Sensitivity of the NEXT experiment to Xe-124 double electron capture

A bstract Double electron capture by proton-rich nuclei is a second-order nuclear process analogous to double beta decay. Despite their similarities, the decay signature quite different, potentially providing new channel measure hypothesized neutrinoless mode of these decays. The Standard-Model-allowed two-neutrino (2 νEC EC ) has been predicted for number isotopes, but only observed in 78 Kr, 130 Ba and, recently, 124 Xe. sensitivity this establishes benchmark ultimate experimental goal, namely potential discover also lepton-number-violating version process, 0 . Here we report on current NEXT-White detector Xe 2 and extrapolation NEXT-100. Using simulated data signal real from operated with Xe-depleted gas as background, define an optimal event selection that maximizes sensitivity. We estimate that, NEXT-100 xenon isotopically enriched 1 kg 5-year run, half-life 6 × 10 22 y (at 90% confidence level) or better can be reached.