A near-threshold proton resonance in ${}^{11}\mathrm{B}$ at $E_{\mathrm{ex}}=11.44\pm 0.04\mathrm{MeV}$ is observed via the reaction ${}^{10}\mathrm{Be}(d,n){}^{11}\mathrm{B}\to {}^{10}\mathrm{Be}+p$ in inverse kinematics, measured with a beam of the radioactive isotope ${}^{10}\mathrm{Be}$. The resonance energy at $E_{\mathrm{res}}=211(40)\mathrm{keV}$ is consistent with a proton signal observed by Ayyad et al. in the $\beta$-delayed proton decay of ${}^{11}\mathrm{Be}$. By comparison to a distorted wave Born approximation calculation, a 0.27(6) spectroscopic factor is extracted and a tentative ($\ell =0$) character is assigned for this resonance. The significant cross section in the proton-transfer $(d,n)$ reaction, as well as the observation of its proton-decay signal, point to the threshold-resonance character of this state. The position of this state, its structure, and strong coupling to the $s$-wave continuum represent an ideal case to study quantum near-threshold many-body dynamics of unstable states. The presence of this state is an important step toward understanding the excessively large beta-delayed proton-decay branch of ${}^{11}\mathrm{Be}$.

• Received 7 February 2022
• Revised 26 April 2022
• Accepted 1 June 2022
• Corrected 16 November 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.012502