[Paper Review] Probing light dark matter with scalar mediator: muon $(g-2)$ deviation, the proton radius puzzle

Bin Zhu, Xuewen Liu|arXiv (Cornell University)|Apr 7, 2021

Dark Matter and Cosmic Phenomena13 citations

TL;DR

This paper proposes a MeV-scale flavor-specific scalar mediator in an effective field theory framework that simultaneously explains the muon g-2 anomaly and the proton radius puzzle via couplings to muons and down-quarks. The model evades high-intensity constraints by routing the mediator predominantly into sub-MeV dark matter, with additional Majoron mediation resolving overproduction and ΔN_eff issues.

ABSTRACT

Flavor specific scalar bosons exist in various proposed extensions of the Standard Model and are motivated to couple to single generation of fermions through a global flavor symmetry breaking mechanism. We propose the MeV flavor specific scalar model in an effective field theory approach, where muon g-2 anomaly and proton radius puzzle can be explained remarkably through the coupling with muon and down-quark simultaneously. The framework is consistent with the null result of high intensity searches. In particular the E137 constraint is safely ignored by assuming mediator decaying into sub-MeV dark matter dominantly. The existence of sub-MeV dark matter results in over-production of dark matter and $\Delta N_{\mathrm{eff}}$ constraint, which can be solved by introducing additional Majoron mediator. We also investigate the searches for mediator and dark matter and their implications on viable parameter space such as supernova cooling on muon coupling, atomic physics constraint, direct detection for light boosted dark matter.

Motivation & Objective

• To resolve the long-standing muon (g-2) anomaly and the proton radius puzzle within a single theoretical framework.
• To construct a flavor-specific scalar model that couples selectively to muons and down-quarks via global flavor symmetry breaking.
• To ensure compatibility with high-intensity experiment null results, particularly E137 constraints.
• To address cosmological overproduction of dark matter and ΔN_eff constraints through an additional Majoron mediator.
• To map viable parameter space using constraints from supernova cooling, atomic physics, and direct detection of boosted light dark matter.

Proposed method

• Employing an effective field theory approach to model a flavor-specific scalar mediator with couplings to muons and down-quarks.
• Assuming the scalar mediator dominantly decays into sub-MeV dark matter to evade E137 constraints from high-intensity experiments.
• Introducing a Majoron mediator to suppress dark matter overproduction and satisfy ΔN_eff bounds from big bang nucleosynthesis and CMB observations.
• Applying constraints from supernova cooling to bound the muon coupling strength.
• Evaluating atomic physics constraints on the mediator's coupling to electrons and light atoms.
• Assessing direct detection prospects for light, boosted dark matter in the context of the model’s parameter space.

Experimental results

Research questions

• RQ1 Can a single MeV-scale scalar mediator simultaneously resolve the muon g-2 anomaly and the proton radius puzzle?
• RQ2 How can the model evade stringent E137 constraints on light scalar mediators while maintaining couplings to muons and quarks?
• RQ3 What mechanism prevents overproduction of sub-MeV dark matter and violation of ΔN_eff bounds?
• RQ4 How do supernova cooling and atomic physics constraints limit the parameter space of the muon coupling?
• RQ5 What are the implications for direct detection of light, boosted dark matter in this model?

Key findings

• The model successfully explains both the muon g-2 deviation and the proton radius puzzle through a single scalar mediator coupling to muons and down-quarks.
• The mediator's dominant decay into sub-MeV dark matter allows the model to safely evade the E137 constraint from high-intensity experiments.
• The inclusion of a Majoron mediator resolves the cosmological overproduction of dark matter and satisfies ΔN_eff constraints.
• Supernova cooling provides a strong bound on the muon coupling, restricting its strength to sub-GeV scale values.
• Atomic physics constraints, particularly from precision spectroscopy, further restrict the parameter space of the mediator coupling to light fermions.
• Direct detection of light, boosted dark matter remains viable in parts of the parameter space, especially when the mediator is light and the dark matter is sufficiently weakly interacting.