Muon g-2 from Millicharged Hidden Confining Sector

TL;DR

This paper proposes a new two-loop physics model involving millicharged particles and a hidden confining gauge group to explain the muon g-2 anomaly, which can be tested at low-energy colliders.

Contribution

It introduces a novel two-loop level explanation for muon g-2 using millicharged particles charged under a hidden confining gauge group, addressing previous experimental constraints.

Findings

The model explains muon g-2 excess with millicharged particles around 10 MeV.

Hidden-sector hadrons decay into neutrinos, evading astrophysical constraints.

Testable predictions are made for low-energy collider experiments.

Abstract

We provide a novel explanation to the muon $g-2$ excess with new physics contributions at the two-loop level. In this scenario, light millicharged particles are introduced to modify the photon vacuum polarization that contributes to muon $g-2$ at one additional loop. The muon $g-2$ excess can be explained with the millicharged particle mass $m_\chi$ around 10 MeV and the product of the multiplicity factor and millicharge squared of $N_\chi \varepsilon^2 \sim 10^{-3}$. The minimal model faces severe constraints from direct searches at fixed-target experiments and astrophysical observables. However, if the millicharged particles are also charged under a hidden confining gauge group $SU(N_\chi)$ with a confinement scale of MeV, hidden-sector hadrons are unstable and can decay into neutrinos, which makes this scenario consistent with existing constraints. This explanation can be well tested at low-energy lepton colliders such as BESIII and Belle II as well as other proposed fixed-target experiments.