β-delayed proton emission from ¹¹Be in effective field theory

Abstract. We calculate the rate of the rare decay branch $^{11}\text{Be} \to {}^{10}\text{Be} + p + e^- + \bar{\nu}_e$ using Halo Effective Field Theory (Halo EFT). This work demonstrates that the experimental branching ratio is consistent with Standard Model predictions without requiring new physics or dark matter decay channels.

In this study, we addressed the anomaly of calculating the β-delayed proton emission from the neutron-rich halo nucleus $^{11}\text{Be}$. This rare decay mode is of significant interest because it involves the emission of a proton from a neutron-rich nucleus, a process that is energetically forbidden for the daughter nucleus $^{11}\text{B}$ in its ground state but becomes possible through a transition to a continuum state.

Using Halo Effective Field Theory (Halo EFT), we constructed a model-independent framework to describe the relevant degrees of freedom. The calculation focuses on the beta decay of the $^{11}\text{Be}$ ground state into the continuum of the $^{10}\text{Be} + p$ system.

Key findings

Our theoretical results for the branching ratio are in good agreement with the experimental value measured by the ISOLDE collaboration at CERN. This agreement suggests that the previously observed high rate of proton emission can be explained by a resonance in the $^{11}\text{B}$ continuum just above the proton separation threshold, rather than exotic mechanisms like neutron dark matter decay.

This work underscores the power of effective field theories in describing loosely bound nuclear systems (halo nuclei) and provides a rigorous, uncertainty-quantified theoretical baseline for interpreting rare decay experiments.

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