Determine the vector vs axial-vector nature of X17

Determine whether the hypothetical X17 particle is of vector (J^π = 1^−) or axial-vector (J^π = 1^+) type by establishing the orbital angular momentum (L = 0 or L = 1) with which it exits the nucleus in the nuclear transitions where anomalous e^+e^− pair angular correlations have been observed, specifically the 18.15 MeV 1^+ → 0^+ transition in 8Be and the 1^− → 0^+ transition in 12C. Ascertain this by measuring the angular distribution of the X17 emission relative to the reaction axis to discriminate isotropic (L = 0) from anisotropic (L = 1) behavior.

Background

The paper reports repeated observations of anomalies in e^+e^− angular correlations consistent with a new boson of mass ~17 MeV (X17) in several nuclear transitions, including 8Be and 12C. Based on spin-parity considerations of the initial and final nuclear states, both vector (J^π = 1^−) and axial-vector (J^π = 1^+) assignments for X17 are kinematically possible depending on whether the particle exits the nucleus with L = 0 or L = 1 orbital angular momentum.

The authors explicitly state that they currently lack experimental information on the orbital momentum of X17 emission, preventing a definitive spin-parity classification. They note that the orbital angular momentum can typically be inferred from the angular distribution of the emitted particle relative to the reaction axis, and highlight that ongoing large-acceptance experiments may provide the necessary data to resolve this question.