On the Extraction of Low-energy Constants of Single- and Double-$\beta$ Decays from Lattice QCD: A Sensitivity Analysis

Lattice quantum chromodynamics (LQCD) has the promise of constraining low-energy constants (LECs) of nuclear effective field theories (EFTs) from first-principles calculations that incorporate the dynamics of quarks and gluons. Given the Euclidean and finite-volume nature of LQCD outputs, complex mappings are developed in recent years to obtain the Minkowski and infinite-volume counterparts of LQCD observables. In particular, as LQCD is moving toward computing a set of important few-nucleon matrix elements at the physical values of the quark masses, it is important to investigate whether the anticipated precision of LQCD spectra and matrix elements will be sufficient to guarantee tighter constraints on the relevant LECs than those already obtained from phenomenology, considering the non-trivial mappings involved. With a focus on the leading-order LECs of the pionless EFT, $L_{1,A}$ and $g_{\nu}^{NN}$, which parametrize, respectively, the strength of the isovector axial two-body current in a single-$\beta$ decay (and other related processes such $pp$ fusion), and of the isotensor contact two-body operator in the neutrinoless double-$\beta$ decay within the light neutrino exchange scenario, the expected uncertainty on future extractions of $L_{1,A}$ and $g_{\nu}^{NN}$ are examined using synthetic data at the physical values of the quark masses. It is observed that achieving small uncertainties in $L_{1,A}$ will be challenging, and (sub)percent-level precision in the two-nucleon spectra and matrix elements is essential in reducing the uncertainty on this LEC compared to the existing constraints. On the other hand, the short-distance coupling of the neutrinoless double-$\beta$ decay, $g_{\nu}^{NN}$, is shown to be less sensitive to uncertainties on both LQCD energies and the matrix element, and can likely be constrained with percent-level precision in the upcoming LQCD calculations.