Does the Sun Shine by $pp$ or CNO Fusion Reactions?

We show that solar neutrino experiments set an upper limit of 7.8% (7.3% including the recent KamLAND measurements) to the fraction of energy that the Sun produces via the CNO fusion cycle, which is an order of magnitude improvement upon the previous limit. New experiments are required to detect CNO neutrinos corresponding to the 1.5% of the solar luminosity that the standard solar model predicts is generated by the CNO cycle.

Figure 1

Experimental bound on $L_{\mathrm{C}\mathrm{N}\mathrm{O}}/L_{\odot }$. The figure shows how the ${\chi }^2$ fit worsens as one increases the assumed fraction of the Sun’s luminosity that arises from CNO reactions. The dotted lines were computed using just solar neutrino data; the solid lines use both solar neutrino experiments and the KamLAND results.The curves labeled LMA were calculated for the favored large mixing angle MSW solution; the curves labeled non-LMA were calculated for the best fit of the LOW, SMA, vacuum, and sterile oscillation solutions. The arrows indicate the predicted 1.5% CNO luminosity from the standard solar model [3] and the $\sim 8\%$ [see Eq. (4)] $3\sigma$ upper bound (1 dof) allowed by the chlorine, gallium, Super-Kamiokande, and SNO solar neutrino data [5, 8, 9, 10, 13, 14] and the KamLAND reactor data [18].