[Paper Review] The (g-2)_mu data and the lightest Higgs boson in 2HDM(II)

Maria Krawczyk|ArXiv.org|Dec 7, 2001

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper uses the new E821 measurement of the muon's anomalous magnetic moment to constrain the lightest Higgs boson in the non-supersymmetric 2HDM(II) model. By applying a two-loop calculation of Higgs contributions to $a_\mu$, it derives tighter limits on the Yukawa couplings of the light scalar $h$ and pseudoscalar $A$, showing that a pseudoscalar with mass between 25–70 GeV and $\tan\beta > 30$ is allowed, while a light scalar is excluded.

ABSTRACT

The present limits on the lightest Higgs boson in 2HDM (II) in light of the new E821 measurement of g-2 for the muon are discussed.

Motivation & Objective

• To re-evaluate constraints on the lightest Higgs boson in 2HDM(II) using the updated E821 measurement of the muon's anomalous magnetic moment.
• To assess the impact of uncertainties in the hadronic vacuum polarization contribution on the allowed new physics contribution to $a_\mu$.
• To apply a two-loop calculation of Higgs contributions to $a_\mu$ to derive improved constraints on the Yukawa couplings of the light scalar $h$ and pseudoscalar $A$.
• To combine the $g-2_\mu$ constraints with existing experimental limits from $Z \to h(A)\gamma$, $\Upsilon$ decays, and LEP searches to derive current bounds on the light Higgs sector.
• To determine whether a light Higgs boson in 2HDM(II) can simultaneously satisfy the $g-2_\mu$ anomaly and other experimental constraints.

Proposed method

• The study uses two representative Standard Model predictions for $a_\mu^{SM}$: one based on Davier and Höcker (DH) for the hadronic contribution (case A), and another from Jegerlehner (J2000) with a larger uncertainty (case B).
• The difference $\Delta a_\mu = a_\mu^{exp} - a_\mu^{SM}$ is calculated with combined experimental and theoretical uncertainties, and 95% confidence level (CL) intervals for the new physics contribution $\delta a_\mu$ are derived.
• A two-loop calculation of the Higgs contributions to $a_\mu$ is performed, including fermionic and bosonic loop diagrams involving $W^+$ and $H^\pm$ for scalar $h$, and $H^\pm$ for pseudoscalar $A$, with $\chi_V^h = 0$ to suppress $W$-loop contributions.
• The analysis assumes a single dominant Higgs exchange (either $h$ or $A$) and evaluates the resulting $\delta a_\mu$ as a function of Higgs mass and $\tan\beta$.
• Constraints from ALEPH, DELPHI, OPAL, and LEP on the Yukawa coupling are combined with the $g-2_\mu$ bounds, and limits from $Z \to h(A)\gamma$, $\Upsilon$ decays, and Tevatron searches are incorporated.
• The results are compared across case A (smaller hadronic uncertainty) and case B (larger uncertainty), with case A yielding a positive $\delta a_\mu$ band and case B allowing both positive and negative contributions.

Experimental results

Research questions

• RQ1 Can the new E821 measurement of $g-2_\mu$ be reconciled with the 2HDM(II) model through contributions from a light Higgs boson?
• RQ2 How do uncertainties in the hadronic vacuum polarization contribution affect the allowed region for new physics in $a_\mu$?
• RQ3 What are the constraints on the Yukawa couplings of the lightest Higgs boson ($h$ or $A$) in 2HDM(II) when a two-loop calculation is used instead of a one-loop approximation?
• RQ4 Which mass ranges and $\tan\beta$ values for the light Higgs bosons are consistent with both the $g-2_\mu$ anomaly and other experimental limits?
• RQ5 Does the combination of $g-2_\mu$ data with other low-energy and LEP constraints allow for a light scalar $h$ or a light pseudoscalar $A$ in 2HDM(II)?

Key findings

• For case A (based on the DH hadronic contribution), the 95% CL interval for $\delta a_\mu$ is $102 \leq \delta a_\mu \leq 750 \times 10^{-11}$, indicating a positive contribution only.
• For case B (based on J2000), the 95% CL interval is $-8.65 \leq \delta a_\mu \leq 733 \times 10^{-11}$, allowing both positive and negative contributions.
• The two-loop analysis shows that a positive $\delta a_\mu$ can arise from a scalar $h$ with mass below 5 GeV or a pseudoscalar $A$ with mass above 3 GeV.
• A negative $\delta a_\mu$ is associated with a scalar $h$ above 5 GeV or a pseudoscalar $A$ below 3 GeV.
• Combining the $g-2_\mu$ constraints with other limits, a pseudoscalar $A$ with mass between 25 GeV and 70 GeV and $\tan\beta > 30$ is allowed, while a light scalar $h$ is excluded.
• The results improve upon previous limits, particularly for $A$ above 50 GeV and $h$ above 10 GeV, when using case B, and provide allowed regions for $h$ below 5 GeV and $A$ above 3 GeV in case A.