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The $pp$-chain of nuclear reactions is the primary route for energyproduction in the Sun. The first step in that reaction sequence converts twoprotons to a deuterium nucleus with the emission of a positron and electronneutrino. This reaction is extremely slow because it is a weak interaction, andsignificantly, it involves quantum tunneling through the Coulomb barrier.Though the reaction rate can be calculated with high confidence in the StandardModel, it has not been measured at solar energies. If there exist interactionsthat are engendered by non-standard mediators then the rate of this reaction inthe Sun could be altered. We probe such non-standard interactions by comparingcalculations of solar evolution to the current solar system age in the presenceand absence of the non-standard mediators. These reveal ranges of non-standardmediator mass and couplings that are inconsistent with measured properties ofthe Sun, including solar neutrino results. Our constraints on thesenon-standard parameters, in many cases overlapping those derived via otherconsiderations, could be extended further with better confidence in the valueof the metalicity of the Sun and the solar neutrino CNO flux. Intriguingly, ourwork reveals a degeneracy between the solar metalicity and the presence of theinvoked non-standard mediators.

A closer look at the $pp$-chain reaction in the Sun: Constraining the coupling of light mediators to protons