A measurement of the diffuse astrophysical neutrino spectrum is presented using IceCube data collected from 2011–2022 (10.3 years). We developed novel detection techniques to search for events with a contained vertex and exiting track induced by muon neutrinos undergoing a charged-current interaction. Searching for these starting track events allows us to not only more effectively reject atmospheric muons but also atmospheric neutrino backgrounds in the southern sky, opening a new window to the sub-100 TeV astrophysical neutrino sky. The event selection is constructed using a dynamic starting track veto and machine learning algorithms. We use this data to measure the astrophysical diffuse flux as a single power law flux (SPL) with a best-fit spectral index of $\gamma =2.58_{-0.09}^{+0.10}$ and per-flavor normalization of ${\varphi }_{\mathrm{per}\text{-flavor}}^{\text{Astro}}=1.68_{-0.22}^{+0.19}\times {10}^{-18}\times {\mathrm{GeV}}^{-1}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ (at 100 TeV). The sensitive energy range for this dataset is 3–550 TeV under the SPL assumption. This data was also used to measure the flux under a broken power law, however we did not find any evidence of a low energy cutoff.

• Received 29 February 2024
• Accepted 4 June 2024

DOI:https://doi.org/10.1103/PhysRevD.110.022001