Search Strategies for Non-Standard Higgs Bosons at Future e^+e^- Colliders

TL;DR

The paper investigates how CP-violating two-Higgs-doublet models (CPV-2HDM) affect Higgs discovery strategies at future $e^+e^-$ colliders. By exploiting sum rules that relate Yukawa and Higgs–Z couplings, the authors show that even if a neutral Higgs has a suppressed $ZZh$ coupling, it must possess sizable Yukawa couplings to either top or bottom quarks, enabling detection via $e^+e^-\to f\bar f h$ processes given sufficient luminosity. They quantify the luminosity required to guarantee discovery across Higgs masses, finding that for moderate $\tan\beta$ the needed luminosity often exceeds planned runs, especially at higher masses, and discuss extensions to singlet-augmented sectors and implications for hadron colliders. Overall, the study emphasizes the importance of including Yukawa-channel searches alongside Higgs-strahlung and pair production to cover the full CPV-2HDM parameter space. The results provide guidance on the luminosity and channel strategy needed to robustly discover non-standard Higgs bosons at future lepton colliders.

Abstract

Already in the simplest two-Higgs-doublet model with CP violation in the Higgs sector, the $3\times3$ mixing matrix for the neutral Higgs bosons can substantially modify their couplings, thereby endangering the ``classical'' Higgs search strategies. However, there are sum rules relating Yukawa and Higgs-Z couplings which ensure that the ZZ, $b\anti b$ and $t\anti t$ couplings of a given neutral 2HDM Higgs boson cannot all be simultaneously suppressed. This result implies that any single Higgs boson will be detectable at an e^+e^- collider if the Z+Higgs, $b\anti b+$Higgs {\it and} $t\anti t+$Higgs production channels are all kinematically accessible {\it and} if the integrated luminosity is sufficient. We explore, as a function of Higgs mass, the luminosity required to guarantee Higgs boson detection, and find that for moderate $\tanβ$ values the needed luminosity is unlikely to be available for all possible mixing scenarios. The additional difficulties for the case when the two-doublet Higgs sector is extended by adding one more singlet are summarized. Implications of the sum rules for Higgs discovery at the Tevatron and LHC are briefly discussed.

Figures (4)

• Figure 1: Contour lines for ${\rm min} [\sigma(e^+e^-\rightarrow h_1 h_2)]$ in fb units, obtained by scanning over the $\alpha_i$ while requiring $\leq 50$$Zh_1$ or $Zh_2$ events for $L=500~{\rm fb}^{-1}$, are plotted in $(m_{h_1},m_{h_2})$ parameter space for the indicated $\sqrt{s}$ values and for $\tan \beta=0.5$. The plots are virtually unchanged for larger values of $\tan \beta$. The contour lines overlap in the inner corner of each plot as a result of excluding mass choices inconsistent with experimental constraints from LEP2 data.
• Figure 2: The minimal and maximal values of $\sigma(b\bar{b} h_1)$ and $\sigma( t\bar{t} h_1)$, obtained by scanning over $\alpha_1$ and $\alpha_2$ (see footnote 6) while requiring $\leq 50$$Zh_1$ events for $L=500~{\rm fb}^{-1}$, are plotted for $\sqrt{s}=500$ and 800 GeV. For a given value of $\tan \beta$, the same type of line (dots for $\tan \beta=0.1$ and $t\overline t h_1$, solid for $\tan \beta=1$, dashes for $\tan \beta=10$, dots for $\tan \beta=50$ and $b\overline bh_1$) is used for the minimal and maximal values of the cross sections. In the case of $b\bar{b} h_1$, the minimal and maximal values of the cross sections are almost the same. Masses of the remaining Higgs bosons are assumed to be $1000\,{\rm GeV}$.
• Figure 3: For $\sqrt s=500\,{\rm GeV}$ and $\sqrt s=800\,{\rm GeV}$, we present as a function of $m_{h_1}$ the value of $\sigma_{\rm min}\equiv {\rm min}_{\tan \beta}\left( {\rm min}_{(\alpha_1,\alpha_2)}\left\{{\rm max}[ \sigma_{\rm min}(b\overline b h_1), \sigma_{\rm min}(t\overline t h_1)]\right\}\right)$, and the corresponding value of $\tan \beta$, as obtained by scanning over $\tan \beta$ and $(\alpha_1,\alpha_2)$ parameter space subject to constraints (I) and (II) --- see text for details. Masses of the remaining Higgs bosons are assumed to be 1000 GeV.
• Figure 4: For $\sqrt s=500\,{\rm GeV}$ (dashes) and $\sqrt s=800\,{\rm GeV}$ (solid) we present as a function of $m_{h_1}$ the maximum and minimum $\tan \beta$ values between which $t\overline t h_1$, $b\overline b h_1$ and $Zh_1$ final states can (for some choice of $(\alpha_1,\alpha_2)$ consistent with constraint (II) --- see text) all have fewer than 50 events assuming (a) $L=1000~{\rm fb}^{-1}$ or (b) $L=2500~{\rm fb}^{-1}$. Masses of the remaining Higgs bosons are assumed to be 1000 GeV.