Search for Fermion-Pair Decays $Q\Qbar \to (\t\Wmp)(\tbar\Wpm)$ in Same-Charge Dilepton Events

Abstract

We report the most restrictive direct limits on masses of fourth-generation down-type quarks $b^{\prime}$, and quark-like composite fermions ($B$ or $T_{5/3}$), decaying promptly to $t W^{\mp}$. We search for a significant excess of events with two same-charge leptons ($e$, $μ$), several hadronic jets, and missing transverse energy. An analysis of data from $p\overline{p}$ collisions with an integrated luminosity of 2.7 fb$^{-1}$ collected with the CDF II detector at Fermilab yields no evidence for such a signal, setting mass limits $m_{b^{\prime}}, m_{B} >$ 338 $\mathrm{GeV}/c^2$ and $m_{T_{5/3}} >$ 365 $\mathrm{GeV}/c^2$ at 95% confidence level.

Figures (4)

• Figure 1: Missing transverse energy in events with same-charge leptons in 2.7$\hbox{,fb}^{-1}$ . The right outer-most bin includes overflow events with ${\not\!\! E_T} > 160$ GeV.
• Figure 2: Number of reconstructed jets for the expected backgrounds. The observed data and the $b^{\prime}$ (or $B$) signal are shown at the best-fit rate for $m_Q=330$ GeV/$c^2$. The fitted size and shape for the $T_{5/3}\xspace+B$ signal is nearly identical.
• Figure 3: Event displays for the observed three-jet, $\mu\mu$ event (a,b) and the five-jet, $e\mu$ event (c,d). Shown in (a) and (c) are views of the events along the beam axis; jets shown as cones, electrons as solid lines, muons as dotted lines and missing transverse energy as an arrow; lengths are proportional to $p_T$ (see Table \ref{['table:evdetails']}). Shown in (b) and (d) are views of the events in $\eta-\phi$; jets shown as open circles, electrons as filled circles and muons as dashed circles; radii are proportional to $p_T$.
• Figure 4: Theoretical cross sections for $b^{\prime}$ (or $B$) and $T_{5/3}\xspace+B$ with expected and observed 95% C.L. limits overlaid.