Searching for Signs of the Second Higgs Doublet

TL;DR

The workDevelops a targeted framework to search for a second Higgs doublet by connecting SM-like Higgs coupling measurements with predictions for additional scalars in flavor- and CP-conserving 2HDMs. It shows that current data favor the alignment limit, constraining the heavy scalars' couplings, yet opens robust discovery channels such as $H\to hh$ and $A\to Zh$, which can dominate heavy scalar decays away from alignment. By mapping the two-parameter 2HDM space to collider observables, the paper identifies accessible final states across various production modes (gluon fusion, VBF, $VH$, and associated production) and provides guidance for future LHC searches. The results underscore the complementarity of precision Higgs coupling measurements and direct heavy-Higgs searches in revealing or excluding extended EWSB sectors.

Abstract

The search for evidence of extended electroweak symmetry breaking has entered a new phase with the discovery of a Standard Model (SM)-like Higgs at the LHC. The measurement of Higgs couplings and direct searches for additional scalars provide complementary avenues for the discovery of new degrees of freedom. This complementarity is particularly sharp in two Higgs doublet models (2HDMs) where the couplings of the SM-like Higgs may be directly related to the LHC signals of additional scalars. In this work we develop a strategy for searching for the second Higgs doublet given the LHC signals of the recently discovered SM-like Higgs. We focus on a motivated parameter space of flavor- and CP-conserving 2HDMs in which the couplings of all scalars to SM states are controlled by two parameters. We construct fits in this parameter space to the signals of the SM-like Higgs and translate these fits into signal expectations for future measurements of both the SM-like Higgs and additional scalars, identifying the most promising search channels for discovery or exclusion of new physics. When kinematically accessible, decays of the heavy neutral scalar Higgs to two light Higgs scalars, $H \to hh$, and decays of the pseudoscalar Higgs to a light Higgs scalar and $Z$ boson, $A \to Zh$, provide promising avenues for discovery even when the couplings of the light Higgs are within a few percent of SM predictions. When the couplings of the light Higgs are exceptionally close to those of the SM, decays of heavier neutral scalars to $γγ$ and $τ^+ τ^-$ become particularly important for discovery.

Figures (24)

• Figure 1: Upper left: Global fit of SM-like Higgs couplings in 2HDM of Type 1. Upper right: Global fit of SM-like Higgs couplings in 2HDM of Type 2. Lower left: Global fit of SM-like Higgs couplings in 2HDM of Type 3. Lower right: Global fit of SM-like Higgs couplings in 2HDM of Type 4.
• Figure 2: Left: Global fit of SM-like Higgs couplings in 2HDM of Type 1. Right: Contributions to the fit coming separately from $h \to \gamma \gamma$, $h\to VV$, and $h \to bb, \tau \tau$.
• Figure 3: Left: Global fit of SM-like Higgs couplings in 2HDM of Type 2. Right: Contributions to the fit coming separately from $h \to \gamma \gamma$, $h\to VV$, and $h \to bb, \tau \tau$. The peninsulas of the $\gamma \gamma$ and $VV$ contours correspond to regions where the bottom coupling has changed sign (i.e. $\alpha >0$) and has a magnitude that is respectively suppressed/unsuppressed relative to the decoupling limit (see Type 2 SM-like Higgs decay rates in Figs. \ref{['fig:htautau']}, \ref{['fig:VBFhtautau']}, \ref{['fig:VBFhgammagamma']} for comparison). Note also that the apparent discontinuity in the $bb+\tau \tau$ contour arises from the requirement $\alpha > -\pi/2$.
• Figure 4: Contours of the inclusive $\sigma \cdot {\rm Br}(h \to \tau^+ \tau^-) / \sigma \cdot {\rm Br}(h_{SM} \to \tau^+ \tau^-)$ for 8 TeV $pp$ collisions for the SM-like Higgs boson, shown as a function of $\cos(\beta - \alpha)$ and $\beta$ for Type 1 (upper left), Type 2 (upper right), Type 3 (lower left), and Type 4 (lower right) 2HDM. The inner (outer) dashed contour denotes the 68% (95%) CL best fit to the signals of the SM-like Higgs.
• Figure 5: Contours of $\sigma \cdot {\rm Br}({\rm VBF \; or \;} Vh \to \tau^+ \tau^-) / \sigma \cdot {\rm Br}({\rm VBF \; or \;} Vh_{SM} \to \tau^+ \tau^-)$ for 8 TeV $pp$ collisions for the SM-like Higgs boson, shown as a function of $\cos(\beta - \alpha)$ and $\beta$ for Type 1 (upper left), Type 2 (upper right), Type 3 (lower left), and Type 4 (lower right) 2HDM. The inner (outer) dashed contour denotes the 68% (95%) CL best fit to the signals of the SM-like Higgs.