Pion decay constraints on exotic 17 MeV vector bosons

Abstract

We derive constraints on the couplings of light vector particles to all first-generation Standard Model fermions using leptonic decays of the charged pion, ${\pi }^{+}\to e^{+}{\nu }_e{X}_{\mu }$. In models where the net charge to which $X_{\mu }$ couples is not conserved, no lepton helicity flip is required for the decay to happen, enhancing the decay rate by factors of $O(m_{\pi }^4/m_e^2{m}_X^2)$. 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 ${}^8$Be, ${}^4$He, and ${}^{12}$C nuclear transitions claimed by the ATOMKI experiment, this limit rules out vector-boson explanations and poses strong limits on axial-vector ones.

Comments: 10 pages, 5 figures. v3: typos and factors of 2 corrected. Matches published version