Search for $H \to γγ$ produced in association with top quarks and constraints on the Yukawa coupling between the top quark and the Higgs boson using data taken at 7 TeV and 8 TeV with the ATLAS detector

TL;DR

This ATLAS study searches for Higgs bosons produced in association with top quarks via the H→γγ decay at 7 and 8 TeV, targeting ttH and tH production to directly probe the top Yukawa coupling κ_t. A data-driven continuum background model combined with MC-predicted Higgs signals is analyzed through an unbinned m_{γγ} likelihood fit across leptonic and hadronic ttH categories. No significant excess is found; the paper sets a 95% CL upper limit on σ(ttH)×BR(H→γγ) of 6.7×SM at m_H ≈ 125.4 GeV and constrains κ_t to −1.3 < κ_t < 8.0, providing important limits on models with non-SM Yukawa structures. These results complement indirect Higgs measurements and help bound scenarios with altered top–Higgs interactions in the early LHC data era.

Abstract

A search is performed for Higgs bosons produced in association with top quarks using the diphoton decay mode of the Higgs boson. Selection requirements are optimized separately for leptonic and fully hadronic final states from the top quark decays. The dataset used corresponds to an integrated luminosity of 4.5 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 7 TeV and 20.3 fb$^{-1}$ at 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess over the background prediction is observed and upper limits are set on the $t\bar{t}H$ production cross section. The observed exclusion upper limit at 95% confidence level is 6.7 times the predicted Standard Model cross section value. In addition, limits are set on the strength of the Yukawa coupling between the top quark and the Higgs boson, taking into account the dependence of the $t\bar{t}H$ and $tH$ cross sections as well as the $H \to γγ$ branching fraction on the Yukawa coupling. Lower and upper limits at 95% confidence level are set at -1.3 and +8.0 times the Yukawa coupling strength in the Standard Model.

Figures (6)

• Figure 1: Feynman diagrams showing examples for $tH{q}{b}$ (a, b) and $WtH$ production (c, d). Higgs boson radiation off top quark and $W$ boson lines is depicted. The $tH{q}{b}$ process is shown in the four-flavor scheme where no $b$-quarks are assumed to be present in the proton Frederix:2012dh.
• Figure 2: Distributions of the diphoton invariant mass, $m_{\gamma\gamma}$, for the leptonic (left) and hadronic (right) category for data at $7 \textrm{Te V}$ (top) and data at $8 \textrm{Te V}$ (bottom). An unbinned signal-plus-background likelihood fit to the full spectra is used to estimate the number of events from continuum background (solid line) as well as from SM Higgs boson production (dashed line). The signal strength, $\mu$, is a parameter common to all categories and its best-fit value is $\mu = 1.4$ for $m_{H} = 125.4\mathrm{\ Ge V} \textrm{Ge V}$.
• Figure 3: Negative log-likelihood scan for the $t\bar{t}H$ cross section times BR($H \rightarrow \gamma\gamma$) relative to the SM expectation, $\mu_{t\bar{t}H}$, at $m_{H} = 125.4\mathrm{\ Ge V} \textrm{Ge V}$, where all other Higgs boson production cross sections, including the cross section for $tH$ production, are set to their respective SM expectations.
• Figure 4: Observed and expected 95% CL upper limits on the $t\bar{t}H$ production cross section times BR($H \rightarrow \gamma\gamma$). All other Higgs boson production cross sections, including the cross section for $tH$ production, are set to their respective SM expectations. While the expected limits are calculated for the case where $t\bar{t}H$ production is not present, the lines denoted by "SM signal injected" show the expected 95% CL limits for a dataset corresponding to continuum background plus SM Higgs boson production. The limits are given relative to the SM expectations and at $m_{H} = 125.4\mathrm{\ Ge V} \textrm{Ge V}$.
• Figure 5: Production cross sections for $t\bar{t}H$ and $tH$ divided by their SM expectations as a function of the scale factor to the top quark-Higgs boson Yukawa coupling, $\kappa_t$. Production of $tH$ comprises the $tH{q}{b}$ and $WtH$ processes. Also shown is the dependence of the BR($H \rightarrow \gamma\gamma$) with respect to its SM expectation on $\kappa_t$.