First direct Be7 electron-capture Q -value measurement toward high-precision searches for neutrino physics beyond the Standard Model

We report the first direct measurement of the nuclear electron-capture (EC) decay Q value of ⁷Be→⁷Li via high-precision Penning trap mass spectrometry (PTMS). This was performed using the LEBIT Penning trap located at the National Superconducting Cyclotron Laboratory/Facility for Rare Isotope Beams (NSCL/FRIB) using the newly commissioned Batch-Mode Ion-Source (BMIS) to deliver the unstable ⁷Be⁺ samples. With a measured value of QEC=861.963(23) keV, this result is three times more precise than any previous determination of this quantity. This improved precision and accuracy of the ⁷Be EC decay Q value is critical for ongoing experiments that measure the recoiling nucleus in this system as a signature to search for beyond the Standard Model (BSM) neutrino physics using ⁷Be-doped superconducting sensors. This experiment has extended LEBIT capabilities, using the first low-energy beam delivered by BMIS at FRIB for PTMS, as well as measuring the lightest-mass isotopes so far with LEBIT.