[Paper Review] Type-I 2HDM under the Higgs and Electroweak Precision Measurements

Ning Chen, Tao Han+4 more|arXiv (Cornell University)|Dec 2, 2019

Particle physics theoretical and experimental studies64 references6 citations

TL;DR

This paper investigates the Type-I Two-Higgs-Doublet Model (2HDM) using future precision measurements at Z-factories and Higgs factories, combining tree-level and one-loop contributions across the full parameter space. It finds that Higgs and electroweak precision data will tightly constrain the model, especially for small and large tan β, yielding |cos(β−α)| < 0.05, |ΔmΦ| < 200 GeV, and tan β ≳ 0.3, with complementary constraints from CEPC, FCC-ee, and ILC on mass splittings and couplings.

ABSTRACT

We explore the extent to which future precision measurements of the Standard Model (SM) observables at the proposed $Z$-factories and Higgs factories may have impacts on new physics beyond the Standard Model, as illustrated by studying the Type-I Two-Higgs-doublet model (Type-I 2HDM). We include the contributions from the heavy Higgs bosons at the tree-level and at the one-loop level in a full model-parameter space. While only small $ anβ$ region is strongly constrained at tree level, the large $ anβ$ region gets constrained at loop level due to $ anβ$ enhanced tri-Higgs couplings. We perform a multiple variable global fit with non-alignment and non-degenerate masses. We find that the allowed parameter ranges could be tightly constrained by the future Higgs precision measurements, especially for small and large values of $ anβ$. Indirect limits on the masses of heavy Higgs bosons can be obtained, which can be complementary to the direct searches of the heavy Higgs bosons at hadron colliders. We also find that the expected accuracies at the $Z$-pole and at a Higgs factory are quite complementary in constraining mass splittings of heavy Higgs bosons. The typical results are $|\cos(β-α)| &lt; 0.05, |Δm_Φ| &lt; 200\ { m GeV}$, and $ anβ\gtrsim 0.3$. The reaches from CEPC, FCCee and ILC are also compared, for both Higgs and $Z$-pole precision measurements. Comparing to the Type-II 2HDM, the 95\% C.L. allowed range of $\cos(β-α)$ is larger, especially for large values of $ anβ$.

Motivation & Objective

• To assess the sensitivity of future Z-factory and Higgs factory measurements to the Type-I 2HDM beyond the Standard Model.
• To evaluate the impact of both tree-level and one-loop contributions—especially tan β-enhanced tri-Higgs couplings—on model constraints.
• To compare the constraining power of different future lepton colliders: CEPC, FCC-ee, and ILC, for Higgs and Z-pole observables.
• To derive indirect limits on heavy Higgs boson masses that complement direct searches at hadron colliders.
• To quantify the complementarity between Z-pole and Higgs factory measurements in constraining mass splittings and alignment parameters.

Proposed method

• Performing a multiple-variable χ² fit to Higgs and electroweak precision observables, including loop corrections from heavy Higgs states.
• Including full one-loop contributions from H±, H, A, and h in the calculation of Higgs couplings (κb, κZ) and Z-boson couplings.
• Using Passarino-Veltman tensor functions (B0, C0, C12) to compute loop integrals in the Type-I 2HDM framework.
• Analyzing the model in three cases: alignment with degenerate heavy Higgs masses, alignment with non-degenerate masses, and non-alignment with degenerate masses.
• Comparing expected sensitivities across CEPC, FCC-ee, and ILC based on projected luminosities and measurement accuracies.
• Applying theoretical constraints from unitarity, perturbativity, and the requirement of a stable electroweak vacuum.

Experimental results

Research questions

• RQ1 How do future Z-factory and Higgs factory measurements constrain the Type-I 2HDM parameter space, especially in the small and large tan β regimes?
• RQ2 What is the relative sensitivity of CEPC, FCC-ee, and ILC to mass splittings between heavy Higgs states (|ΔmΦ|) and alignment parameters (cos(β−α))?
• RQ3 To what extent do loop corrections, particularly tan β-enhanced tri-Higgs couplings, improve constraints in the large tan β region?
• RQ4 How do Z-pole and Higgs precision measurements complement each other in constraining heavy Higgs masses and couplings?
• RQ5 How does the Type-I 2HDM compare to the Type-II 2HDM in terms of allowed cos(β−α) ranges at 95% confidence level, especially for large tan β?

Key findings

• The allowed parameter space for the Type-I 2HDM is tightly constrained by future precision measurements, with |cos(β−α)| < 0.05 and tan β ≳ 0.3 at 95% confidence level.
• The large tan β region is significantly constrained at the one-loop level due to tan β-enhanced tri-Higgs couplings, despite weaker tree-level constraints.
• Mass splittings between heavy Higgs states are constrained to |ΔmΦ| < 200 GeV, especially when deviating from the alignment limit.
• Z-pole and Higgs factory measurements are complementary: Z-pole data better constrains mass splittings, while Higgs precision data better constrains cos(β−α) and tan β.
• FCC-ee shows slightly better reach in Z-pole precision measurements, while ILC with varying center-of-mass energy offers slightly better sensitivity in Higgs precision fits.
• Compared to Type-II 2HDM, the Type-I 2HDM allows a larger 95% C.L. range for cos(β−α), particularly in the large tan β region, due to different fermion coupling structures.