High-precision $Q$-value measurement confirms the potential of ${}^{135}$Cs for antineutrino-mass detection

Abstract

The ground-state-to-ground-state $\beta$-decay $Q$-value of ${}^{135}\text{Cs}(7/2^{+})\to {}^{135}\text{Ba}(3/2^{+})$ was directly measured for the first time utilizing the Phase-Imaging Ion-Cyclotron Resonance (PI-ICR) technique at the JYFLTRAP Penning-trap setup. It is the first direct determination of this $Q$-value and its value of 268.66(30)\,keV is a factor of three more precise than the currently adopted $Q$-value in the Atomic Mass Evaluation 2016. Moreover, the $Q$-value deduced from the $\beta$-decay endpoint energy has been found to deviate from our result by approximately 6 standard deviations. The measurement confirms that the first-forbidden unique ${\beta }^{-}$-decay transition ${}^{135}\text{Cs}(7/2^{+})\to {}^{135}\text{Ba}(11/2^{-})$ is a candidate for antineutrino-mass measurements with an ultra-low $Q$-value of $0.44(31)$ keV. This $Q$-value is almost an order of magnitude smaller than in any presently running or planned direct (anti)neutrino-mass experiment.

5 pages, 6 figures, 2 tables Submitted for publication to Physical Review Letters (PRL)