Astrophysicists at the Borexino experiment in Italy have recently claimed that they have detected CNO-cycle neutrinos coming from the Sun.

It was the Cover story for the November 26 issue of Nature.

I read the whole Paper, and accompanying editorial, yet unless I missed something, they do not say what makes a CNO-neutrino different from an ordinary pp-neutrino.

They go to great lengths to describe how they excluded all types of background noise, but did not say whether CNO-neutrinos are higher or lower in energy than 'proton-proton chain' neutrinos....

Presumably, CNO neutrinos are higher in energy than pp ones? How many eV or keV?


1 Answer 1


Nuclear reactions can have particular energy spectra, which arise based on how energy and momentum are distributed among the product particles. This holds true for neutrinos produced in the many different stages of the CNO cycle, as well as the pp chain. The authors began the paper with a figure showing the theoretical neutrino fluxes for the various components of the CNO cycle:

Diagram showing neutrino energy spectra for pp chain and CNO cycle

The dominant CNO neutrino producers are the steps involving $^{13}\mathrm{N}$ and $^{15}\mathrm{O}$. The peak of the nitrogen spectrum lies in (or at the edge of) a region where the ratio of CNO neutrinos to background is the highest:

Neutrino spectra

Fitting the observed spectra with models of CNO neutrino emission indicated that such neutrinos were indeed present. I'd draw an analogy with making electromagnetic observations: if your statistics are significantly better when, say, you add a black body component to a non-thermal power law fit, then you have evidence of black body emission. It's basically the same thing here - adding another component to a spectral fit.

  • $\begingroup$ What is 5N_h? 'Events per 5Nh'? Was it explained in the Paper? I hope I didn't lose that issue.... $\endgroup$
    – Kurt Hikes
    Feb 24, 2021 at 0:55

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