Publications by Subir Sarkar


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 Sarkar, H Liu, P Mertsch, S Hausegger

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.


The prompt atmospheric neutrino flux in the light of LHCb

JHEP Springer Berlin Heidelberg 02 (2016) 130-

R Gauld, L Rottoli, J Rojo, 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, M Ackermann, K Abraham, J Adams, JA Aguilar, M Ahlers, M Ahrens, D Altmann, T Anderson, I Ansseau, C Arguelles, M Archinger, TC Arlen, J Auffenberg, X Bai, SW Barwick, V Baum, R Bay, JJ Beatty, K-H Becker, JB Tjus, 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

M Zoll, JB Tjus, E Beiser, K-H Becker, P Berghaus, S Benzvi, D Berley, E Bernardini, A Bernhard, G Binder, DZ Besson, 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

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.


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.


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

S Sarkar, M Aartsen, K Abraham, M Ackermann, 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

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, C Pfendner, D Pieloth, J Posselt, E Pinat, PB Price, GT Przybylski, M Quinnan

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, S Sarkar, M Ackermann

© 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.


Dark matter annihilation and decay in dwarf spheroidal galaxies: The classical and ultrafaint dSphs

Monthly Notices of the Royal Astronomical Society Oxford University Press 453 (2015) 849-867

V Bonnivard, C Combet, E others, M Daniel, S Funk, JA Hinton, A Geringer-Sameth, D Maurin, JI Read, S Sarkar, MG Walker, MI Wilkinson

Dwarf spheroidal (dSph) galaxies are prime targets for present and future Γ-ray telescopes hunting for indirect signals of particle darkmatter. The interpretation of the data requires careful assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimized spherical Jeans analysis to reconstruct the 'astrophysical factor' for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimize biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 'classical' and 13 'ultrafaint' dSphs, with the quoted uncertainties directly linked to the sample size; themore flexible parametrizationwe use results in changes compared to previous calculations. This translates into our ranking of potentiallybrightest and most robust targets-namely Ursa Minor, Draco, Sculptor-and of the more promising, but uncertain targets-namely Ursa Major 2, Coma-for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new 'ultrafaint' satellites that are beginning to be discovered in southern sky surveys.


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.


Search for dark matter annihilation in the Galactic Center with IceCube-79

European Physical Journal C Springer Berlin Heidelberg 75 (2015) 492

J Luenemann, J Madsen, G Maggi, KBM Mahn, R Maruyama, K Mase, HS Matis, F McNally, R Maunu, K Meagher, M Medici, A Meli, T Menne, G Merino, S Miarecki, T Meures, E Middell, E Middlemas, J Miller, T Montaruli, L Mohrmann, R Morse, R Nahnhauer, U Naumann, H Niederhausen

The Milky Way is expected to be embedded in a halo of dark matter particles, with the highest density in the central region, and decreasing density with the halo-centric radius. Dark matter might be indirectly detectable at Earth through a flux of stable particles generated in dark matter annihilations and peaked in the direction of the Galactic Center. We present a search for an excess flux of muon (anti-) neutrinos from dark matter annihilation in the Galactic Center using the cubic-kilometer-sized IceCube neutrino detector at the South Pole. There, the Galactic Center is always seen above the horizon. Thus, new and dedicated veto techniques against atmospheric muons are required to make the southern hemisphere accessible for IceCube. We used 319.7 live-days of data from IceCube operating in its 79-string configuration during 2010 and 2011. No neutrino excess was found and the final result is compatible with the background. We present upper limits on the self-annihilation cross-section, (Formula presented.), for WIMP masses ranging from 30 GeV up to 10 TeV, assuming cuspy (NFW) and flat-cored (Burkert) dark matter halo profiles, reaching down to ≃4·10-24 cm3 s-1, and ≃2.6·10-23 cm3 s-1 for the νν¯ channel, respectively.


The detection of a SN IIn in optical follow-up observations of IceCube neutrino events

Astrophysical Journal American Astronomical Society 811 (2015) 1-17

M Ackermann, K Abraham, S Sarkar

The IceCube neutrino observatory pursues a follow-up program selecting interesting neutrino events in real-time and issuing alerts for electromagnetic follow-up observations. In 2012 March, the most significant neutrino alert during the first three years of operation was issued by IceCube. In the follow-up observations performed by the Palomar Transient Factory (PTF), a Type IIn supernova (SN IIn) PTF12csy was found 0.°2 away from the neutrino alert direction, with an error radius of 0.°54. It has a redshift of z = 0.0684, corresponding to a luminosity distance of about 300 Mpc and the Pan-STARRS1 survey shows that its explosion time was at least 158 days (in host galaxy rest frame) before the neutrino alert, so that a causal connection is unlikely. The a posteriori significance of the chance detection of both the neutrinos and the SN at any epoch is 2.2σ within IceCube's 2011/12 data acquisition season. Also, a complementary neutrino analysis reveals no long-term signal over the course of one year. Therefore, we consider the SN detection coincidental and the neutrinos uncorrelated to the SN. However, the SN is unusual and interesting by itself: it is luminous and energetic, bearing strong resemblance to the SN IIn 2010jl, and shows signs of interaction of the SN ejecta with a dense circumstellar medium. High-energy neutrino emission is expected in models of diffusive shock acceleration, but at a low, non-detectable level for this specific SN. In this paper, we describe the SN PTF12csy and present both the neutrino and electromagnetic data, as well as their analysis.


Evidence for Astrophysical Muon Neutrinos from the Northern Sky with IceCube

Physical Review Letters American Physical Society 115 (2015) 081102-

S Vanheule, J Veenkamp, M Vehring, M Voge, M Vraeghe, C Walck, M Wallraff, N Wandkowsky, C Weaver, C Wendt, S Westerhoff, BJ Whelan, N Whitehorn, C Wichary, K Wiebe, CH Wiebusch, L Wille, Williams, H Wissing, TR Wood, M Wolf, K Woschnagg, DL Xu, XW Xu, Y Xu

Results from the IceCube Neutrino Observatory have recently provided compelling evidence for the existence of a high energy astrophysical neutrino flux utilizing a dominantly Southern Hemisphere data set consisting primarily of ν(e) and ν(τ) charged-current and neutral-current (cascade) neutrino interactions. In the analysis presented here, a data sample of approximately 35,000 muon neutrinos from the Northern sky is extracted from data taken during 659.5 days of live time recorded between May 2010 and May 2012. While this sample is composed primarily of neutrinos produced by cosmic ray interactions in Earth's atmosphere, the highest energy events are inconsistent with a hypothesis of solely terrestrial origin at 3.7σ significance. These neutrinos can, however, be explained by an astrophysical flux per neutrino flavor at a level of Φ(E(ν))=9.9(-3.4)(+3.9)×10(-19)  GeV(-1) cm(-2) sr(-1) s(-1)(E(ν)/100  TeV(-2), consistent with IceCube's Southern-Hemisphere-dominated result. Additionally, a fit for an astrophysical flux with an arbitrary spectral index is performed. We find a spectral index of 2.2(-0.2)(+0.2), which is also in good agreement with the Southern Hemisphere result.


A combined maximum-likelihood analysis of the high-energy astrophysical neutrino flux measured with ICECUBE

Astrophysical Journal IOP Publishing 809 (2015) 98-98

K Abraham, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Ahrens, JB Tjus, K-H Becker, E Beiser, S BenZvi, D Bindig, E Blaufuss, C Bohm, F Bos, D Bose, S Boeser, O Botner, J Braun, L Brayeur, D Chirkin, K Clark, S Coenders, AHC Silva, J Daughhetee

Evidence for an extraterrestrial flux of high-energy neutrinos has now been found in multiple searches with the IceCube detector. The first solid evidence was provided by a search for neutrino events with deposited energies ≳30 TeV and interaction vertices inside the instrumented volume. Recent analyses suggest that the extraterrestrial flux extends to lower energies and is also visible with throughgoing, νμ-induced tracks from the Northern Hemisphere. Here, we combine the results from six different IceCube searches for astrophysical neutrinos in a maximum-likelihood analysis. The combined event sample features high-statistics samples of shower-like and track-like events. The data are fit in up to three observables: energy, zenith angle, and event topology. Assuming the astrophysical neutrino flux to be isotropic and to consist of equal flavors at Earth, the all-flavor spectrum with neutrino energies between 25 TeV and 2.8 PeV is well described by an unbroken power law with best-fit spectral index -2.50 ± 0.09 and a flux at 100 TeV of (6.7-1.2 +1.1) × 10-18 GeV-1 s-1cm-2. Under the same assumptions, an unbroken power law with index -2 is disfavored with a significance of 3.8σ (p = 0.0066%) with respect to the best fit. This significance is reduced to 2.1σ (p = 1.7%) if instead we compare the best fit to a spectrum with index .2 that has an exponential cut-off at high energies. Allowing the electron-neutrino flux to deviate from the other two flavors, we find a νe fraction of 0.18 ± 0.11 at Earth. The sole production of electron neutrinos, which would be characteristic of neutron-decay-dominated sources, is rejected with a significance of 3.6σ (p = 0.014%).


Contributions to ICRC 2015 Part I: Point Source Searches

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

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


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

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

M Ackermann, K Abraham, 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 Lünemann, J Madsen, N Kurahashi, T Kuwabara, J Kunnen, M Labare, JL Lanfranchi, MJ Larson, M Lesiak-Bzdak


The IceCube Neutrino Observatory - Contributions to ICRC 2015 Part V: Neutrino Oscillations and Supernova Searches

34th International Cosmic Ray Conference Proceedings of Science 2015 (2015)

TC Arlen, J Auffenberg, SW Barwick, X Bai, V Baum, JJ Beatty, R Bay, JB Tjus, K-H Becker, E Beiser, S Benzvi, P Berghaus, D Berley, E Bernardini, A Bernhard, M Ackermann, K Abraham, J Adams, JA Aguilar, M Ahlers, M Ahrens, G Binder, D Bindig

Papers on neutrino oscillations and supernova searches submitted to the 34th International Cosmic Ray Conference (ICRC 2015, The Hague) by the IceCube Collaboration.


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.

Pages