The proton-proton (pp) chain and the carbon-nitrogen-oxygen (CNO) fusion cycle process are two processes understood theoretically how stars converted hydrogen to helium. Neutrinos emitted in such processes are the only direct probe of the deep interior of the Sun. Borexino is a liquid scintillator detector targeted at measuring solar neutrinos. It is hosted by the underground INFN Laboratori Nazionali del Gran Sasso in L’Aquila, Italy and has achieved unprecedented radio purity of liquid scintillator. Since 2015, the Borexino has made a major effort in stabilizing the thermal condition of the detector aimed at measuring the CNO solar neutrinos, including installation of a passive thermal insulation system and an active temperature control system. It was observed that the movement of the Po-210 backgrounds was significantly suppressed and a low rate region in the center was stably present since 2016. After the Po-210 low-rate-region tracking method was improved, an upper limit of the Bi-210, the critical background of the CNO neutrino signal, was determined. Last fall, Borexino published in Nature the results excluding the absence of the CNO neutrino signal with a significance of five sigmas. The results quantify the relative contribution of CNO fusion in the Sun to be of the order of one percent. It is also the first experimental evidence of the CNO cycle process, the primary mechanism for the stellar conversion of hydrogen in the Universe. Additionally, solar models built from heliuseismoeogy (SSM-HZ) and from spectroscopy (SSM-LZ) predicted inconsistent solar metallicities, known as the “solar metallicity problem”. The CNO neutrino fluxes depend on the carbon and nitrogen abundances. So this work paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. The details of the detector stabilization, the strategy to track the Po-210 low-rate-region, and the strategy to break the correlation between CNO neutrino signals and its backgrounds will be presented.
|First author||XueFeng Ding|
|Collaboration / Activity||Borexino|