First direct determination of the 48Ca double-β decay Q value

The low-energy beam and ion trap Penning trap mass spectrometer was used for an improved determination of the Ca-48 double-beta decay Q value: Q(beta beta) = 4268.121(79) keV. The new value is 1.2 keV greater than the value in the 2012 atomic mass evaluation [Chin. Phys. C 36, 1603 (2012)], a shift of three sigma, and is a factor of 5 more precise. Accurate knowledge of this Q value is important for experimental searches to observe neutrinoless double-beta decay (0 nu(beta beta)) in Ca-48 and is essential for extracting the effective mass of the electron neutrino if the Ca-48 half-life of 0 nu(beta beta) was experimentally determined.