Search for resonant top quark plus jet production in $t\overline{t}\mathbf{+}\mathrm{\text{jets}}$ events with the ATLAS detector in $pp$ collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$

This paper presents a search for a new heavy particle produced in association with a top or antitop quark. Two models in which the new heavy particle is a color singlet or a color triplet are considered, decaying, respectively, to $\overline{t}q$ or $tq$, leading to a resonance within the $t\overline{t}+\mathrm{\text{jets}}$ signature. The full 2011 ATLAS $pp$ collision data set from the LHC ($4.7{\mathrm{fb}}^{-1}$) is used to search for $t\overline{t}$ events produced in association with jets, in which one of the $W$ bosons from the top quarks decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation, and a new particle with mass below 430 Gev for both $W^{\prime }$ boson and color triplet models is excluded at 95% confidence level, assuming unit right-handed coupling.

Figure 1

Example production and decay Feynman diagrams for the (a) $W^{\prime }$ and (b) $\varphi$ models.

Figure 2

The leading jet $p_{\mathrm{T}}$ in the four-jet $t\overline{t}$ control region (a), and $m_{tj}$ in the five-jet $t\overline{t}$ control region (b). The example signal-only distributions are overlaid for comparison, where unit coupling for the new physics process is assumed. The total uncertainty shown on the ratio includes both statistical and systematic effects. The “other” background category includes single top production, diboson production, and multijet events.

Figure 3

Expected and observed distribution of $m_{tj}$ in the $W+\mathrm{\text{jets}}$ control region. The example signal-only distributions are overlaid for comparison, where unit coupling for the new physics process is assumed. The total uncertainty shown on the ratio includes both statistical and systematic effects. The other background category includes single top production, diboson production, and multijet events.

Figure 4

Expected and observed distributions of (a) $m_{tj}$ and (b) $m_{\overline{t}j}$ in the signal region. The example signal distributions assume unit coupling for the new physics process. The total uncertainty shown on the ratio includes both statistical and systematic effects. The other background category includes single top production, diboson production, and multijet events.

Figure 5

Expected and observed 95% C.L. upper limits on the (a) $W^{\prime }$ and (b) $\varphi$ model cross sections. The CDF result is documented in Ref. [10]. The $W^{\prime }$ cross sections are NLO calculations, and the $\varphi$ cross sections are LO calculations.

Figure 6

Expected and observed 95% C.L. upper limits on the (a) $W^{\prime }$ and (b) $\varphi$ model cross sections assuming a cross section that scales with $g_R^2$. The hatched area shows the region of parameter space excluded by this search at 95% C.L. The CDF result is documented in Ref. [10]. The $W^{\prime }$ cross sections are NLO calculations, and the $\varphi$ cross sections are LO calculations. The region favored by the Tevatron $A_{FB}$ and ${\sigma }_{t\overline{t}}$ measurements is shown as the dark band [42].