Observation and Characterization of a Cosmic Muon Neutrino Flux From the Northern Hemisphere Using Six Years of Icecube Data

Abstract

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

Use of ANTARES and IceCube Data to Constrain a Single Power-law Neutrino Flux

“…This means that fixing a certain threshold in p-value for rejecting the null hypothesis, the addition of the ANTARES data set to the IceCube one enlarges the range of spectral indices for which the null hypothesis is disfavored. Moreover, we observe that the interpretation of the six-year up-going muon neutrinos as a single power-law with γ=2.13±0.13 (Aartsen et al 2016) is almost statistically incompatible (p-value smaller than 10 −2 ) with the same interpretation for the whole data sample. We underline that the benchmark prediction of Fermi acceleration mechanism γ=2.0 has a p-value equal to 2.6 10 1.8 3.6 5 -+ -for χ 2 and 1.6 10 1.0 2.1 6 -+ -for KS statistical tests, where the errors correspond to a ±20% uncertainty on the conventional atmospheric background.…”

“…The two-component scenario is also strongly motivated by the analyses performed on the up-going muon neutrinos (Aartsen et al 2016). Indeed, the recent six-year data sample points toward a harder power-law with spectral index of 2.13±0.13 (Aartsen et al 2016). Such a value is in a3.3σ tension with the combined analysis of different IceCube data samples (Aartsen et al 2015a).…”

“…On the other hand, we do not take into account the prompt atmospheric background (neutrinos produced by the decays of charmed mesons; Enberg et al 2008), according to the IceCube conclusions contained in (Aartsen et al 2014a(Aartsen et al , 2015b(Aartsen et al , 2016 In the present analysis, the combined fit is performed by maximizing the binned multi-Poisson likelihoods A,IC  , with expressions that are given by Baker & Cousins (1984) …”

“…For instance, Waxman and Bahcall(1999) predicted a cosmic neutrino flux proportional to E −2.0 , according to the standard Fermi acceleration mechanism at shock fronts (Fermi 1949 g have suggested the presence of an additional component dominating at lower energies (E ν 200 TeV), on top of a single unbroken power-law flux (see Chen et al 2015;Vincent et al 2016;Anchordoqui et al 2017;see Kopper 2016 for studies of a neutrino flux with two power-laws and Kimura et al 2015;Murase et al 2016;Senno et al 2016 for viable hidden astrophysical candidates). The two-component scenario is also strongly motivated by the analyses performed on the up-going muon neutrinos (Aartsen et al 2016). Indeed, the recent six-year data sample points toward a harder power-law with spectral index of 2.13±0.13 (Aartsen et al 2016).…”

“…Once a hard power-law is considered according to the analyses of up-going muon neutrinos (Aartsen et al 2016), a low-energy excess in the range 40-200TeV arises in different IceCube data samples (Aartsen et al 2015b;. The statistical characterization of such a ∼2σ excess, with respect to a power-law with spectral index 2.0, was highlighted in Chianese et al (2016) by analyzing the four-year HESE data .…”