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 ($\sim 90\%$) 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 $\gamma =2.53\pm 0.07$ and a flux normalization for each neutrino flavor of ${\varphi }_{\text{astro}}=1.66_{-0.27}^{+0.25}$ at $E_0=100\mathrm{TeV}$, 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 $\gamma \le 2.28$ at $\ge 3\sigma$ 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 $\sim 100\mathrm{TeV}$ 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 $\ge 0.06$). The sizable and smooth flux measured below $\sim 100\mathrm{TeV}$ 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.

中文翻译：

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六年期IceCube高能级联数据的弥散天体电子和Tau中微子通量的特征

我们报告了2010-2015年IceCube数据中使用粒子阵雨（级联）对天体中微子通量的首次测量。假设存在标准振荡，则该专用级联样品中的天体中微子占主导地位（$〜90％$）的电子和牛头味。在从16 TeV到2.6 PeV的敏感能量范围内观察到的通量，与单功率定律模型相一致，这是天体物理上高能粒子的费米型加速所期望的。我们发现通量谱指数为$\gamma =2.53\pm 0.07$ 以及每种中微子味的通量归一化 ${\varphi }_{\text{天文}}=1.66_{-0.27}^{+0.25}$ 在 ${Ë}_0=100\mathrm{电视}$，与IceCube的互补性μon中微子结果和全中微子风味拟合结果一致。在测得的能量范围内，我们拒绝光谱指数$\gamma \le 2.28$ 在 $\ge 3\sigma$显着性水平。由于中微子的能量分辨率高且大气中微子的背景低，因此该分析提供了以下能量下中微子通量的最详细表征$〜100\mathrm{电视}$与以前的IceCube结果相比。假设更复杂的中微子通量模型的拟合结果表明，高能量时通量软化，而低能量时通量硬化（$p$ 值 $\ge 0.06$）。下方测得的通量和平滑通量$〜100\mathrm{电视}$仍然是一个难题。为了不违反费米大面积望远镜测量的各向同性扩散伽马射线背景，建议存在天文学的中微子源，其特点是对伽玛射线不透明的密集环境。