Searching for Higgs Bosons in Association with Top Quark Pairs in the H -> bb Decay Mode

TL;DR

The study evaluates the feasibility of observing a light Higgs via associated production with top quarks, focusing on $H\to b\bar{b}$ in $t\bar{t}H$ events. It develops a likelihood-based reconstruction framework that leverages $b$-tagging and mass constraints to correctly assign jets and reconstruct resonances, with CMS detector parametrisations to model response. Results show that, despite large $t\bar{t}b\bar{b}$ backgrounds, the signal can be extracted with favorable $S/B$ and significant $S/\sqrt{B}$ for realistic luminosities, enabling Higgs mass and top Yukawa measurements. The MSSM prospects are also favorable in substantial portions of parameter space, and performance studies indicate robustness to detector variations, albeit requiring higher luminosities if acceptance or tagging degrade.

Abstract

Search for the Higgs Boson is one of the prime goals of the LHC. Higgs bosons lighter than 130 GeV decay mainly to a b-quark pair. While the detection of a directly produced Higgs boson in the bb channel is impossible because of the huge QCD background, the channel ttH -> lnqqbbbb is very promising in the Standard Model and the MSSM. We discuss an event reconstruction and selection method based on likelihood functions. The CMS detector response is performed with parametrisations obtained from detailed simulations. Various physics and detector performance scenarios are investigated and the results are presented. It turns out that excellent b-tagging performance and good mass resolution are essential for this channel.

Figures (6)

• Figure 1: One example of a $t\bar{t} H^0 \rightarrow l^\pm \nu q\bar{q} b\bar{b} b\bar{b}$ event at LO.
• Figure 2: Invariant resonance masses of the $t\bar{t} H^0 \rightarrow l^\pm \nu q\bar{q} b\bar{b} b\bar{b}$ signal: Higgs boson, leptonic top, hadronic top and hadronic $W^\pm$. The leptonic $W^\pm$ is not reconstructed but its nominal mass is used to calculate $p_Z(\nu)$. The generated masses are: $m_{H^0} =$ 115 $GeV/c^2$, $m_{t} =$ 175 $GeV/c^2$ and $m_{W^\pm} =$ 80.3427 $GeV/c^2$.
• Figure 3: Simulated invariant mass distribution of signal (dark shaded, $m_{H^0} =$ 115 $GeV/c^2$) plus background for $L_{int} =$ 30 $fb^{-1}$. The dashed curve is obtained from the fit of the background without signal, the solid line describes the fit of signal plus background.
• Figure 4: $S / B$, $S / \sqrt{B}$, $L_{int}$ (required for $S / \sqrt{B} =$ 5) and $\Delta y_t / y_t$ versus generated Higgs mass in the SM. Two k-factor scenarios (k$_{t\bar{t} H^0 ,\ t\bar{t} Z^0} =$ k and k$_{t\bar{t} q\bar{q}} =$ 1.9) are shown: k = 1.0 (dots) and k = 1.5 (boxes).
• Figure 5: Discovery contours in the MSSM ("maximum $m_h$" scenario) parameter space for $L_{int} =$ 30 $fb^{-1}$ (left) and for $L_{int} =$ 100 $fb^{-1}$ (right). $S / \sqrt{B} \ge$ 5 to the shaded side of the solid line. The dotted and dashed lines are the isomass curves for $m_{h^0} =$ 125 $GeV/c^2$ and $m_{h^0} =$ 115 $GeV/c^2$, respectively.