Probing the explanation of the muon (g-2) anomaly and thermal light dark matter with the semi-visible dark photon channel

TL;DR

This paper investigates a dark photon model that could explain the muon g-2 anomaly and dark matter relic abundance, using re-analyzed NA64 data to exclude large parts of the parameter space.

Contribution

It provides the first combined analysis linking the muon g-2 discrepancy with thermal dark matter via a semi-visible dark photon channel, using existing experimental data.

Findings

Excluded significant parameter space for dark photon masses from 2$m_e$ to 390 MeV.

Constrained mixing parameter $\epsilon$ between $3 imes10^{-5}$ and $2 imes10^{-2}$.

Supported the viability of the model to explain muon g-2 and dark matter relic density.

Abstract

We report the results of a search for a new vector boson ($A'$) decaying into two dark matter particles $\chi_1 \chi_2$ of different mass. The heavier $\chi_2$ particle subsequently decays to $\chi_1$ and $A' \to e^- e^+$. For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay $A'\to \chi \overline{\chi}$ and axion-like or pseudo-scalar particles $a \to \gamma \gamma$. With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for $A'$ masses from 2$m_e$ up to 390 MeV and mixing parameter $\epsilon$ between $3\times10^{-5}$ and $2\times10^{-2}$.