Gaussian process error modeling for chiral effective-field-theory
  calculations of $np\leftrightarrow d\gamma$ at low energies
  We calculate the energy-dependent cross section of the $np\leftrightarrow
d\gamma$ process in chiral effective field theory and apply state-of-the-art
tools for quantification of theory uncertainty. We focus on the low-energy
regime, where the magnetic dipole and the electric dipole transitions cross
over, including the range relevant for big-bang nucleosynthesis. Working with
the leading one- and two-body electromagnetic currents, we study the
order-by-order convergence of this observable in the chiral expansion of the
nuclear potential. We find that the Gaussian process error model describes the
observed convergence very well, allowing us to present Bayesian credible
intervals for the truncation error with correlations between the cross sections
at different energies taken into account. We obtain a 1$\sigma$ estimate of
about 0.2\% for the uncertainty from the truncation of the nuclear potential.
This is an important step towards calculations with statistically interpretable
uncertainties for astrophysical reactions involving light nuclei.
