Speaker
Description
The Sun offers us the opportunity to make detailed observations of the exterior and interior of a typical star in its mid-life phase. A unique way to study the innermost regions of our star is offered by solar neutrinos, emitted by the thermonuclear fusion reactions in the Sun. The Borexino experiment, located in Laboratori Nazionali del GranSasso in Italy and widely known for its rich Solar Neutrino physics program, is now in his high-purity Phase II, thanks to intense purification campaigns of scintillator that were very successful in further reducing the already low backgrounds, leading to unprecedented radiopurity levels reached in its active volume (<10^19 g/g for 238U/232Th chains).
In this talk I will review the latest result released, the first simultaneous measurement of the neutrino interaction rates from the proton-proton chain, and I will present the strategies for measuring the neutrino flux from the CNO cycle, the last one not yet measured. The detection of these neutrinos has important implications in astrophysics, as it would be the first direct evidence of the nuclear process that is believed to fuel massive stars.
Despite the unprecedented low levels of background reached in the detector, the CNO solar neutrinos detection remains a very challenging task because of the almost degenerate spectral shapes of the signal due to CNO neutrinos and the 210Bi background, so that spectral fits are not able to disentangle the two contributions. A realistic measurement of CNO neutrinos can be performed only using a multivariate spectral fit supported by a sensitive pulse shape discrimination. The 210Bi background can be reduced by further purifications of the scintillator, but anyway it must be also constrained independently by measuring the rate of decay of the daughter nucleus, the 210Po. The difficulty of this analysis lies both in the low signal to noise ratio and in the contamination of the 210Po in the vessel, which diffuses towards the center of the detector. Such transport was strongly suppressed after the detector was equipped with thermal insulation yet this effect remains non-negligible.