Parameter-free effective field theory calculation for the solar proton-fusion and hep processes

Abstract

Spurred by the recent complete determination of the weak currents in two-nucleon systems up to $\mathcal{O}(Q^3)$ in heavy-baryon chiral perturbation theory, we carry out a parameter-free calculation of the threshold $S$-factors for the solar $pp$ (proton-fusion) and $hep$ processes in an effective field theory that {\it combines} the merits of the standard nuclear physics method and systematic chiral expansion. The power of the EFT adopted here is that one can correlate in a unified formalism the weak-current matrix elements of two-, three- and four-nucleon systems. Using the tritium ${\hat{I}}^2$-decay rate as an input to fix the only unknown parameter in the theory, we can evaluate the threshold $S$ factors with drastically improved precision; the results are $S_{pp}(0)=3.94\times (1\pm 0.004)\times 10^{-25}MeV-b$ and $S_{hep}(0)=(8.6\pm 1.3)\times 10^{-20}keV-b$. The dependence of the calculated $S$-factors on the momentum cutoff parameter $\hat{I}›$ has been examined for a physically reasonable range of $\hat{I}›$. This dependence is found to be extremely small for the $pp$ process, and to be within acceptable levels for the $hep$ process, substantiating the consistency of our calculational scheme.

22 pages, 4 eps figures, ReVTeX