Search for an excess of events in the Super-Kamiokande detector in the directions of the astrophysical neutrinos reported by the IceCube Collaboration
Super-Kamiokande Collaboration, K. Abe, C. Bronner, G. Pronost, Y. Hayato, M. Ikeda, K. Iyogi, J. Kameda, Y. Kato, Y. Kishimoto, Ll. Marti, M. Miura, S. Moriyama, M. Nakahata, Y. Nakano, S. Nakayama, Y. Okajima, A. Orii, H. Sekiya, M. Shiozawa, Y. Sonoda, A. Takeda, A. Takenaka, H. Tanaka, S. Tasaka, T. Tomura, R. Akutsu, T. Kajita, K. Kaneyuki, Y. Nishimura, K. Okumura, K. M. Tsui, L. Labarga, P. Fernandez, F. d. M. Blaszczyk, J. Gustafson, C. Kachulis, E. Kearns, J. L. Raaf, J. L. Stone, L. R. Sulak, S. Berkman, S. Tobayama, M. Goldhaber, M. Elnimr, W. R. Kropp, S. Mine, S. Locke, P. Weatherly, M. B. Smy, H. W. Sobel, V. Takhistov, K. S. Ganezer, J. Hill, J. Y. Kim, I. T. Lim, R. G. Park, A. Himmel, Z. Li, E. O'Sullivan, K. Scholberg, C. W. Walter, T. Ishizuka, T. Nakamura, J. S. Jang, K. Choi, J. G. Learned, S. Matsuno, S. N. Smith, J. Amey, R. P. Litchfield, W. Y. Ma, Y. Uchida, M. O. Wascko, S. Cao, M. Friend, T. Hasegawa, T. Ishida, T. Ishii, T. Kobayashi, T. Nakadaira, K. Nakamura, Y. Oyama, K. Sakashita, T. Sekiguchi, T. Tsukamoto, KE. Abe, M. Hasegawa, Y. Nakano, A. T. Suzuki, Y. Takeuchi, T. Yano, S. V. Cao, T. Hayashino, T. Hiraki, S. Hirota, K. Huang, M. Jiang, A. Minamino, KE. Nakamura, et al. (63 additional authors not shown)
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SEARCH FOR AN EXCESS OF EVENTS IN THE SUPER-KAMIOKANDE DETECTOR IN THE DIRECTIONSOF THE ASTROPHYSICAL NEUTRINOS REPORTED BY THE ICECUBE COLLABORATION
K. Abe,
1, 38
C. Bronner, G. Pronost, Y. Hayato,
1, 38
M. Ikeda, K. Iyogi, J. Kameda,
1, 38
Y. Kato, Y. Kishimoto,
1, 38
Ll. Marti, M. Miura,
1, 38
S. Moriyama,
1, 38
M. Nakahata,
1, 38
Y. Nakano, S. Nakayama,
1, 38
Y. Okajima, A. Orii, H. Sekiya,
1, 38
M. Shiozawa,
1, 38
Y. Sonoda, A. Takeda,
1, 38
A. Takenaka, H. Tanaka, S. Tasaka, T. Tomura,
1, 38
R. Akutsu, T. Kajita,
2, 38
K. Kaneyuki,
2, 38, ∗ Y. Nishimura, K. Okumura,
2, 38
K. M. Tsui, L. Labarga, P. Fernandez, F. d. M. Blaszczyk, J. Gustafson, C. Kachulis, E. Kearns,
4, 38
J. L. Raaf, J. L. Stone,
4, 38
L. R. Sulak, S. Berkman, S. Tobayama, M. Goldhaber, ∗ M. Elnimr, W. R. Kropp, S. Mine, S. Locke, P. Weatherly, M. B. Smy,
7, 38
H. W. Sobel,
7, 38
V. Takhistov, † K. S. Ganezer, J. Hill, J. Y. Kim, I. T. Lim, R. G. Park, A. Himmel, Z. Li, E. O’Sullivan, K. Scholberg,
10, 38
C. W. Walter,
10, 38
T. Ishizuka, T. Nakamura, J. S. Jang, K. Choi, J. G. Learned, S. Matsuno, S. N. Smith, J. Amey, R. P. Litchfield, W. Y. Ma, Y. Uchida, M. O. Wascko, S. Cao, M. Friend, T. Hasegawa, T. Ishida, T. Ishii, T. Kobayashi, T. Nakadaira, K. Nakamura,
16, 38
Y. Oyama, K. Sakashita, T. Sekiguchi, T. Tsukamoto, KE. Abe, M. Hasegawa, A. T. Suzuki, Y. Takeuchi,
17, 38
T. Yano, S. V. Cao, T. Hayashino, T. Hiraki, S. Hirota, K. Huang, M. Jiang, A. Minamino, KE. Nakamura, T. Nakaya,
18, 38
B. Quilain, N. D. Patel, R. A. Wendell,
18, 38
L. H. V. Anthony, N. McCauley, A. Pritchard, Y. Fukuda, Y. Itow,
21, 22
M. Murase, F. Muto, P. Mijakowski, K. Frankiewicz, C. K. Jung, X. Li, J. L. Palomino, G. Santucci, C. Vilela, M. J. Wilking, C. Yanagisawa, ‡ S. Ito, D. Fukuda, H. Ishino, A. Kibayashi, Y. Koshio,
25, 38
H. Nagata, M. Sakuda, C. Xu, Y. Kuno, D. Wark,
27, 33
F. Di Lodovico, B. Richards, R. Tacik,
29, 42
S. B. Kim, A. Cole, L. Thompson, H. Okazawa, Y. Choi, K. Ito, K. Nishijima, M. Koshiba, Y. Totsuka, ∗ Y. Suda, M. Yokoyama,
37, 38
R. G. Calland, M. Hartz, K. Martens, C. Simpson,
38, 27
Y. Suzuki, M. R. Vagins,
38, 7
D. Hamabe, M. Kuze, T. Yoshida, M. Ishitsuka, J. F. Martin, C. M. Nantais, H. A. Tanaka, A. Konaka, S. Chen, L. Wan, Y. Zhang, A. Minamino, and R. J. Wilkes The Super-Kamiokande Collaboration Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu 506-1205, Japan Research Center for Cosmic Neutrinos, Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan Department of Theoretical Physics, University Autonoma Madrid, 28049 Madrid, Spain Department of Physics, Boston University, Boston, MA 02215, USA Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T1Z4, Canada Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697-4575, USA Department of Physics, California State University, Dominguez Hills, Carson, CA 90747, USA Department of Physics, Chonnam National University, Kwangju 500-757, Korea Department of Physics, Duke University, Durham NC 27708, USA Junior College, Fukuoka Institute of Technology, Fukuoka, Fukuoka 811-0295, Japan Department of Physics, Gifu University, Gifu, Gifu 501-1193, Japan GIST College, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA Department of Physics, Imperial College London , London, SW7 2AZ, United Kingdom High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan Department of Physics, University of Liverpool, Liverpool, L69 7ZE, United Kingdom
Corresponding author: Erin O’[email protected] a r X i v : . [ a s t r o - ph . H E ] J a n Abe et al. Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan Institute for Space-Earth Enviromental Research, Nagoya University, Nagoya, Aichi 464-8602, Japan Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya, Aichi 464-8602, Japan National Centre For Nuclear Research, 00-681 Warsaw, Poland Department of Physics and Astronomy, State University of New York at Stony Brook, NY 11794-3800, USA Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Physics, Oxford University, Oxford, OX1 3PU, United Kingdom School of Physics and Astronomy, Queen Mary University of London, London, E1 4NS, United Kingdom Department of Physics, University of Regina, 3737 Wascana Parkway, Regina, SK, S4SOA2, Canada Department of Physics, Seoul National University, Seoul 151-742, Korea Department of Physics and Astronomy, University of Sheffield, S10 2TN, Sheffield, United Kingdom Department of Informatics in Social Welfare, Shizuoka University of Welfare, Yaizu, Shizuoka, 425-8611, Japan STFC, Rutherford Appleton Laboratory, Harwell Oxford, and Daresbury Laboratory, Warrington, OX11 0QX, United Kingdom Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study,University of Tokyo, Kashiwa, Chiba 277-8583, Japan Department of Physics,Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan Department of Physics, University of Toronto, ON, M5S 1A7, Canada TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T2A3, Canada Department of Engineering Physics, Tsinghua University, Beijing, 100084, China Faculty of Engineering, Yokohama National University, Yokohama, 240-8501, Japan Department of Physics, University of Washington, Seattle, WA 98195-1560, USA (Received July 26, 2017; Revised October 18, 2017; Accepted January 8, 2018)
Submitted to ApJABSTRACTWe present the results of a search in the Super-Kamiokande (SK) detector for excesses of neutrinos with energiesabove a few GeV that are in the direction of the track events reported in IceCube. Data from all SK phases (SK-Ithrough SK-IV) were used, spanning a period from April 1996 to April 2016 and corresponding to an exposure of 225kilotonne-years . We considered the 14 IceCube track events from a data set with 1347 livetime days taken from 2010to 2014. We use Poisson counting to determine if there is an excess of neutrinos detected in SK in a 10 degree searchcone (5 degrees for the highest energy data set) around the reconstructed direction of the IceCube event. No significantexcess was found in any of the search directions we examined. We also looked for coincidences with a recently reportedIceCube multiplet event. No events were detected within a ±
500 s time window around the first detected event, andno significant excess was seen from that direction over the lifetime of SK. ∗ Deceased. † also at Department of Physics and Astronomy, UCLA, CA 90095-1547, USA. ‡ also at BMCC/CUNY, Science Department, New York, New York, USA. ceCube coincidence search in Super-Kamiokande INTRODUCTIONNeutrino astronomy is a burgeoning field, bridging the gap between astronomy and particle physics. Neutrinos travelundistorted from their source, and are therefore a valuable probe of the inner workings of astrophysical phenomena.The first definitive measurement of high energy extragalactic neutrinos was made by the IceCube experiment in 2013(IceCube Collaboration 2013), where they were able to reject the atmospheric-neutrino-only hypothesis at greater than4 σ .Detecting neutrinos that are astrophysical in origin has raised many questions: Where are these neutrinos comingfrom? What process is creating them? There has not yet been any significant evidence to suggest that these neutrinosare pointing to a particular region of the sky. The many searches for counterparts to the neutrino signal have beenlargely unsuccessful, including searches for coincidences with photons from Fermi LAT (Peng & Wang 2017), fast radiobursts (Fahey et al. 2016), as well as a search for coincidences with a large catalogue of candidate sources (Aartsen etal. 2017). One search (Kadler et al. 2016) found a coincidence between a PeV neutrino detected in IceCube and anoutburst of the blazar PKS B1424418, giving a hint at a possible origin of these astrophysical neutrinos.IceCube uses detected events with energies above a few hundred TeV to look for astrophysical neutrinos. In thisenergy region, the atmospheric neutrino background is expected to be low. However, now that astrophysical neutrinocandidates have been identified, one can search in the lower energy data for an excess of events coming from thesame direction. Given the unknown origin of these neutrinos, searching this previously unexplored energy region is ofinterest.Super-Kamiokande (SK), a water Cherenkov detector located in Japan, detects atmospheric neutrinos in the energyrange of 30 MeV to several TeV. For events with energies above a few GeV, the direction of the incoming neutrinocan be reconstructed as it is well correlated with the direction of the detected outgoing lepton.SK has performed a number of astrophysical neutrino searches in the past. Recent searches include a general, all-skyastrophysical search (Thrane et al. 2009b; Abe et al. 2006; Swanson et al. 2006), searches for coincidences with gammaray bursts, supernova remnants, and other potential sources of astrophysical neutrinos (Thrane et al. 2009a; Desai etal. 2006; Thrane et al. 2009b), dark matter searches (Tanaka et al. 2011; Choi et al. 2011), and a search for coincidenceswith the recent detection of gravitational wave signals (Abe et al. 2016).In this paper, we look for excesses of neutrino events in Super-Kamiokande in the direction of the IceCube eventsfrom their data release in IceCube Collaboration (2015). We use the IceCube events that have the best pointingaccuracy, known as track events, and determine if there is an excess of events in the full SK high energy dataset.Given the uncertainty about the origins of these astrophysical neutrinos, we perform a model-blind search, withoutassuming an energy spectrum a priori. Since we have no observational or theoretical motivation for the time durationover which these neutrinos are emitted, we do not require any timing correlation between the IceCube and SK events.For this simple estimate, we omit discussions of systematic errors. We also report the search for coincidences with therecent multiplet event reported in Aartsen et al. (2017).An estimate of the number of events anticipated in the SK sample can be derived by extending the point sourcelimits from Aartsen et al. (2016) down to lower energies. Assuming the standard E − spectrum, we would expectapproximately 0.5 events in the UPMU sample and approximately 0.002 events in FC and PC samples. If we assume acutoff of E ν <
100 TeV (which is below the deposited energies of IceCube events 4, 9, and 12), we would expect around5 events in the UPMU sample and around 0.02 events in the FC and PC samples. More events could be expected inthe SK topologies if there is a softer spectral index (in IceCube Collaboration (2015) the best fit index is 2.58 ± THE SUPER-KAMIOKANDE EXPERIMENTThe SK detector is a 50-kilotonne (22.5 kilotonne fiducial) water Cherenkov detector located in the Mozumi minein the Gifu prefecture of Japan. The cylindrical detector is optically separated into an inner detector (ID) volume,which is viewed by ∼ ∼ Abe et al. an electronics upgrade in 2008, the current phase of the experiment, SKIV, began. Data from all four phases of theexperiment are used in this analysis, spanning April 1996 - April 2016 and corresponding to 225 kilotonne-years.Detected neutrino events at energies above 30 MeV can have three different topologies in the SK detector. Thefirst, known as fully-contained (FC) events, have a reconstructed vertex inside the fiducial volume, with little lightseen in the OD. Events that have a reconstructed vertex inside the fiducial volume, but have interaction productsthat produce light in the OD, are known as partially-contained (PC) events. Finally, muon neutrinos that interact inthe surrounding rock and produce penetrating muons are known as upward-going muon (UPMU) events. We requirethese events to be coming from below the horizon in order to distinguish them from cosmic muons. These topologiesroughly represent increasing, though overlapping, energy regions. For the atmospheric neutrino energy spectrum, FCevents have an average energy of about 1 GeV, PC events have an average energy of about 10 GeV, and UPMU eventshave an average energy of about 100 GeV. More information on the SK topologies, including the selection cuts usedin the data reduction, can be found in (Ashie et al. 2005). SEARCH METHODThe IceCube search directions were taken from the October 2015 data release (IceCube Collaboration 2015). Thisdata set contains neutrino candidates with two different topologies: track and shower. Track events are mainly frommuon neutrino charged-current interactions, while shower events are mainly from neutral current interactions, as wellas electron and tau neutrino charged-current interactions. Only IceCube track events were used in this analysis.Shower events typically have poor angular resolution, with an error up to 20 ◦ for the (IceCube Collaboration 2015)data set, making them unsuitable for a coincidence analysis using only spatial information. Track events, on the otherhand, have a median angular resolution of better than 1 ◦ , allowing us to perform a spatial coincident search. Table 3lists the properties of the IceCube events. Event Number Declination (degrees) RA (degrees) Deposited Energy (TeV)1 -31.2 127.9 78.72 -0.4 110.6 71.43 -21.2 182.4 32.64 40.3 67.9 252.75 -24.8 345.6 31.56 -13.2 208.7 82.27 -71.5 164.8 46.18 20.7 167.3 30.89 14.0 93.3 200.510 -22.0 206.6 46.511 0.0 336.7 84.612 -86.3 219.0 429.913 67.4 209.4 74.314 -37.7 239.0 27.6
Table 1.
Information on the track events from IceCube used in this coincidence search. The data were taken from IceCubeCollaboration (2015).
To ensure that the detected lepton points back to the incoming neutrino, a low energy threshold was imposed onthe FC and PC datasets. A minimum energy requirement was determined by calculating the lowest energy such that50% of the reconstructed lepton directions of that energy agreed to within 10 ◦ of the incoming neutrino direction inthe simulated data set from our Monte Carlo (MC) code. This threshold was determined to be 3.8 GeV for FC eventsand 2.1 GeV for PC events. No explicit upper energy cut was applied.A search cone, centered at the reconstructed direction of each IceCube event, was defined with a half-angle opening of10 ◦ for FC and PC events and 5 ◦ for UPMU events. The UPMU events are higher in energy than the other topologies ceCube coincidence search in Super-Kamiokande N B ) is, L = e − NB (1 − α ) N ! (cid:18) N B − α (cid:19) N , (1)where α represents the fraction of the N events that are from signal and is the parameter over which we maximize.The background to this search is from atmospheric neutrinos. The SK MC code, along with a scaling for neutrinooscillations and the overall normalization of the flux, was used to determine the number of background events ( N B ) weexpect to see in our search cone. The MC code used Geant3 (Brun et al. 1987) to simulate particle interactions andtracking. We used 500 years of simulated atmospheric neutrino events for each SK phase. The truth information wasgenerated using nuclear interaction models used in NEUT (Hayato 2009). We scaled the MC sample for each phase tothe appropriate livetime, and then summed them together. Events were assigned right ascensions assuming a flat localsidereal time, and so the resulting N B was assumed to depend only on declination. The combined sample was thenscaled on an event-by-event basis to the all-sky, best-fit value from data, which accounts for the flux normalizationand oscillations.Our test statistic, Λ, is then, Λ = 2 log L ( α fitted ) L ( α = 0) , (2)where α fitted is obtained from maximizing Equation 1. This is the final indicator of the statistical significance for thenumber of measured neutrino events in our search cone. RESULTSFigure 1 shows the spatial distribution of the detected neutrino events in the region of each search direction. Thedensity of the events are dependent on declination. This can be seen most clearly in the UPMU data sample, wherethere is a high density near the southern pole and no events at declinations above 54 ◦ (see event 13 in Figure 1). Thedensity of detected neutrino events does not appear to be significantly higher inside any of the search regions comparedwith the density around the search regions.Figure 2 shows the background expectations for the three topologies considered in this search. The backgroundexpectation is assumed to be independent of the right ascension. The FC and PC topologies extend to all declinationsand have a slightly positive slope due to oscillations in the upward-going neutrinos. The two peaks are due to theatmospheric neutrinos coming from near the horizon. Here, the cosmic rays are more likely to interact due to the factthat the atmosphere is thicker and that path length for traversing this region is longer and so there is a greater chancefor the cosmic ray to decay. The UPMU topology requires that the neutrino events come from below the horizon,and thus there are no events where the reconstructed direction has declinations above 54 ◦ . Neutrinos coming fromdeclinations of greater than -54 ◦ spend increasingly more time above the horizon, and so there is a decreasing trendabove this declination. The maximum number of UPMUs are at -54 ◦ . This is because these neutrinos are near, butalways below the horizon, where a greater flux of atmospheric neutrinos is expected. Abe et al.
Figure 1.
The detected FC (black circle), PC (blue x) and UPMU (red +) events in and around the search window. Theposition of events are shown in equatorial coordinates with right ascension on the x-axis and declination on the y-axis. Thedark grey disk shows the 5 degree search cone used for UPMU events, while the light grey disk shows the 10 degree search coneused for FC and PC events. ceCube coincidence search in Super-Kamiokande N ) in the search cone for each of the IceCubesearch directions. Figure 2 shows the number of detected neutrino events in the search cone compared with thebackground expectation. Figure 2.
The number of events expected in the search cone (solid line) for FC (black, left), PC (blue, center), and UPMU(red, right) topologies, shown with the number of events found in the search cone from the data (points). The errors shownhere are √ N . Event 13 (declination of 67.4 ◦ ) was not visible to the UPMU data set since it is always above the horizon in theSK detector. To determine if there were any statistically significant excesses in our data, we calculated the test statistic, Λ,for each search direction using Equation 2. The expected distribution of Λ was also calculated using the 500-yearsimulated MC data set. The UPMU topology was used and the declination of -31.2 ◦ was assumed. The events werefirst randomly assigned a right ascension assuming a flat local sidereal time. The number of events in a 5 ◦ search conewas then determined, scaled for the appropriate livetime. This was compared to the background expectation N B forthis declination and Λ was calculated using Equation 2. This algorithm was repeated 1 × times, randomly assigningnew right ascension values to the data each time. The expected distributions for the different topologies, as well asthe different declinations, were checked and found to be the same.Figure 3 shows the test statistic for all three topologies for each search direction. Λ was found to be zero most often,signifying that the number of events in the search cone best fit to the expected background, or that there were fewerevents in the search cone than the expected background would suggest. No excess of greater than 2 σ was found forany of the topologies in any of the search directions. No search direction jointly yielded a significantly higher Λ in allthree of the topologies.The most significant event had a calculated Λ of 1.1, which corresponds to a significance of about 1.1 σ from theMC background prediction. This was in the UPMU data sample in the direction of event number 10, corresponding toa declination of -22 ◦ . In this search direction, we observed 22 events and expected 13.7 events from the atmosphericbackground MC prediction.Figure 4 shows the distribution of the test statistic separately for each of the three topologies considered, along withthe expected test statistic distribution calculated using simulated data from our Monte Carlo code. As seen here, nostatistically significant excesses are seen and the distribution of the test statistic from the data matches the backgroundexpectation. The confidence levels are determined using the MC test statistic distribution by calculating the Λ where68.3%, 95.4%, and 99.7% of the test statistic prediction is enclosed for 1 σ , 2 σ , and 3 σ , respectively.4.1. Searching for a coincidence with the IceCube multiplet event
In addition to the search for excesses in the direction of the IceCube track events, we separately searched forcoincidence events with the IceCube multiplet event reported in Aartsen et al. (2017). On February 17, 2016, IceCubeobserved three neutrino candidate events within less than 100s separated by 3.6 ◦ . This type of multiplet event wouldbe expected to occur once every 13.7 years. No optical counterparts were found in the follow up searches discussed inAartsen et al. (2017).In SK, we searched for neutrino candidate events in the FC, PC, and UPMU data sets in a 500-s time windowaround the time of the first detected neutrino in the multiplet event. No SK candidate events were detected in any ofthe three topologies.We also performed a spatial coincidence search over all SK phases using the same method used for the IceCube trackevents reported in the other sections of this paper. In the direction of the triplet event (dec = 39.5 ◦ , RA = 26.1 ◦ ), we Abe et al.
Figure 3.
The test statistic for each search direction, plotted for the FC (black circle), PC (blue x), and UPMU (red +)topologies. The dashed line represents the 1 σ expectation calculated from the atmospheric MC. No trend can be seen for anexcess in a particular search direction. (a) (b) (c) Figure 4.
The binned Λ values obtained for each of the 14 (13 for UPMUs) search directions for (a) FC, (b) PC, and (c)UPMU topologies. Also shown is the expected test statistic distribution as predicted from our MC simulation. The confidencelevels are determined by enclosing 68.3 %, 95.4 %, and 99.7% of the test statistic prediction for 1 σ (dark blue), 2 σ (mediumblue), and 3 σ (light blue), respectively. detected (expected) 24 (24.5) FC events, 26 (23.2) PC events, and 16 (12.9) UPMU events. The likelihood ratio (Λ)for the three topologies was calculated to be 0 for FC, 0.14 for PC, and 0.30 for UPMU, which is all less than the onesigma value determined using the atmospheric MC background. CONCLUSIONSWe performed a search for SK neutrino detections coincident in direction with the IceCube track events. UsingPoisson statistics, we used SK data taken from April 1996 - April 2016 to determine if there was any excess of eventsin each of the search directions. The detected numbers of SK neutrino events in each of the search directions wereconsistent with the background expectations. The most significant Λ was 1.1 at a declination of -22 ◦ in the UPMUdata set, which corresponds to a significance of about 1.1 σ using the atmospheric MC prediction.We also looked for coincidence events with the IceCube multiplet event reported in (Aartsen et al. 2017). In thetime coincidence search, no events were found within a ±
500 s time window from the first detected IceCube event.In the direction coincidence search, the number of events detected over the lifetime of SK from the direction of theIceCube multiplet event was consistent with atmospheric background for the FC, PC, and UPMU topologies. ceCube coincidence search in Super-Kamiokande − , for example) for the flexibility of being modelindependent. This search is useful for constraining the behaviour of astrophysical neutrinos in the lower energy regimeand guiding new models which predict neutrino behaviours at lower energies. ACKNOWLEDGEMENTSWe gratefully acknowledge the cooperation of the Kamioka Mining and Smelting Company. The SuperKamiokandeexperiment has been built and operated from funding by the Japanese Ministry of Education, Culture, Sports, Scienceand Technology, the U.S. Department of Energy, and the U.S. National Science Foundation. Some of us have beensupported by funds from the National Research Foundation of Korea NRF20090083526 (KNRC) funded by the Ministryof Science, ICT, and Future Planning, the European Union H2020 RISEGA641540SKPLUS, the Japan Society for thePromotion of Science, the National Natural Science Foundation of China under Grants No. 11235006, the NationalScience and Engineering Research Council (NSERC) of Canada, the Scinet and Westgrid consortia of Compute Canada,and the National Science Centre, Poland (2015/17/N/ST2/04064, 2015/18/E/ST2/00758).REFERENCES
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