Prospects for Observing an Invisibly Decaying Higgs Boson in the t anti-t H Production at the LHC

TL;DR

This study evaluates the prospects for observing an invisibly decaying Higgs in ttH production at the LHC. Using matrix-element–based simulations at 14 TeV, it defines a distinctive signature with an isolated lepton, a hadronically reconstructed top, two b-jets, and large MET, and rigorously models Standard Model backgrounds, finding tt production to be the dominant challenge. With optimized top reconstruction and suppression of events where one top decays to tau, the analysis projects potential excesses of roughly 10–100% over background for Higgs masses between 100 and 200 GeV, though sensitivity is limited by background modeling and tau contamination. The results indicate that while ttH with invisible Higgs decays is less sensitive than vector-boson fusion in the low-mass region, it remains a complementary channel that gains power with larger data samples and improved background control.

Abstract

The prospects for observing an invisibly decaying Higgs boson in the t anti-t H production at LHC are discussed. An isolated lepton, reconstructed hadronic top-quark decay, two identified b-jets and large missing transverse energy are proposed as the final state signature for event selection. Only the Standard Model backgrounds are taken into account. It is shown that the t anti-t Z, t anti-t W, b anti-b Z and b anti-b W backgrounds can individually be suppressed below the signal expectation. The dominant source of background remains the t anti-t production. The key for observability will be an experimental selection which allows further suppression of the contributions from the t anti-t events with one of the top-quarks decaying into a tau lepton. Depending on the details of the final analysis, an excess of the signal events above the Standard Model background of about 10% to 100% can be achieved in the mass range m_H= 100-200 GeV.

Figures (5)

• Figure 1: Reconstructed transverse mass of the lepton and $\not \space E_T$ system in the $t \bar{t}H$ events (top plot) and in the $t \bar{t}$ events (bottom plot). The dashed line denotes the distributions calculated from the true invisible energy of the primary products of W boson decays in these events, obtained by using the generator level information. The distributions are normalised to the number of events expected for an integrated luminosity of $30 fb^{-1}$.
• Figure 2: The relative fractions of the $t \bar{t}$ decay modes are listed for signal and $t \bar{t}$ background simulated with PYTHIA (top plot). The $R_{\mathrm{jj}}$ cone separation between jets used in the $W \to jj$ reconstruction; the distributions are normalised to the number of events expected for an integrated luminosity of $30 fb^{-1}$ (bottom plot).
• Figure 3: Reconstructed transverse momenta, $p_T$ (top plot) and the rapidity (absolute value),$\eta$ (bottom plot), of the two light jets used in the $W \to q \bar{q}$ reconstruction, originating either in theISR/FSR, $W \to \tau \nu$ or true $W \to jj$ decays in the $t \bar{t}$ background. The distributions are normalised to one.
• Figure 4: Reconstructed missing transverse energy $\not \space E_T$ (top) and sum of the transverse momenta of reconstructed objects $\sum p_T^{\mathrm{rec}}$ (bottom). The solid line denotes the $ttH$ signal with $m_H = 120$ GeV, the dashed one the $t \bar{t}$ background prediction. The distributions before the last selection step specified in Table \ref{['T2:a']} are shown. The distributions are normalised to the number of events expected for an integrated luminosity of $30 fb^{-1}$.
• Figure 5: Integrated number if events from the $ttH$ signal with $m_H$ = 100, 120, 140, 160 GeV (histograms) and the $t \bar{t}$ background (stars) as a function of the $\not \space E_T$ threshold.The results for $t \bar{t}$ events simulated with PYTHIA (top) and HERWIG (bottom) generators are shown. The distributions are normalised to the number of events expected for an integrated luminosity of $30 fb^{-1}$.