Improved measurement of ${}^8$B solar neutrinos with 1.5 kt y of Borexino exposure

We report on an improved measurement of the ${}^8$B solar neutrino interaction rate with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. Neutrinos are detected via their elastic scattering on electrons in a large volume of liquid scintillator. The measured rate of scattered electrons above 3 MeV of energy is $0.223\substack{+0.015 \ -0.016},(stat),\substack{+0.006 \ -0.006},(syst)$ cpd/100 t, which corresponds to an observed solar neutrino flux assuming no neutrino flavor conversion of $\Phi\substack{\rm ES \ ^8\rm B}=2.57\substack{+0.17 \ -0.18}(stat)\substack{+0.07\ -0.07}(syst)\times$10$^6$cm$^{-2},$s$^{-1}$. This measurement exploits the active volume of the detector in almost its entirety for the first time, and takes advantage of a reduced radioactive background following the 2011 scintillator purification campaign and of novel analysis tools providing a more precise modeling of the background. Additionally, we set a new limit on the interaction rate of solar $hep$ neutrinos, searched via their elastic scattering on electrons as well as their neutral current-mediated inelastic scattering on carbon, ${}^{12}$C($\nu ,{\nu }^{\mathrm{\prime }}$)${}^{12}$C* ($E_{\gamma }$= 15.1 MeV).