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Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand.
DESY, D-15738 Zeuthen, Germany.
Phys Rev Lett. 2020 Sep 18;125(12):121104. doi: 10.1103/PhysRevLett.125.121104.
We report on the first measurement of the astrophysical neutrino flux using particle showers (cascades) in IceCube data from 2010-2015. Assuming standard oscillations, the astrophysical neutrinos in this dedicated cascade sample are dominated (∼90%) by electron and tau flavors. The flux, observed in the sensitive energy range from 16 TeV to 2.6 PeV, is consistent with a single power-law model as expected from Fermi-type acceleration of high energy particles at astrophysical sources. We find the flux spectral index to be γ=2.53±0.07 and a flux normalization for each neutrino flavor of ϕ_{astro}=1.66_{-0.27}^{+0.25} at E_{0}=100 TeV, in agreement with IceCube's complementary muon neutrino results and with all-neutrino flavor fit results. In the measured energy range we reject spectral indices γ≤2.28 at ≥3σ significance level. Because of high neutrino energy resolution and low atmospheric neutrino backgrounds, this analysis provides the most detailed characterization of the neutrino flux at energies below ∼100 TeV compared to previous IceCube results. Results from fits assuming more complex neutrino flux models suggest a flux softening at high energies and a flux hardening at low energies (p value ≥0.06). The sizable and smooth flux measured below ∼100 TeV remains a puzzle. In order to not violate the isotropic diffuse gamma-ray background as measured by the Fermi Large Area Telescope, it suggests the existence of astrophysical neutrino sources characterized by dense environments which are opaque to gamma rays.
我们报告了利用2010 - 2015年冰立方数据中的粒子簇射（级联）对天体物理中微子通量进行的首次测量。假设标准振荡，在这个专门的级联样本中的天体物理中微子主要由电子和τ子味主导（约90%）。在16 TeV至2.6 PeV的灵敏能量范围内观测到的通量与单个幂律模型一致，这正如天体物理源处高能粒子的费米型加速所预期的那样。我们发现通量谱指数为γ = 2.53±0.07，在E₀ = 100 TeV时，每种中微子味的通量归一化值为ϕ₍astro₎ = 1.66₋₀.₂₇⁺₀.₂₅，这与冰立方的互补μ子中微子结果以及所有中微子味拟合结果一致。在测量的能量范围内，我们在≥3σ显著性水平下拒绝谱指数γ≤2.28。由于中微子能量分辨率高且大气中微子背景低，与之前冰立方的结果相比，该分析提供了能量低于约100 TeV时中微子通量最详细的特征描述。假设更复杂中微子通量模型的拟合结果表明，在高能时通量变软，在低能时通量变硬（p值≥0.06）。在约100 TeV以下测量到的可观且平滑的通量仍然是一个谜题。为了不违反费米大面积望远镜测量的各向同性弥漫伽马射线背景，这表明存在以对伽马射线不透明的致密环境为特征的天体物理中微子源。
