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 mχ around 10 MeV and the product of the multiplicity factor and millicharge squared of Nχε2 ∼ 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(Nχ) 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.
Highlights
For the first R-ratio way, experimental data for the e+e− → γ∗ → hadrons are collected and used to derive the photon vacuum polarization from a dispersion relation [5,6,7,8,9]
We provide a novel explanation to the muon g − 2 excess with new physics contributions at the two-loop level
We have provided a new scenario to explain the muon g − 2 excess based on millicharged particle contributions to the photon vacuum polarization
Summary
For millicharged particles with a mass below ∼ 1 MeV, the Big Bang nucleosynthesis (BBN) physics provides very stringent constraints on the charge [20, 21]. The upper limit from J/ψ invisible decay is very similar to the above one Another indirect search for mCPs is to use signatures of mono-photon plus missing energy or e+e− → γ + E/ at a lepton collider. Searches for events with large missing energy in the fixed-target experiment with an electron beam can be used to constrain the millicharged particles, if one recasts the limits on produced on-shell invisible dark photon into the signals with two mCPs in the NA64 experiment [27, 28]. For the free-streaming case, the luminosity of the mCPs ∝ Nχε and provides a constraint Nχε − for mχ ∼ 15 MeV, which excludes the muon g − 2 preferred region. Combining with the other bound Nχ > 7.4 × 109, this excludes the muon g − 2 preferred parameter region Nχε2 ∼ 10−3
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