X17 discovery potential from $\gamma D \to e^+ e^- p n$ with neutron tagging

TL;DR

This paper proposes a new experimental approach to directly search for the hypothetical X17 particle via gamma-deuteron interactions, aiming to verify its existence and properties amidst ongoing nuclear decay experiments.

Contribution

It introduces a novel direct search method using gamma-deuteron reactions with neutron tagging, providing a way to confirm X17 and determine its quantum numbers independently.

Findings

X17 signal is detectable above background in proposed kinematics.

The method can distinguish X17 quantum numbers from nuclear decay results.

Optimized for the MAGIX experiment at MESA.

Abstract

We propose a novel direct search experiment for X17 using the reaction $\gamma D \to e^+ e^- pn$. X17 is a hypothetical particle conjectured by the ATOMKI collaboration to explain anomalous signals around 17 MeV in excited ${}^8$Be, ${}^4$He and ${}^{12}$C nuclear decays via internal pair creation. It has been subject to a global experimental and theoretical research program. The proposed direct search in $\gamma D \to e^+ e^- pn$ can verify the existence of X17 through the production on a quasi-free neutron, and determine its quantum numbers separate from ongoing and planned nuclear-decay experiments. This is especially timely in view of the theoretical tension between results from the ${}^{12}$C and ${}^8$Be measurements. Using the plane-wave impulse approximation, we quantify the expected signal and background for pseudoscalar, vector and axial-vector X17 scenarios. We optimize the kinematics for the quasi-free neutron region with the upcoming MAGIX experiment at MESA in mind and show that for all three scenarios the X17 signal is clearly visible above the QED background.