An effective field theory technique that combines the standard nuclear physics approach and chiral perturbation theory is applied to the hen process , ^ { 3 } \mbox { He } + n \rightarrow { } ^ { 4 } \mbox { He } + \gamma .
For the initial and final nuclear states , high-precision wave functions are generated via the variational Monte Carlo method using the Argonne v _ { 14 } potential and Urbana VIII trinucleon interactions , while the relevant transition operators are calculated up to { \cal O } ( Q ^ { 4 } ) in HB \chi PT .
The imposition of the renormalization condition that the magnetic moments of ^ { 3 } \mbox { He } and ^ { 3 } \mbox { H } be reproduced allows us to carry out a parameter-free calculation of the hen cross section .
The result , \sigma = ( 60 \pm 3 \pm 1 ) ~ { } \mu b , is in reasonable agreement with the experimental values , ( 54 \pm 6 ) ~ { } \mu b and ( 55 \pm 3 ) ~ { } \mu b .
This agreement demonstrates the validity of the calculational method previously used for estimating the reaction rate of the solar hep process .