Pion decay constraints on exotic 17 MeV vector bosons

TL;DR

This paper uses pion decay data to set strong constraints on hypothetical 17 MeV vector bosons, ruling out certain explanations for nuclear transition anomalies and providing new bounds on their couplings.

Contribution

It introduces novel constraints on light vector particles from pion decay experiments, impacting models explaining nuclear transition anomalies.

Findings

Severely constrains vector boson explanations for ATOMKI anomalies

Rules out axial-vector explanations for the 17 MeV particle

Provides new bounds on couplings of light vector particles to fermions

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 ${O}(m_\pi^4/m_e^2m_X^2)$. 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.