Spurred by the recent complete determination of the weak currents in two-nucleon systems up to { \cal O } ( Q ^ { 3 } ) in heavy-baryon chiral perturbation theory , we carry out a parameter-free calculation of the solar proton fusion rate in an effective field theory that combines the merits of the standard nuclear physics method and systematic chiral expansion .
Using the tritium \beta -decay rate as an input to fix the only unknown parameter in the effective Lagrangian , we can evaluate with drastically improved precision the ratio of the two-body contribution to the well established one-body contribution ; the ratio is determined to be ( 0.86 \pm 0.05 ) \% .
This result is essentially independent of the cutoff parameter for a wide range of its variation ( 500 { \mbox { MeV } } \leq \Lambda \leq 800 { \mbox { MeV } } ) , a feature that substantiates the consistency of the calculation .