Constraining the axial-vector X17 interpretation with ¹²C data

Recent findings of an unexpected, narrow resonance in the e⁺e⁻ decay spectra of excited states of ⁸Be, ⁴He and ¹²C by the ATOMKI collaboration have received considerable experimental and theoretical attention, whereby a new, 17-MeV vector-like or axial-vector-like boson termed X17 was conjectured as an explanation of the anomaly. Further analysis of all existing constraints disfavors a vector X17 scenario. For a similar analysis of the axial-vector scenario, a calculation of the reduced matrix element of a spin-dipole operator between the excited nuclear state ¹²C(17.23) and the carbon ground state is required. In the present work, we compute the aforementioned reduced matrix element under the assumption that the state ¹²C(17.23) is well represented by the 2s1/21p⁻¹3/2 particle-hole shell-model excitation of the ground state, as supported by experimental data. Within such a framework, our results indicate that, like the vector scenario, the axial-vector interpretation of X17 shows strong tensions with the other existing constraints on the nucleon coupling of a conjectured X17.