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Register a new userChallenges for an axion explanation of the muon $g-2$ measurement
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The discrepancy between the muon $g-2$ measurement and the Standard Model prediction points to new physics around or below the weak scale. It is tantalizing to consider the loop effects of a heavy axion (in the general sense, also known as an axion-like particle) coupling to leptons and photons as an explanation for this discrepancy. We provide an updated analysis of the necessary couplings, including two-loop contributions, and find that the new physics operators point to an axion decay constant on the order of 10s of GeV. This poses major problems for such an explanation, as the axion couplings to leptons and photons must be generated at low scales. We outline some possibilities for how such couplings can arise, and find that these scenarios predict new charged matter at or below the weak scale and new scalars can mix with the Higgs boson, raising numerous phenomenological challenges. These scenarios also all predict additional contributions to the muon $g-2$ itself, calling the initial application of the axion effective theory into question. We conclude that there is little reason to favor an axion explanation of the muon $g-2$ measurement relative to other models postulating new weak-scale matter.
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The muon anomalous magnetic moment measurement has, for more than a decade, been a long-standing anomaly hinting the physics beyond the Standard Model (BSM). The recently announced results from muon $g-2$ collaboration, corresponding to 3.3$sigma$ deviation from Standard Model value (4.2$sigma$ in combination with previous measurement) are strengthening the need for new physics coupled to muons. In this letter, we propose a novel scenario in which Standard Model (SM) is augmented by an axion-like particle (ALP) and vector-like fermions. We find that such a model admits an excellent interpretation of recent muon $g-2$ measurement through quantum process featuring ALP interacting with muons and newly introduced fermions. Previously proposed explanations with ALPs utilize interactions with photons and/or SM fermions. Therefore, in this letter we complement and extend such scenarios. We also discuss collider prospects for the model as well as the possibility that ALP is long lived or stable dark matter (DM) candidate.
We propose simple models with a flavor-dependent global $U(1)_ell$ and a discrete $mathbb{Z}_2$ symmetries to explain the anomalies in the measured anomalous magnetic dipole moments of muon and electron, $(g-2)_{mu,e}$, while simultaneously accommodating a dark matter candidate. These new symmetries are introduced not only to avoid the dangerous lepton flavor-violating decays of charged leptons, but also to ensure the stability of the dark matter. Our models can realize the opposite-sign contributions to the muon and electron $g-2$ via one-loop diagrams involving new vector-like leptons. Under the vacuum stability and perturbative unitarity bounds as well as the constraints from the dark matter direct searches and related LHC data, we find suitable parameter space to simultaneously explain $(g-2)_{mu,e}$ and the relic density. In this parameter space, the coupling of the Higgs boson with muons can be enhanced by up to $sim 38%$ from its Standard Model value, which can be tested in future collider experiments.
We argue that a large region of so far unconstrained parameter space for axion-like particles (ALPs), where their couplings to the Standard Model are of order $(0.01!-!1),mbox{TeV}^{-1}$, can be explored by searches for the exotic Higgs decays $hto Za$ and $hto aa$ in Run-2 of the LHC. Almost the complete region in which ALPs can explain the anomalous magnetic moment of the muon can be probed by searches for these decays with subsequent decay $atogammagamma$, even if the relevant couplings are loop suppressed and the $atogammagamma$ branching ratio is less than~1.
We construct a model to explain the muon anomalous magnetic moment, without considering any lepton flavor violations, in the modular $A_4$ symmetry. We have investigated a predictive radiative seesaw model including dark matter candidate at favorable fixed point of $tau=omega$ obtained by recent analysis of the stabilized moduli values from the possible configurations of the flux compactifications. In the result, we show our predictions on the Dirac CP and Majorana phases, the neutrino masses, the mass range of dark matter as well as the muon anomalous magnetic moment through the $chi^2$ analysis.
We consider lepton flavor violating Higgs decay, specifically $h to mutau$, in a leptoquark model. We introduce two scalar leptoquarks with the $SU(3)_c times SU(2)_L times U(1)_Y$ quantum numbers, $(3,2,7/6)$ and $(3,2,1/6)$, which do not generate the proton decay within renormalizable level. They can mix with each other by interactions with the standard model Higgs. The constraint from the charged lepton flavor violating process, $tau^{-} to mu^{-} gamma$, is very strong when only one leptoquark contribution is considered. However, we demonstrate that significant cancellation is possible between the two leptoquark contributions. We show that we can explain the CMS (ATLAS) excess in $h to mu tau$. We also show that muon $(g-2)$ anomaly can also be accommodated.
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