Narrow Results

We review the first results from the Laboratory for Experimental Nuclear Astrophysics (LENA), which is a dedicated accelerator facility for measuring reactions of astrophysical interest. We also briefly describe the facility itself and the detector system. The reactions that have been measured have relevance for both stellar evolution and for classical nova explosions.

Borexino measured with unprecedented accuracy the fluxes of solar neutrinos emitted at all the steps of the pp fusion chain. Still missing is the measurement of the flux of neutrinos produced in the CNO cycle. A positive measurement of the CNO neutrino flux is of fundamental importance for understanding the evolution of stars and addressing the unresolved controversy on the solar abundances. The measurement of the CNO neutrino flux in Borexino is challenging because of the low intensity of this component (CNO cycle accounts for about 1% of the energy emitted by Sun), the lack of prominent spectral features and the presence of background sources. The main background component is ²¹⁰Bi decaying in the liquid scintillator of Borexino that creates events with an energy distribution very close to the one of CNO neutrino interactions. Since 2015 the collaboration undertook significant efforts to achieve an independent measurement of the background affecting a CNO measurement, whose impact on the sensitivity to a CNO signal will be discussed.

The Borexino has recently reported the first experimental evidence of neutrinos from the CNO cycle. Since this process accounts only for about 1% of the total energy production in the Sun, the associated neutrino flux is extremely low as compared with the one from the pp-chain, the dominant process of hydrogen burning. This experimental evidence of the CNO neutrinos was obtained using the highly radio-pure liquid scintillator of Borexino. Improvements in the thermal stabilization of the detector over the last five years enabled us to exploit a method to constrain the rate of ²¹⁰Bi background. Since the CNO cycle is dominant in massive stars, this result gives the first experimental proof of the primary mechanism for stellar conversion of hydrogen into helium in the Universe.

Borexino has recently observed CNO solar neutrinos. This measurement confirms an energy production mechanism in stars predicted about a century ago. The CNO cycle in the Sun is sub-dominant with respect to the pp-chain energy production. However, it is definitely important in more massive stars. We describe the main characteristics of the CNO cycle in the Sun and in massive stars.