  We observed, for the first time, solar neutrinos in the 1.0-1.5 MeV energy
range. We measured the rate of pep solar neutrino interactions in Borexino to
be [3.1+-0.6(stat)+-0.3(syst)] counts/(day x 100 ton) and provided a constraint
on the CNO solar neutrino interaction rate of <7.9 counts/(day x 100 ton) (95%
C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L.,
while the absence of the pep signal is disfavored at 98% C.L. This
unprecedented sensitivity was achieved by adopting novel data analysis
techniques for the rejection of cosmogenic 11C, the dominant background in the
1-2 MeV region. Assuming the MSW-LMA solution to solar neutrino oscillations,
these values correspond to solar neutrino fluxes of [1.6+-0.3]x10^8 cm^-2s-1
and 7.7x10^8 cm^-2s-1 (95% C.L.), respectively, in agreement with the Standard
Solar Model. These results represent the first measurement of the pep neutrino
flux and the strongest constraint of the CNO solar neutrino flux to date.
