Quantifying uncertainties due to irreducible three-body forces in deuteron-nucleus reactions

\noindent{\bf Background:} Deuteron-induced nuclear reactions are an essential tool for probing the structure of nuclei as well as astrophysical information such as $(n,\gamma)$ cross sections. The deuteron-nucleus system is typically described within a Faddeev three-body model consisting of a neutron ($n$), a proton ($p$), and the target nucleus ($A$) interacting through pairwise phenomenological potentials. While Faddeev techniques enable the exact description of the three-body dynamics, their predictive power is limited in part by the omission of irreducible neutron-proton-nucleus three-body force ($n$-$p$-$A$ 3BF). {\bf Results:} By comparing the Faddeev and NCSM/RGM results, we show that the irreducible $n$-$p$-$\alpha$ 3BF has a non-negligible effect on bound state and scattering observables alike. Specifically, the Faddeev approach %are yields a $^6$Li ground state that is approximately $600$~keV shallower than the one obtained with the NCSM/RGM. Additionally, the Faddeev calculations for $d$+$\alpha$ scattering yield a $3^+$ resonance that is located approximately $400$~keV higher in energy compared to the NCSM/RGM result. The shape of the $d$+$\alpha$ angular distributions computed using the two approaches also differ, owing to the discrepancy in the predictions of the $3^+$ resonance energy.