Pion decay constraints on exotic 17 MeV vector bosons

We derive constraints on the couplings of light vector particles to all first-generation Standard Model fermions using leptonic decays of the charged pion, π⁺→e⁺νₑX_μ. In models where the net charge to which X_μ couples to is not conserved, no lepton helicity flip is required for the decay to happen, enhancing the decay rate by factors of O(m_π⁴/mₑ²mX²). A past search at the SINDRUM-I spectrometer severely constrains this possibility. In the context of the hypothesized 17 MeV particle proposed to explain anomalous ⁸Be, ⁴He, and ¹²C nuclear transitions claimed by the ATOMKI experiment, this limit rules out vector-boson explanations and poses strong limits on axial-vector ones.