About the parity-non-conserving asymmetry in n+p--->d+gamma

Abstract

The parity-non-conserving (pnc) asymmetry in \vec{n} + p \to d+γat thermal energies has recently been calculated using effective field-theory methods. A comparison of this calculation with much more elaborate calculations performed in the 70's is made. This allows one to assess the validity of this new approach as presently used. It is found to overshoot the almost exact calculations by a factor close to 2 for the contribution involving the ^3S_1 component of both the initial and final states. This is much larger than anticipated by the authors. This discrepancy is analyzed and found to originate from the over-simplified description of the deuteron and capture states which underlies the new approach. The claim that earlier determinations of the sign would be in error is also examined. It is found that the sign discrepancy is most probably due, instead, to the fact that the pion-nucleon interaction referred to by the authors corresponds to a parity-non-conserving potential with a sign opposite to what is currently used. Some estimates and constraints relative to the pnc piNN coupling, h^1_π, which the above asymmetry is dependent on, are reviewed. Further details are given in an Appendix.