Neutrinos limit role of CNO cycle

The Sun burns through various nuclear reactions with the same net effect – the fusion of four protons to form a nucleus of helium, with the release of energy. The two main sets of reactions are the p-p chain, which involves the lightest nuclei, and the CNO cycle, in which the heavier nuclei of carbon, nitrogen and oxygen play a role. Calculations based on the standard solar model suggest that most of the Sun’s energy comes from the p-p chain, with only 1.5% from the CNO cycle. This prediction is widely assumed to be correct, but can it be tested experimentally?

This was indeed an early goal of solar neutrino experiments. However, the solar neutrinos that proved the least difficult to detect have been the high-energy neutrinos from the ⁸B decays in a variant of the p-p chain. The low-energy neutrinos from ¹³N and ¹⁵O decays in the CNO cycle are much more problematic. Moreover, electron-neutrinos emitted by the Sun are now known to escape detection in many experiments through oscillation to another variety. Indeed, before the data from the Sudbury Neutrino Observatory (SNO) and SuperKamiokande became available, calculations in which the CNO cycle contributed as much as 99.95% of the solar energy would fit the data.

Now John Bahcall and Carlos Peña-Garay of the Institute of Advanced Study, Princeton, and M C Gonzalez-Garcia of CERN, SUNY and IFIC Valencia, have revisited the problem using all existing solar neutrino data, including that from SNO and SuperKamiokande, as well as the recent reactor data from KamLAND. Their extensive analysis involves 10 free parameters, which include neutrino oscillation parameters as well as various solar neutrino fluxes – including ¹³N and ¹⁵O decays. Their best fit indicates that the energy from the CNO cycle must be less than 7.3% at the 3ρ level. To improve the limits to the 1.5% level of the solar model predictions will be very challenging, but would, say Bahcall and colleagues, provide a stringent test of the theory of stellar evolution.