Cosmic neutrino background absorption line in the neutrino spectrum at IceCube

The IceCube experiment has recently reported a high energy neutrino spectrum between the TeV and PeV scales. The observed neutrino flux can be as a whole well fitted by a simple power law of the neutrino energy $E_{\nu }$, $E_{\nu }^{-{\gamma }_{\nu }}$ (${\gamma }_{\nu }\simeq 2$). As a notable feature of the spectrum, however, it has a gap between 500 TeV and 1 PeV. Although the existence of the gap in the neutrino spectrum is not statistically significant at this point, it is very enticing to ask whether it might hint at some physics beyond the Standard Model. In this paper, we investigate a possibility that the gap can be interpreted as an absorption line in the power-law spectrum by the cosmic neutrino background through a new resonance in the MeV range. We also show that the absorption line has rich information about not only the MeV scale new particle but also the neutrino masses as well as the distances to the astrophysical sources of the high energy neutrinos. Viable models to achieve this possibility are also discussed.

Figure 1

Left panel: The neutrino-(anti)neutrino scattering cross sections for the center of mass energy $\sqrt{S}$. The black solid line is the SM. The red solid, the red dashed, the red dotted, the blue dashed, and the green dotted lines depict the contributions from the interaction of Eq. (1) for several parameter samples of the coupling $g$ and the scalar particle mass. Right panel: The neutrino mean free path $\lambda$ as a function of the energy of the neutrino flux.

Figure 2

The neutrino mean free path for various neutrino masses. The numbers shown in the boxes are the mean free path in the unit Mpc for each coupling. The scalar boson mass is set to be $M_s=1\mathrm{MeV}$, 2 MeV, 3 MeV, and 4 MeV in the upper left, upper right, bottom left, and bottom right panels, respectively.

Figure 3

Absorption line with the sample parameters assuming the source distance to be $O(1)\mathrm{Gpc}$. Left panel: The neutrino mass dependences. The black dots with error bars are observed data, and the best-fit power law is $E_{\nu }^2{\mathrm{\Phi }}_{\nu }(E_{\nu })=1.5\times 10^{-8}(E_{\nu }/100\mathrm{TeV}{)}^{-0.3}\mathrm{GeV}/{\mathrm{cm}}^2/\mathrm{s}/\mathrm{sr}$ [2]. Right panel: The dependence on the resonance mass. In both figures, we assumed $g\equiv g_{11}^{\mathrm{eff}}=O(10^{-3})$.

Figure 4

Absorption line with the sample parameters assuming the source distance to be $O(10)\mathrm{kpc}$. Here, we have taken $g_{\tau \tau }^{\mathrm{eff}}=0.5$.