Cold Neutron Capture on the Deuteron around Wigner-SU(4) limit in Pionless Effective Field Theory

Radiative capture of cold neutron by deuteron ($nd\to \gamma t$) is interesting because its isospin-mirror process is important in big bang nucleosynthesis and its time reversed process is relevant for experiments at the Triangle Universities Nuclear Laboratory (TUNL)/High Intensity Gamma Source (HIGS). We compute the cross section of this process ($\sigma_{nd\to\gamma t}$) strictly perturbatively in pionless effective field theory (EFT) and expand it around the Wigner-SU(4) (spin-isospin) limit. We show that, although the Wigner-SU(4) symmetric part of the triton wave function dominates the full triton wave function in the zero-recoil limit [1], its contribution to $\sigma_{nd\to\gamma t}$ is suppressed relative to the contribution from energy-dependent three-body forces, which only appears starting at next-to-next-to-leading-order (NNLO) in pionless EFT. I will discuss the expansion around the Wigner-SU(4) limit and its impact on the power counting of this calculation.

[1]Vanasse, J., Phillips, D.R. Three-Nucleon Bound States and the Wigner-SU(4) Limit. Few-Body Syst 58, 26 (2017). https://doi.org/10.1007/s00601-016-1173-2