Speaker
Description
We study the feasibility of producing monochromatic, low-energy electron antineutrinos via bound-$\beta$ decay of fully stripped radioactive ions and detecting them via resonant antineutrino-induced electron capture on the corresponding isobaric ``mirror'' nucleus. This approach exploits the time-reversed relationship to electron capture and, at resonance, can yield capture cross sections many orders of magnitude larger than conventional (non-resonant) antineutrino detection, e.g.\ inverse-$\beta$ decay. In the source concept, a $\beta$-unstable parent nucleus is fully stripped and stored in a ring. In bound-$\beta$ decay the emitted electron is created directly into a bound orbital of the daughter ion, producing an electron antineutrino with a well-defined energy $E_{\bar{\nu}_e} \simeq Q_{\beta} - B_{\rm src}$ up to small recoil and atomic corrections. Resonant detection is achieved by directing this monochromatic beam onto a neutral target containing the mirror nucleus, where capture occurs when the antineutrino energy in the target rest frame matches the inverse electron-capture resonance; in practice this requires a precisely controlled Doppler shift, i.e. a well-defined source velocity and a narrow beam momentum spread. A promising experimental signature combines a prompt atomic relaxation signal from the capture event (soft X-rays and/or Auger electrons from the created inner-shell vacancy) with a delayed radioactive decay of the produced daughter nucleus. This prompt--delayed coincidence provides strong background rejection. Bound-$\beta$ decay of fully stripped ions has been experimentally demonstrated, establishing the feasibility of the monochromatic source. However, resonant antineutrino capture has not yet been observed. The key challenges for a realistic experiment are achieving sufficiently high stored-ion intensity, deploying a large and isotopically pure target mass, and maintaining the required energy/velocity resolution and long-term stability to remain on resonance. If these technical requirements can be met, the method enables symmetry and new-physics tests in the charged-current sector, including a detailed-balance/time-reversal consistency test (and hence a CPT consistency test under standard assumptions): the bound-$\beta$ decay rate and the inverse resonant capture rate depend on the same nuclear matrix element, so a precise comparison provides a sensitive Standard-Model consistency test and constraints on exotic charged-current interactions.