in the Sun with the 79-string IceCube detector and implications for supersymmetry

Journal of Cosmology and Astroparticle Physics IOP PUblishing 2016 (2016) 022-022

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

We present an improved event-level likelihood formalism for including neutrino telescope data in global fits to new physics. We derive limits on spin-dependent dark matter-proton scattering by employing the new formalism in a re-analysis of data from the 79-string IceCube search for dark matter annihilation in the Sun, including explicit energy information for each event. The new analysis excludes a number of models in the weak-scale minimal supersymmetric standard model (MSSM) for the first time. This work is accompanied by the public release of the 79-string IceCube data, as well as an associated computer code for applying the new likelihood to arbitrary dark matter models.


Characterization of the atmospheric muon flux in IceCube

Astroparticle Physics Elsevier 78 (2016) 1-27

C Pfendner, D Pieloth, E Pinat, J Posselt, PB Price, GT Przybylski, M Quinnan, N Whitehorn, C Wichary, K Wiebe, CH Wiebusch, L Wille, DR Williams, H Wissing, M Wolf, TR Wood, K Woschnagg, DL Xu, XW Xu, Y Xu, JP Yáñez, G Yodh, S Yoshida, P Zarzhitsky, M Zoll

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 in the zenith angle distribution of events were found which indicate the presence of an unexplained effect outside the currently applied range of detector systematics. The underlying cause could be related to the hadronic interaction models used to describe muon production in air showers.


Searches for relativistic magnetic monopoles in IceCube

European Physical Journal C Springer Verlag 76 (2016) 1-16

K Abraham, M Ackermann, S Sarkar

© 2016, The Author(s). Various extensions of the Standard Model motivate the existence of stable magnetic monopoles that could have been created during an early high-energy epoch of the Universe. These primordial magnetic monopoles would be gradually accelerated by cosmic magnetic fields and could reach high velocities that make them visible in Cherenkov detectors such as IceCube. Equivalently to electrically charged particles, magnetic monopoles produce direct and indirect Cherenkov light while traversing through matter at relativistic velocities. This paper describes searches for relativistic ((Formula presented.)) and mildly relativistic ((Formula presented.)) monopoles, each using one year of data taken in 2008/2009 and 2011/2012, respectively. No monopole candidate was detected. For a velocity above (Formula presented.) the monopole flux is constrained down to a level of (Formula presented.). This is an improvement of almost two orders of magnitude over previous limits.


Search for features in the spectrum of primordial perturbations using Planck and other datasets

Journal of Cosmology and Astroparticle Physics IOP Publishing 2015 (2015) 052-052

P Hunt, S Sarkar

We reconstruct the power spectrum of primordial curvature perturbations by applying a well-validated non-parametric technique employing Tikhonov regularisation to the first data release from the Planck satellite. To improve the reconstruction on small spatial scales we include data from the ground-based ACT and SPT experiments, the WiggleZ galaxy redshift survey, the CFHTLenS tomographic weak lensing survey, and spectral analysis of the Lyman-α forest. The reconstructed scalar spectrum (assuming the standard ΛCDM cosmology) is not scale-free but has an infrared cutoff at k ≲ 5 × 10-4 Mpc-1 and several (2-3)σ features, of which two at wavenumber k/Mpc-1 0.0018 and 0.057 had been seen already in WMAP data. A higher significance feature at k ∼ 0.12 Mpc-1 is indicated by Planck data, but may be sensitive to the systematic uncertainty around multipole ℓ ∼ 1800 in the 217×217 GHz cross-spectrum. In any case accounting for the 'look elsewhere' effect decreases its global significance to ∼2σ.


Contributions to ICRC 2015 Part I: Point Source Searches

Proceedings of Science Scuola Internazionale Superiore di Studi Avanzati (2015)

K Abraham, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Dunkman, R Eagan, B Eberhardt, T Ehrhardt, B Eichmann, S Euler, PA Evenson, O Fadiran, S Fahey, AR Fazely, A Fedynitch, J Feintzeig, J Felde, K Filimonov, C Finley, T Fischer-Wasels, S Flis, C-C Fösig, TK Gaisser


Contributions to ICRC 2015: The Next Generation Neutrino Observatory at the South Pole

Proceedings of Science Scuola Internazionale Superiore di Studi Avanzati (2015)

K Abraham, M Ackermann, J Adams, S Axani, X Bai, I Bartos, SW Barwick, V Baum, R Bay, JJ Beatty, JB Tjus, K-H Becker, E Beiser, S Benzvi, A Bernhard, J Kunnen, N Kurahashi, T Kuwabara, M Labare, JL Lanfranchi, MJ Larson, M Lesiak-Bzdak, J Lünemann, J Madsen


Contributions to ICRC 2015 Part IV: Searches for Dark Matter and Exotic Particles

Proceedings of Science Scuola Internazionale Superiore di Studi Avanzati (2015)

K Abraham, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Ahrens, D Altmann, T Anderson, I Ansseau, M Archinger, C Arguelles, TC Arlen, J Auffenberg, X Bai, SW Barwick, V Baum, R Bay, JJ Beatty, JB Tjus, K-H Becker, E Beiser, S Benzvi, P Berghaus, D Berley


The IceCube Neutrino Observatory - Contributions to ICRC 2015 Part II: Atmospheric and Astrophysical Diffuse Neutrino Searches of All Flavors

Proceedings of Science Sissa Medialab (2015) 1-66

K Abraham, M Ackermann, J Adams, C Arguelles, TC Arlen, JB Tjus, K-H Becker, E Beiser, A Bernhard, DZ Besson, G Binder, D Bindig, M Bissok, J Blumenthal, E Blaufuss, C Bohm, M Börner, F Bos, D Bose, S Böser, O Botner, J Braun, L Brayeur

Papers on atmospheric and astrophysical diffuse neutrino searches of all flavors submitted to the 34th International Cosmic Ray Conference (ICRC 2015, The Hague) by the IceCube Collaboration.

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