  We report the measurement of sub-MeV solar neutrinos through the use of their
associated Cherenkov radiation, performed with the Borexino detector at the
Laboratori Nazionali del Gran Sasso. The measurement is achieved using a novel
technique that correlates individual photon hits of events to the known
position of the Sun. In an energy window between 0.54 MeV to 0.74 MeV, selected
using the dominant scintillation light, we have measured 10887$^{+2386}_{-2103}
(\mathrm{stat.})\pm 947 (\mathrm{syst.})$ ($68\%$ confidence interval) solar
neutrinos out of 19904 total events. This corresponds to a $^{7}$Be neutrino
interaction rate of 51.6$^{+13.9}_{-12.5}$ counts/(day$\cdot$ 100 ton), which
is in agreement with the Standard Solar Model predictions and the previous
spectroscopic results of Borexino. The no-neutrino hypothesis can be excluded
with $>$5$\sigma$ confidence level. For the first time, we have demonstrated
the possibility of utilizing the directional Cherenkov information for sub-MeV
solar neutrinos, in a large-scale, high light yield liquid scintillator
detector. This measurement provides an experimental proof of principle for
future hybrid event reconstruction using both Cherenkov and scintillation
signatures simultaneously.
