Search for single production of a vectorlike $T$ quark decaying into a Higgs boson and top quark with fully hadronic final states using the ATLAS detector

A search is made for a vectorlike $T$ quark decaying into a Higgs boson and a top quark in 13 TeV proton-proton collisions using the ATLAS detector at the Large Hadron Collider with a data sample corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$. The Higgs-boson and top-quark candidates are identified in the all-hadronic decay mode, where $H\to b\overline{b}$ and $t\to bW\to bq{\overline{q}}^{\prime }$ are reconstructed as large-radius jets. The candidate Higgs boson, top quark, and associated $B$ hadrons are identified using tagging algorithms. No significant excess is observed above the background, so limits are set on the production cross section of a singlet $T$ quark at 95% confidence level, depending on the mass $m_T$ and coupling ${\kappa }_T$ of the vectorlike $T$ quark to Standard Model particles. In the considered mass range between 1.0 and 2.3 TeV, the upper limit on the allowed coupling values increases with $m_T$ from a minimum value of 0.35 for $1.07<m_T<1.4\mathrm{TeV}$ to 1.6 for $m_T=2.3\mathrm{TeV}$.

Figure 1

A Feynman diagram illustrating the $W$-mediated production of a single $T$ quark, shown in red, decaying into a top quark and Higgs boson.

Figure 2

The $9\times 9$ matrix that represents the 81 exclusive event-tagging states defined by the nine possible tagging states of each large-$R$ jet. Each event can be in only one of the 81 event-tagging states. The red event-tagging states comprise the events in the signal region ($SR$), the blue event-tagging states comprise the $t\overline{t}$ normalization region ($NR$), and the yellow event-tagging states are validation regions labeled $V1$ through $V1$. The gray event-tagging states are regions used to estimate the multijet background, as described in Sec. 5a.

Figure 3

Invariant mass distributions for (a) the leading small-$R$ jet associated with the leading large-$R$ jet in the $t\overline{t}$ normalization region where both the leading and second-leading large-$R$ jets are top-quark tagged with at least $1b$-tag and (b) the leading large-$R$ jet, in the same region. The predicted distributions include the estimated backgrounds and a hypothetical $T$-quark signal with $m_T=1.6\mathrm{TeV}$ and ${\kappa }_T=0.5$. The blue hashed lines correspond to the sum in quadrature of the statistical and systematic uncertainties of the prediction in a given bin. The lower panels show the ratio of the data to the prediction, along with the uncertainty in the ratio. A ratio outside the bounds of the axis is represented by a blue arrow. The last bin includes the event overflows. Contributions to the predicted yield are stacked in the same order as they appear in the legend.

Figure 4

Invariant mass distributions for the leading small-$R$ jet associated with the leading large-$R$ jet for (a) $VR1$ defined by requiring the leading large-$R$ jet be top-quark tagged with $1b$-tag and the second-leading jet is Higgs-boson tagged with $1b$-tag, (b) $V1$ defined by requiring the leading large-$R$ jet be top-quark tagged with $1b$-tag and the second-leading jet is neither Higgs-boson tagged nor top-quark tagged with $1b$-tag, (c) $V1$ defined by requiring the leading large-$R$ jet be top-quark tagged with $1b$-tag and the second-leading jet is top-quark tagged with no $b$-tag, and (d) $V1$ defined by requiring the leading large-$R$ jet be top-quark tagged with $1b$-tag and the second-leading jet is Higgs-boson tagged with no $b$-tag. The predicted distribution includes the estimated backgrounds and a hypothetical $T$-quark signal with $m_T=1.6\mathrm{TeV}$ and ${\kappa }_T=0.5$. The blue hashed lines correspond to the sum in quadrature of the statistical and systematic uncertainties of the prediction in a given bin. The lower panels show the ratio of the data to the prediction, along with the uncertainty in the ratio. A ratio outside the bounds of the axis is represented by a blue arrow. The last bin includes the event overflows. Contributions to the predicted yield are stacked in the same order as they appear in the legend.

Figure 5

Dijet invariant mass distributions for the two large-$R$ jets in four validation regions: (a) $VR5$ defined by requiring a leading jet Higgs-boson tagged with $\ge 2b$-tags and second-leading jet top-quark tagged with no associated $b$-tag, (b) $VR6$ defined by requiring a leading jet top-quark tagged with no associated $b$-tag and second-leading jet Higgs-boson tagged with $\ge 2b$-tags, (c) $VR7$ defined by requiring a leading jet top-quark tagged with $\ge 2b$-tags and second-leading jet neither top-quark tagged nor Higgs-boson tagged with $1b$-tag, and (d) $VR8$ defined by requiring a leading jet neither top-quark tagged nor Higgs-boson tagged with $1b$-tag and second-leading jet top-quark tagged with $\ge 2b$-tags. The predicted distributions include the estimated backgrounds and a hypothetical $T$-quark signal with $m_T=1.6\mathrm{TeV}$ and ${\kappa }_T=0.5$. The blue hashed lines correspond to the sum in quadrature of the statistical and systematic uncertainties of the prediction in a given bin. The lower parts of each panel show the ratio of the data to the prediction, along with the uncertainty in the ratio. A ratio outside the bounds of the axis is represented by a blue arrow. The last bin includes the event overflows. Contributions to the predicted yield are stacked in the same order as they appear in the legend.

Figure 6

Dijet invariant mass distributions in (a) the $SR$ and (b) the $NR$ before the fit of the signal and background model to the data. A $T$-quark hypothesis with $m_T=1.6\mathrm{TeV}$ and ${\kappa }_T=0.5$ is used in these plots. The blue hashed lines correspond to the sum in quadrature of the statistical and systematic uncertainties of the prediction in a given bin. The lower panels show the ratio of the data to the prediction, along with the uncertainty in the ratio. A ratio outside the bounds of the axis is represented by a blue arrow. The last bin includes the event overflows. Contributions to the predicted distributions are stacked in the same order as they appear in the legend.

Figure 7

Dijet invariant mass distributions for (a) the $SR$ and (b) the $NR$ showing the results of the model when fitted to the data. A $T$-quark hypothesis with $m_T=1.6\mathrm{TeV}$ and ${\kappa }_T=0.5$ is used in the fit. Since the central value of the fitted $T$-quark cross section is negative, the predicted $SR$ mass distribution shows no contribution from the signal. The blue hashed lines correspond to the sum in quadrature of the statistical and systematic uncertainties of the prediction. The lower panels show the ratio of the data to the prediction, along with the uncertainty in the ratio. A ratio outside the bounds of the axis is represented by a blue arrow. The last bin includes the event overflows. Contributions to the predicted distributions are stacked in the same order as they appear in the legend.

Figure 8

Observed and expected 95% C.L. upper limits on the single $T$-quark production cross section as a function of $m_T$ for values of ${\kappa }_T$ ranging from 0.1 to 1.1 in steps of 0.2 for figures (a) through (f), respectively. The green (yellow) band is the 68% (95%) confidence interval around the median expected limit, as determined using pseudo-experiments. The predicted cross sections of single $T$-quark production are shown in red.

Figure 9

Observed and expected 95% C.L. upper limits on the single $T$-quark coupling ${\kappa }_T$ as a function of $m_T$ are shown as solid and dashed lines, respectively. The green (yellow) band is the 68% (95%) confidence interval around the median expected limit, as determined using pseudo-experiments. All values of ${\kappa }_T$ above the solid line are excluded. The dashed curves represent contours of fixed $\mathrm{\Gamma }/m_T$.

Figure 10

Observed (a) and expected (b) 95% C.L. lower limits on the $T$-quark mass as a function of the $T$-quark width-to-mass ratio and the branching fraction of the $T\to Ht$ decay (${\mathrm{\Gamma }}_T$ is the $T$-quark width). The branching fractions ($\mathcal{B}$) for $T\to Ht$ and $T\to Zt$ are kept equal. The branching fraction for $T\to Wb$ is $1-\mathcal{B}(T\to Ht)-\mathcal{B}(T\to Zt)$. The color scale on the right side of each plot defines the 95% C.L. limit on the $T$-quark mass. Masses below the observed limit are excluded. The dashed white contour lines denote isolines of equal exclusion on the mass in units of TeV.