Impact of theoretical constraints in the sensitivity estimation for neutrinoless double beta decay

The next generation of neutrinoless double[Formula: see text] decay (0[Formula: see text]) experiments are expected to probe the inverted mass hierarchy (IH) region in the near future, as well as enter into the normal hierarchy (NH) region. It is possible to expect such long-awaited result if the theoretical uncertainties of the axial-vector coupling constant, phase space factor, and particularly the nuclear physics matrix elements are negligible or if the nature follows the most optimistic expectation. Conversely, if the axial-vector coupling constant is maximally quenched, nuclear physics matrix elements follows the lowest values of nuclear models, and the nature follows its most conservative scenario, then even covering the IH region will require tremendous efforts. In this scenario, it requires an exposure of [Formula: see text](1)kton-yr and [Formula: see text](100)Mton-yr to cover the IH and NH, respectively, at the current best achieved background level of [Formula: see text]1 count/FWHM-ton-yr. It is unrealistic and unfeasible to expect the upcoming generation of experiments to be able to withstand such a large and costly exposure. This paper will quantitatively address the implications of these theoretical constraints on experiments and their current expectations in 0[Formula: see text] searches.