First Detection of Solar Neutrinos from the CNO Cycle with Borexino

Abstract: Stars are fuelled by the fusion of hydrogen into helium via two processes: the ‘‘pp chain’’ and the ‘‘CNO cycle.’’ Neutrinos emitted in the core of the Sun are the only direct probe for the investigation of such processes. The Borexino experiment has performed a complete spectroscopy of the neutrinos from the ‘‘pp chain,’’ and has recently reported the first experimental evidence of neutrinos from the CNO cycle, never observed before, being this process subdominant in the Sun (about 1% of the solar energy). This experimental evidence of the CNO neutrinos was obtained using the highly radiopure large-volume liquid-scintillator detector of Borexino. Advances in the thermal stabilization of the detector over the last five years enabled us to exploit a method to constrain the rate of Bi-210 contaminating the scintillator, the main background for such a measurement. Since the CNO cycle is dominant in massive starts, this result proves the evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe.