Pion decay constraints on exotic 17 MeV vector bosons
Matheus Hostert, Maxim Pospelov
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, ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}{X}_{\ensuremath{\mu}}$. In models where the net charge to which ${X}_{\ensuremath{\mu}}$ couples to is not conserved, no lepton helicity flip is required for the decay to happen, enhancing the decay rate by factors of $\mathcal{O}({m}_{\ensuremath{\pi}}^{4}/{m}_{e}^{2}{m}_{X}^{2})$. 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 $^{8}\mathrm{Be}$, $^{4}\mathrm{He}$, and $^{12}\mathrm{C}$ nuclear transitions claimed by the ATOMKI experiment, this limit rules out vector-boson explanations and poses strong limits on axial-vector ones.