[Paper Review] Lepton-Specific Two Higgs Doublet Model as a Solution of Muon $g-2$ Anomaly

Tomohiro Abe, Ryosuke Sato+1 more|arXiv (Cornell University)|Apr 27, 2015

Particle physics theoretical and experimental studies24 references11 citations

TL;DR

This paper proposes the lepton-specific (Type-X) two Higgs doublet model (2HDM) as a solution to the muon $g-2$ anomaly, showing that loop contributions from extra Higgs bosons can reduce the discrepancy between experiment and the Standard Model to the 2$\sigma$ level. It incorporates constraints from $\tau$ lepton decays, flavor physics, and renormalization group evolution, finding viable parameter regions compatible with current data and future LHC searches.

ABSTRACT

We discuss the Type-X (lepton-specific) two Higgs doublet model as a solution of the anomaly of the muon $g-2$. We consider various experimental constraints on the parameter space such as direct searches for extra Higgs bosons at the LEP II and the LHC Run-I, electroweak precision observables, the decay of $B_s o μ^+μ^-$, and the leptonic decay of the tau lepton. We find that the measurement of the tau decay provides the most important constraint, which excludes the parameter region that can explain the muon $g-2$ anomaly at the 1$σ$ level. We then discuss the phenomenology of extra Higgs bosons and the standard model-like Higgs boson ($h$) to probe the scenario favored by the $g-2$ data at the collider experiments. We find that the $4τ$, $3τ$ and $4τ+ W/Z$ signatures are expected as the main signal of the extra Higgs bosons at the LHC. In addition, we clarify that the value of the $hττ$ coupling is predicted to be the standard model value times about $-1.6$ to $-1.0$, and the branching fraction of the $h o γγ$ mode deviates from the standard model prediction by $-30\%$ to $-15\%$. Furthermore, we find that the exotic decay mode, $h$ decaying into the $Z$ boson and a light CP-odd scalar boson, is allowed, and its branching fraction can be a few percent. These deviations in the property of $h$ will be tested by the precision measurements at future collider experiments.

Motivation & Objective

• To investigate whether the lepton-specific (Type-X) 2HDM can resolve the long-standing muon $g-2$ anomaly beyond the Standard Model.
• To include stringent constraints from leptonic $\tau$ decays, which were previously neglected in similar analyses.
• To assess viability under renormalization group evolution (RGEs), ensuring quartic couplings remain perturbative up to 10 TeV.
• To explore collider signatures, including $h \to ZA$ and exotic Higgs decays, for future LHC discovery.

Proposed method

• One-loop calculation of the muon anomalous magnetic moment ($a_\mu$) in the Type-X 2HDM, including two-loop Barr-Zee contributions from the $A$ and $H^\pm$ bosons.
• Evaluation of the $Z\tau\tau$ vertex and leptonic $\tau$ decay rates at one-loop level to constrain the model using precise experimental data.
• Application of renormalization group equations (RGEs) to evolve scalar quartic couplings up to 10 TeV, ensuring perturbativity and unitarity.
• Incorporation of constraints from LEP II, LHC Run-I, $B_s \to \mu^+\mu^-$ decay, and electroweak precision observables.
• Simulation of parton-level cross sections for Higgs pair production ($HA$, $H^+H^-$, $H^+A$) at the LHC using effective couplings and luminosity functions.
• Analysis of the $h \to ZA$ decay branching fraction, identifying it as a key signature for testing the model.

Experimental results

Research questions

• RQ1 Can the Type-X 2HDM reduce the muon $g-2$ discrepancy to within 2$\sigma$ of the experimental value while satisfying all current constraints?
• RQ2 How do leptonic $\tau$ decays constrain the parameter space of the Type-X 2HDM in the context of the $g-2$ anomaly?
• RQ3 What are the implications of renormalization group evolution for the stability and perturbativity of the scalar sector in the model?
• RQ4 What are the dominant collider signatures for the extra Higgs bosons in the $g-2$-favorable parameter region?
• RQ5 Can the $h \to ZA$ decay mode serve as a distinctive signal for this model at the LHC?

Key findings

• The Type-X 2HDM can reduce the $g-\!2$ discrepancy to the 2$\sigma$ level, but not below 1$\sigma$, due to the interplay of loop contributions and constraints.
• The inclusion of leptonic $\tau$ decay constraints significantly restricts the viable parameter space, especially for large $\tan\beta$ and light $A$ bosons.
• The model remains perturbative up to 10 TeV under RGE evolution, provided the scalar quartic couplings do not trigger Landau poles.
• The branching fraction for $h \to ZA$ can reach a few percent in the favored region, offering a distinctive collider signature.
• The $h \to ZA$ decay is enhanced when $m_A \lesssim m_h/2$, making it a promising channel for discovery at the LHC.
• The model predicts measurable deviations in $h$ couplings to $\gamma\gamma$, $ZZ$, $WW$, and $\tau\tau$, which can be tested in ongoing and future LHC runs.