  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 $\gamma = 2.58 ^{+0.10}_{-0.09}$ and per-flavor normalization
of $\phi^{\mathrm{Astro}}_{\mathrm{per-flavor}} = 1.68 ^{+0.19}_{-0.22} \times
10^{-18} \times \mathrm{GeV}^{-1} \mathrm{cm}^{-2} \mathrm{s}^{-1}
\mathrm{sr}^{-1}$ (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.
