[Paper Review] Probing loop effects in wrong-sign Yukawa region of 2HDM

Wei Su|arXiv (Cornell University)|Oct 14, 2019

Particle physics theoretical and experimental studies35 references4 citations

TL;DR

This paper investigates the wrong-sign Yukawa region in Type-II Two Higgs Doublet Models (2HDM) up to one-loop level using direct LHC searches and indirect Higgs precision measurements. It finds that loop corrections significantly alter constraints: regions with small λv² are more strongly excluded, while large λv² allows larger surviving wrong-sign regions, especially for mA = 800 GeV and √λv² = 400 GeV, where cos(β−α) ∈ (0, 0.3) remains allowed under combined constraints.

ABSTRACT

In the framework of 2HDM, we explore the wrong-sign Yukawa region with direct and indirect searches up to one-loop level. The direct searches include the latest $H/A o f\bar f, VV, Vh, hh$ reports at current LHC, and the study of indirect Higgs precision measurements works with current LHC, future HL-LHC and CEPC. At tree level of Type-II 2HDM, for degenerate heavy Higgs mass $m_A=m_H=m_{H^\pm}<800$ GeV, the wrong-sign Yukawa regions are excluded largely except for the tiny allowed region around $\cos(β-α)\in(0.2,0.3)$ under the combined constraints. The excluded region is also nearly independent of parameter $m_{12}$ or $λv^2=m_A^2-m_{12}^2/(\sin β\cos β)$. The situation changes a lot after including loop corrections to the indirect searches, for example $m_A=800 $ GeV, the region with $λv^2<0$ will be stronger constrained to be totally excluded. Whilst $λv^2>0$ would get larger survived wrong-sign region, such as $\cos(β-α) \in (0,0.3)$ for $\sqrt{λv^2}=400$ GeV, under combined restrictions.

Motivation & Objective

• To test the viability of the wrong-sign Yukawa region in Type-II 2HDM using current and future Higgs precision measurements.
• To assess how one-loop corrections modify the constraints on the wrong-sign region compared to tree-level results.
• To evaluate the impact of the parameter λv² = m²A − m²12/(sinβ cosβ) on the allowed parameter space.
• To compare constraints from direct LHC searches (H/A → ff, VV, Vh, hh) with indirect global fits from LHC Run-II, HL-LHC, and CEPC.
• To determine whether the tree-level relation cos(β−α) ≈ 2/tanβ holds at one-loop level in the wrong-sign region.

Proposed method

• Performs a global fit of Higgs precision measurements from LHC Run-II, HL-LHC, and CEPC at one-loop level using a χ²-likelihood approach.
• Applies direct search limits from H/A → f̄f, VV, Vh, hh at tree level, using σ × Br constraints from LHC Run-I and Run-II.
• Uses the parameterization λv² = m²A − m²12/(sinβ cosβ) to explore the dependence of constraints on the soft Z₂-breaking term m²12.
• Analyzes the cos(β−α)−tanβ and mΦ−m12 planes to test the tree-level relation cos(β−α) = 2/tanβ at one-loop level.
• Evaluates the impact of loop corrections on the sign of Yukawa couplings, particularly for bottom quarks.
• Compares allowed regions at 95% C.L. across different collider phases (LHC Run-II, HL-LHC, CEPC) to assess evolving constraints.

Experimental results

Research questions

• RQ1 How do loop corrections affect the allowed parameter space in the wrong-sign Yukawa region of Type-II 2HDM?
• RQ2 What is the role of λv² in modifying the constraints on the wrong-sign region, especially for mA = 800 GeV?
• RQ3 Does the tree-level relation cos(β−α) = 2/tanβ remain valid at one-loop level in the wrong-sign region?
• RQ4 How do future Higgs precision measurements at HL-LHC and CEPC further constrain the wrong-sign region compared to current LHC data?
• RQ5 To what extent do direct LHC searches and indirect precision measurements jointly exclude the wrong-sign Yukawa region?

Key findings

• For mA = mH = mH± < 800 GeV, the wrong-sign Yukawa region is largely excluded at tree level, except for a tiny allowed region around tanβ ∈ (8, 10).
• The excluded region is nearly independent of m12 or λv² at tree level, indicating that m12 does not significantly affect constraints in the absence of loop corrections.
• After including one-loop corrections, regions with λv² < 0 are more strongly constrained and eventually fully excluded for mA = 800 GeV.
• For λv² > 0, the allowed wrong-sign region expands; for √λv² = 400 GeV, cos(β−α) ∈ (0, 0.3) remains viable under combined constraints.
• At mA = 1500 GeV, the region with λv² ≤ 0 is completely excluded, and large λv² (>50 GeV²) shifts the allowed region to the right, reducing constraints.
• The tree-level relation cos(β−α) = 2/tanβ holds approximately at one-loop level for tanβ > 20, but breaks down for tanβ < 20 due to complex loop cancellations.