Searches for Extra Higgs Bosons using $t\bar{t}+$Higgs{$(\to b\bar b)$} Events within 2HDMs: Direct versus Indirect Probes

TL;DR

The paper addresses the challenge of discovering and characterizing extra Higgs states in a CP-conserving two-Higgs-doublet model by exploiting inclusive $t\bar t$ production recoiling against a neutral Higgs. It develops an indirect, mass- and CP-sensitive approach using the $t\bar t$ system, via observables like $m_{t\bar t}$, $p_T^{t\bar t}$, and CP-sensitive angular variables $b_2$ and $b_4$, comparing its effectiveness to direct $H_{2,3}\to b\bar b$ reconstruction. The study demonstrates that $H_2$ (CP-even) and $H_3$ (CP-odd) imprint distinct kinematics on the $t\bar t$ system, enabling mass and CP determinations even when both states are present, and applies the method to benchmark points in Composite 2HDM (C2HDM). It provides HL-LHC projections showing accessible regions of parameter space and argues that the inclusive recoil strategy offers robustness to systematics while delivering competitive discovery and characterization capabilities for extended Higgs sectors.

Abstract

We study the possibility of establishing the production of additional Higgs states in the process $gg,q\bar q\to t\bar t Φ$, where $Φ$ = $H_{2,3}$, with $H_2$ being CP-even and $H_3$ being CP-odd, at the Large Hadron Collider (LHC), by solely exploiting the kinematic features of the reconstructed $t\bar t$ system. We adopt as reference theoretical framework a generic CP-Conserving 2-Higgs Doublet Model (2HDM), which also accommodate a Standard Model (SM)-like Higgs state $H_1$. We show that the masses $m_{H_{2,3}}$ exhibit clear correlations with the $t\bar{t}$ system properties and could, in principle, be extracted from these. Moreover, the CP properties of the $H_{2,3}$ states can be determined, even when both states are produced simultaneously. We then compare the results produced using this method with those obtained from a full kinematic reconstruction of the $H_{2,3}$ decays in the most studied $b\bar b$ channel (we take $m_{H_{2,3}}< 2m_t$), thus proving the superiority of the approach here proposed. This paves the way to both the discovery and characterization of additional Higgs states produced {\sl inclusively} in association with top-antitop quark pairs, thereby dispensing of the complications intrinsic to the {\sl exclusive} reconstruction of such states from their decay products. We test this by establishing the sensitivity of our approach in the case of a Composite 2HDM (C2HDM), describing the Higges as pseudo-Nambu Goldstone Bosons (pNGBs) and naturally predicting Higgs mass spectra in the range of sensitivity of the described analysis.

Figures (10)

• Figure 1: Distributions of $t\bar{t}H_2$ (top-left) and $t\bar{t}H_3$ (top-right) for $b_2$ and $b_4$ are shown in the ($b_2$,$b_4$) plane, evaluated in the LAB frame at parton level at LO. In the bottom-left(right) plot we show $t\bar{t}h_{\rm SM}$ case at parton level at LO(NLO).
• Figure 2: Same as as in Figure \ref{['fig:b2_vs_b4']} but using the CoM frame.
• Figure 3: Sensitivity of key kinematic observables to the mass of the additional CP-even Higgs boson ($m_{H_2}$) in $pp \to t\bar{t}H_2$ events, fixing the mass of the $H_3$ to $m_{H_3}=150$ GeV. (Left) The median (50th) of the $m_{t\bar{t}}$ (top) and $p_T^{t\bar{t}}$ (bottom) distributions are shown as a function of $m_{H_2}$. The uncertainty bars represent the precision with which the medians are extracted from the current simulation (see the text for details). (Right) The corresponding distributions normalized to the cross section for several $m_{H_2}$ values, illustrating the sensitivity of the observables to $|m_{H_2}-m_{H_3}|$.
• Figure 4: Examples of distributions between partons and charged leptons. The distributions where particles are coming from the same (wrong) mother particle, are represented in Blue (Red) and labeled Signal (Background). Distributions are normalized with their bin size.
• Figure 5: The Receiver Operating Characteristic (ROC) curve for several multivariate algorithms (left) and the best performing one (right), the BDTD, are shown.