Search for heavy, top-like quark pair production in the dilepton final state in pp collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study searches for pair production of a heavy top-like quark $t'$ decaying to $bW$ in the dilepton final state, using 5.0 fb$^{-1}$ of pp collision data at $ oot ext{-} ext{s} = 7$ TeV. The analysis selects events with two isolated leptons, jets with two $b$-tags, and significant missing energy, and defines a signal region with $M_{ ext{lb}}^{ ext{min}} > 170$ GeV to separate $t'ar{t'}$ from SM $tar{t}$ events. Backgrounds are estimated with data-driven mistag and lepton-misidentification methods, supplemented by simulation, and the observed yield is consistent with SM expectations. No evidence for $t'ar{t'}$ is found, and 95% CL upper limits on the production cross section are set as a function of $M_{t'}$, excluding $M_{t'}<557$ GeV/$c^2$ under BR$(t' o bW)=1$ (expected 547 GeV/$c^2$). The result strengthens constraints on fourth-generation quark scenarios at the LHC.

Abstract

The results of a search for pair production of a heavy, top-like quark, t', in the decay mode (t' anti-t') to (b anti-W anti-b W) to (b anti-lepton neutrino anti-b lepton anti-neutrino) are presented. The search is performed with a data sample corresponding to an integrated luminosity of 5.0 inverse femtobarns in pp collisions at a center-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the expectation from standard model processes, and no evidence of t' anti-t' production is found. Upper limits on the production cross section as a function of t' mass are presented, and t' masses below 557 GeV/c^2 are excluded at the 95% confidence level.

Figures (2)

• Figure 1: Comparison between the data and the simulated background for $M_{\ell{b}}^{\text{min}}$. The signal region is defined by $M_{\ell{b}}^{\text{min}}$$>170$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$. The Category I simulated background yield in the signal region is scaled so that it matches the yield estimated from control regions in data, as given in Table \ref{['tab:results']}. Outside the signal region the simulated background yields are taken without rescaling. One event is observed in the signal region. The expected distribution for a ${t'}\overline{{t'}}$ signal with $M_{{t'}}$$=450$${\,\text{Ge\spaceV\space/\space}c^\text{2}}$ is also shown.
• Figure 2: The 95% CL upper limits on the production cross section of ${t'}\overline{{t'}}$ as a function of ${t'}$ mass. The observed (expected) 95% CL lower bound on $M_{{t'}}$ is 557 (547) ${\,\text{Ge\spaceV\space/\space}c^\text{2}}$.