[Paper Review] $H^\pm o cb$ in models with two or more Higgs doublets

A. G. Akeroyd, Stefano Moretti+1 more|arXiv (Cornell University)|Sep 26, 2014

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper proposes enhancing the sensitivity to charged Higgs bosons ($H^{/pm}$) in models with two or more Higgs doublets by tagging the bottom quark from $H^{/pm} \to cb$ decays, which can dominate up to 80% of the width in 3HDMs with natural flavour conservation. The method improves signal-to-background ratio by a factor of ~2.13 over standard $H^{/pm} \to cs$ searches, enabling tighter constraints on fermionic couplings and potentially discovering $H^{/pm} \to cb$ at $m_{H^{/pm}} < m_t$. The approach is directly applicable to aligned 2HDMs and could reveal new physics beyond the Standard Model.

ABSTRACT

Searches for light $H^\pm$s via $t o H^\pm b$ are being carried out at the LHC. Herein, it is normally assumed that the dominant decay channels are $H^\pm o τν$ and $H^\pm o cs$ and separate data analyses are performed with comparable sensitivity to the underlying model assumptions. However, the $H^\pm o cb$ decay rate can be as large as $80\%$ in models with two or more Higgs doublets with natural flavour conservation, while satisfying the constraint from $b o sγ$ for $m_{H^\pm}&lt; m_t$. Despite the current search strategy for $H^\pm o cs$ is also sensitive to $H^\pm o cb$, a significant gain in sensitivity could be obtained by tagging the $b$ quark from the decay $H^\pm o cb$.

Motivation & Objective

• To address the limited sensitivity of current LHC searches for $H^\pm \to cb$ decays in multi-Higgs doublet models.
• To propose a practical extension of existing $t \to H^\pm b$ and $H^\pm \to cs$ search strategies by tagging the $b$ quark from $H^\pm \to cb$ decays.
• To demonstrate that $b$-tagging significantly improves sensitivity to $H^\pm \to cb$ when the branching ratio is large, especially in 3HDMs with natural flavour conservation.
• To provide a framework for constraining the $X$ and $Y$ Yukawa coupling parameters in 3HDMs using $H^\pm \to cb$ decay information.

Proposed method

• The study analyzes the 3Higgs Doublet Model (3HDM) with a democratic Yukawa structure, where fermion masses arise from three distinct vacuum expectation values (VEVs).
• The charged Higgs mass matrix is diagonalized via a $3 \times 3$ unitary mixing matrix $U$, with $H^\pm_2$ identified as the lightest charged Higgs state.
• Yukawa couplings are parameterized via $X = U_{12}/U_{11}$, $Y = -U_{22}/U_{21}$, and $Z = U_{32}/U_{31}$, with constraints from $b \to s\gamma$ and $Z \to b\bar{b}$ processes.
• The signal is $t \to H^\pm b$ followed by $H^\pm \to cb$, with the $b$-quark from $H^\pm$ decay tagged to suppress backgrounds.
• A sensitivity gain estimate is derived using $S/\sqrt{B}$ with $b$-tagging efficiency $\epsilon_b = 0.5$, $c$-quark mistag rate $\epsilon_c = 0.1$, and light quark mistag rate $\epsilon_j = 0.01$, yielding a factor of ~2.13 improvement.
• The analysis compares signal regions with and without $b$-tagging, showing that $b$-tagging enables sensitivity to $\text{BR}(t \to H^\pm b) < 0.5\%$.

Experimental results

Research questions

• RQ1 Can $b$-quark tagging in $t \to H^\pm b$ events significantly improve sensitivity to $H^\pm \to cb$ decays in multi-Higgs doublet models?
• RQ2 What is the quantitative gain in signal-to-background ratio when $b$-tagging is applied to $H^\pm \to cb$ decays in the context of $t \to H^\pm b$ searches at the LHC?
• RQ3 In which regions of the $[|X|, |Y|]$ parameter space can $H^\pm \to cb$ dominate while satisfying $b \to s\gamma$ and $Z \to b\bar{b}$ constraints?
• RQ4 How do current LHC limits on $\text{BR}(t \to H^\pm b)$ compare with constraints from $b \to s\gamma$ when $H^\pm \to cb$ is dominant?
• RQ5 Can $b$-tagging enable discovery or tighter constraints on $H^\pm \to cb$ in 3HDMs and aligned 2HDMs where $\text{BR}(H^\pm \to cb)$ can reach 80%?

Key findings

• The $b$-tagging method improves the signal-to-background ratio by a factor of approximately 2.13 compared to standard $H^\pm \to cs$ searches, due to reduced mistagging from light quarks and $c$-quarks.
• In the 3HDM, $H^\pm \to cb$ can dominate with a branching ratio as high as 80% when $|X| \gg |Y|, |Z|$, while satisfying $b \to s\gamma$ constraints for $m_{H^\pm} < m_t$.
• Current LHC limits on $\text{BR}(t \to H^\pm b) < 0.02$ rule out regions of the $[|X|, |Y|]$ parameter space that are not excluded by $b \to s\gamma$, particularly for $15 < |X| < 40$ and $0 < |Y| < 0.04$, and for $0 < |X| < 4$ and $0.3 > |Y| > 0.8$.
• Tagging the $b$ quark from $H^\pm \to cb$ could extend sensitivity to $\text{BR}(t \to H^\pm b) < 0.5\%$, enabling discovery of $H^\pm \to cb$ in otherwise inaccessible regions.
• The results are directly applicable to the aligned 2HDM (A2HDM), where large $\text{BR}(H^\pm \to cb)$ is also possible with $m_{H^\pm} < m_t$, while the same enhancement is not feasible in Type I, II, or IV 2HDMs due to small $V_{cb}$.
• The method provides a viable path to definitively test the viability of 3HDMs and A2HDMs by constraining the $X$ and $Y$ Yukawa couplings through $H^\pm \to cb$ decay signatures.