Atmospheric and Astrophysical Neutrinos above 1 TeV Interacting in IceCube

TL;DR

This study analyzes 1 TeV to 1 PeV neutrino interactions contained within IceCube over 641 days to extend southern-sky sensitivity to 10 TeV. Using a veto-based containment approach and a forward-folding likelihood, it decomposes the observed events into conventional and prompt atmospheric neutrinos, penetrating muons, and an isotropic astrophysical flux, finding a per-flavor astrophysical normalization of about 2.06×10^-18 and a spectral index of roughly 2.46. The results indicate astrophysical neutrinos dominate at high energies with no statistically significant charm-initiated atmospheric component, constraining prompt charm production to 1.52× the ERS prediction at 90% CL under isotropy; the analysis also shows the spectrum is softer than the E^-2 benchmark. These findings underscore the power of veto-based, contained-event analyses to probe diffuse astrophysical neutrinos across broad sky regions and energy ranges, with future data expected to tighten constraints and probe potential anisotropies.

Abstract

The IceCube Neutrino Observatory was designed primarily to search for high-energy (TeV--PeV) neutrinos produced in distant astrophysical objects. A search for $\gtrsim 100$~TeV neutrinos interacting inside the instrumented volume has recently provided evidence for an isotropic flux of such neutrinos. At lower energies, IceCube collects large numbers of neutrinos from the weak decays of mesons in cosmic-ray air showers. Here we present the results of a search for neutrino interactions inside IceCube's instrumented volume between 1~TeV and 1~PeV in 641 days of data taken from 2010--2012, lowering the energy threshold for neutrinos from the southern sky below 10 TeV for the first time, far below the threshold of the previous high-energy analysis. Astrophysical neutrinos remain the dominant component in the southern sky down to 10 TeV. From these data we derive new constraints on the diffuse astrophysical neutrino spectrum, $Φ_ν = 2.06^{+0.4}_{-0.3} \times 10^{-18} \left({E_ν}/{10^5 \,\, \rm{GeV}} \right)^{-2.46 \pm 0.12} {\rm {GeV^{-1} \, cm^{-2} \, sr^{-1} \, s^{-1}} } $, as well as the strongest upper limit yet on the flux of neutrinos from charmed-meson decay in the atmosphere, 1.52 times the benchmark theoretical prediction used in previous IceCube results at 90\% confidence.

Figures (12)

• Figure 1: Fluxes of vertically-downgoing muons and neutrinos detectable in IceCube. The upper line shows the flux of penetrating, single atmospheric muons at the depth of IceCube, while the remaining lines show neutrino fluxes multiplied by the probability that a neutrino of the given energy would interact in 1 km of glacial ice. The dotted lines show the total interacting flux of atmospheric neutrinos of all flavors Honda:2006Sarcevic:2008, while the corresponding solid lines show the interacting flux that arrives at the depth of IceCube without accompanying muons above 1 TeV UncorrelatedVeto. Accompanying muons suppress the effective $\nu_{\mu}$ flux from $\pi$ and $K$ decay below the level of the effective $\nu_{e}$ flux from $K$ decay at 50 TeV, producing a kink in the spectrum. The $E^{-2}$ astrophysical neutrino flux, shown here with the normalization of HESE_3year, always arrives without accompanying muons.
• Figure 2: Distribution of photon counts per event after each stage of the event selection. The total number of collected photons is on average proportional to the total deposited energy; for example, $10^3$ photons correspond to roughly 10 TeV deposited energy. The stepped lines show the prediction from Monte Carlo simulation of penetrating atmospheric muons (blue) atmospheric neutrinos (red), while the points show experimental data. Left: Pre-selected events transmitted from the South Pole (Sec. \ref{['sub:pre_selection']}). Center: Removed events with veto hits (Sec. \ref{['sub:veto']}). Right: Fiducial volume scaled with photon count (Sec. \ref{['sub:fiducial_volume_scaling']}).
• Figure 3: An illustration of the incoming-muon veto procedure described in Sec. \ref{['sub:veto']}. Each panel shows a snapshot in time with the current position of the muon marked by the blue arrowhead and the position of the reconstructed vertex marked by a green star. \ref{['fig:veto_sketch:incoming_before']} shows a penetrating muon before its largest energy loss with a photon detection that counts towards the veto, while \ref{['fig:veto_sketch:incoming_after']} shows the same configuration after the largest energy loss with an ambiguous photon detection that does not count towards the veto. \ref{['fig:veto_sketch:starting']} shows how the technique can be inverted to detect starting tracks.
• Figure 4: Fraction of pre-selected penetrating muon background events (Sec. \ref{['sub:pre_selection']}) that pass the veto conditions (Sec. \ref{['sub:veto']}), derived from MC simulation. The outer-layer veto reduces the rate of the highest-energy muons by $10^4$, but degrades rapidly at lower energies. The incoming-track veto scales in a similar way with respect to energy, but is more sensitive because it considers isolated photon detections. In contrast to the outer-layer veto, its efficiency also improves with increasing distance $d$ from the detector border of the reconstructed vertex.
• Figure 5: Fiducial volume scaling function evaluated at four different photon counts. Top: Overhead view, showing the positions of the IceCube strings and the boundaries of the fiducial volume for events with a given total photon count. Bottom: Side view, showing the modules along strings.