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

TL;DR

This paper investigates how hypothetical non-standard mediators could influence the proton-proton chain reaction in the Sun, using solar evolution models to constrain their properties based on solar observations and neutrino data.

Contribution

It introduces a method to constrain non-standard light mediators affecting solar nuclear reactions by comparing solar evolution models with observational data.

Findings

Constraints on non-standard mediator mass and couplings that are inconsistent with solar observations.

Degeneracy identified between solar metallicity and non-standard mediator effects.

Potential for tighter constraints with improved measurements of solar metallicity and neutrino flux.

Abstract

The $pp$-chain of nuclear reactions is the primary route for energy production in the Sun. The first step in that reaction sequence converts two protons to a deuterium nucleus with the emission of a positron and electron neutrino. This reaction is extremely slow because it is a weak interaction, and significantly, it involves quantum tunneling through the Coulomb barrier. Though the reaction rate can be calculated with high confidence in the Standard Model, it has not been measured at solar energies. If there exist interactions that are engendered by non-standard mediators then the rate of this reaction in the Sun could be altered. We probe such non-standard interactions by comparing calculations of solar evolution to the current solar system age in the presence and absence of the non-standard mediators. These reveal ranges of non-standard mediator mass and couplings that are inconsistent with measured properties of the Sun, including solar neutrino results. Our constraints on these non-standard parameters, in many cases overlapping those derived via other considerations, could be extended further with better confidence in the value of the metalicity of the Sun and the solar neutrino CNO flux. Intriguingly, our work reveals a degeneracy between the solar metalicity and the presence of the invoked non-standard mediators.