The X17 with Chiral Couplings

In recent years, the ATOMKI collaboration has performed a series of measurements of excited nuclei, observing a resonant excess of electron-positron pairs at large opening angles compared to the Standard Model prediction. The excess has been hypothesized to be due to the production of a new spin-1 or spin-0 particle, X17, with a mass of about 17 MeV. Recently, the PADME experiment has reported an excess in the $e^+e^-$ cross section at center-of-mass energies near 17 MeV, perhaps further hinting at the existence of a new state. Studies of the spin-1 case have hitherto focused on either vector {\em or} axial-vector couplings to quarks and leptons, whereas UV theories more naturally produce {\em both} vector and axial-vector (\textit{i.e.} chiral) couplings, analogous to the Standard Model weak interactions. We consider the ATOMKI anomalies in the context of an $X$ with chiral couplings to quarks and explore the parameter space that can explain the ATOMKI anomalies, contrasting them with experimental constraints. We find that it is possible to accommodate the reported ATOMKI signals. However, the $99%$ CL region is in tension with null results from searches for atomic parity violation and direct searches for new low mass physics coupled to electrons. This tension is found to be driven by the magnitude of the reported excess in the transition of $^{12}{\rm C}(17.23)$, which drives the best-fit region towards excluded couplings.

💡 Research Summary

The paper investigates whether the hypothesized 17 MeV boson X17, which has been invoked to explain anomalous electron‑positron pair production observed in several nuclear transitions by the ATOMKI collaboration, can be accommodated within a model that features chiral (both vector and axial‑vector) couplings to quarks. Earlier phenomenological studies have largely focused on pure vector or pure axial‑vector interactions, but ultraviolet‑complete theories naturally generate a mixture of the two, much like the Standard Model weak interaction.

The authors extend the Standard Model by a new spin‑1 field Xμ whose interactions with nucleons are described by six effective couplings: vector (εV) and axial‑vector (εA) couplings to protons and neutrons, plus a vector coupling to electrons (εVe). The nucleon‑level Lagrangian (Eq. 1) is matched onto nuclear transition amplitudes using an effective field theory (EFT) built from operators that respect parity and angular‑momentum conservation. Two representative operators, O5 (vector) and O3 (axial‑vector), encode the relevant nuclear matrix elements ⟨N|Jμ|N*⟩. The vector matrix elements are related to the well‑measured photon‑emission widths, while the axial‑vector elements must be estimated from nuclear‑structure calculations: N‑body methods for 8 Be, shell‑model calculations for 12 C, and literature values for 4 He. Because the axial matrix elements for carbon and helium are poorly known, the authors assign conservative uncertainties of 50 % and 100 % and also explore a factor‑5 variation to test robustness.

The experimental inputs comprise ATOMKI measurements of four excited nuclear states: 8 Be(18.15 MeV), 8 Be(17.64 MeV), 12 C(17.23 MeV), and a combined 4 He(20.21 MeV + 21.01 MeV) transition. For each, the branching ratio B_X ≡ Γ_X/Γ_γ (assuming 100 % X→e⁺e⁻) is reported, with the 12 C transition showing a particularly large excess (B_X = (3.6 ± 0.3) × 10⁻⁶). The recent PADME result, which observes an excess in e⁺e⁻ production near the same invariant mass, is also taken into account.

A host of null‑search constraints must be satisfied simultaneously: (i) rare pion decay limits from SINDRUM‑I, which impose a stringent bound on a particular linear combination of quark, neutrino, and electron couplings; (ii) the “protophobic” bound from NA48 on the proton vector coupling (|εVp| < 8 × 10⁻⁴); (iii) limits on neutron vector couplings from long‑range force searches; (iv) electron‑coupling limits from KLOE‑2 (|εVe|² + |εAe|² < 2 × 10⁻³ e²) and NA64 (εVe > 6.8 × 10⁻⁴); and (v) atomic parity‑violation (APV) constraints, notably from cesium, which bound the product |εAp εVCs| < 6 × 10⁻⁸. To evade the SINDRUM‑I bound, the authors enforce εVe = (εVu + εAu) − (εVd + εAd), effectively canceling the dangerous combination.

The parameter scan proceeds by varying the neutron couplings (εVn, εAn) over a grid. For each point, the proton couplings (εVp, εAp) are fitted to the ATOMKI data while respecting the NA48 protophobic limit, and the electron vector coupling is set by the cancellation condition. The X mass is fixed at 16.9 MeV because the combined likelihood from Ref.