Proton Removal from ²¹F: Probing the Z = 8 Shell Gap and the ⟨F|O+p⟩ overlap using HELIOS

Neutron-rich light nuclei serve as a critical testing ground for modern nuclear structure interactions; however, while neutron single-particle states have been extensively studied via transfer reactions, proton states remain poorly constrained due to the experimental challenges of (d,³He) and (³He,d) reactions. Understanding the evolution of the Z = 8 proton shell gap is essential to benchmark cross-shell interactions, particularly in the p-sd model space where even state-of-the-art shell-model calculations face limitations. Recent knockout studies using (p,2p) and proton-removal reactions such as ⁹Be(²⁵F, ²⁴O)X have revealed a reduced overlap between ground states compared to shell-model predictions, whereas other reactions like ¹²C(²⁴F, ²³O)X show strong agreement with theoretical expectations. These contrasting results raise important questions about the evolution of proton structure and the reliability of model descriptions across isotopic chains. In this context, we performed the first ²¹F(d,³He)²⁰O measurement using the HELIOS spectrometer at Argonne National Laboratory. This study will also shed light on the coupling between the ²⁰O core and the valence proton—helping to determine whether a similar overlap mismatch, as seen in the 〈²⁵F|²⁴O + p〉 system, emerges in lighter nuclei. This presentation will show the preliminary results obtained in the experiment run in march 2025.