Ab initio calculation of the $β$ decay spectrum of $^6$He

Abstract

We calculate the β spectrum in the decay of 6 He using Quantum Monte Carlo methods with nuclear interactions derived from chiral Effective Field Theory and consistent weak vector and axial currents. We work at second order in the multipole expansion, retaining terms suppressed by O(q 2 /m 2 π ), where q denotes low-energy scales such as the reaction's Q-value or the electron energy, and m π the pion mass. We go beyond the impulse approximation by including the effects of two-body vector and axial currents. We estimate the theoretical error on the spectrum by using four potential models in the Norfolk family of local two-and three-nucleon interactions, which have different cut-off, fit two-nucleon data up to different energies and use different observables to determine the couplings in the three-body force. We find the theoretical uncertainty on the β spectrum, normalized by the total rate, to be well below the permille level, and to receive contributions of comparable size from first and second order corrections in the multipole expansion. We consider corrections to the β decay spectrum induced by beyond-the-Standard Model charged-current interactions in the Standard Model Effective Field Theory, with and without sterile neutrinos, and discuss the sensitivity of the next generation of experiments to these interactions.

The Width of a Beta-decay-induced Antineutrino Wavepacket

“…58. This can presently only be calculated for very light nuclei [35,36] where convergent calculations exist, and this is of only marginal relevance for the problem of antineutrinos from nuclear reactors. Nevertheless, the derivations in this section demonstrate that the relevant scale for localization in the radioactive decay scenario lies in a range between the shortest nucleonnucleon correlated distance to the nuclear diameter, already a specific enough range to make interesting and testable predictions for future reactor experiments.…”

High-precision measurement of the He6 half-life

“…Experiments using trapped 6 He + ions [3] or atoms [4] have been performed to address the β-neutrino angular correlation and ancillary measurements have been carried out to study atomic effects following β decay [5,6]. New techniques have been proposed to determine the Fierz interference term from the β-energy spectrum [7][8][9] and selected observables of the transition have been the object of detailed theoretical studies to assess the sensitivity of future experiments to new physics [10,11].…”