Search for 4th Generation at CMS

The CMS experiment has performed a feasibility study of a search for a 4th generation quark $b^\prime$. For light $b^\prime$ masses, the search is based on the process $b^\prime {\overline{b^\prime}} \rightarrow cWbZ$ with a trilepton final state signature. One can expect to find evidence for the existence of a light $b^\prime$ quark with 1 fb$^{-1}$ of accumulated data. In the absence of a signal, a 95\% C.L. exclusion limit can be set for the $b^\prime$ production cross section for various $b^\prime$ masses. If the mass of the $b^\prime$ quark is above the $m_t + m_W \approx$ 255 GeV/c$^2$ threshold, the decay $b^\prime \rightarrow tW$ is allowed and the search is performed using same-sign dilepton and trilepton events. With a data set of 200 pb$^{-1}$, a $b^\prime {\overline{b^\prime}} \rightarrow t {\overline{t}}W^{-}W^{+}$ signal can be discovered with a significance of 3.7 standard deviations for M$(b^\prime)\approx$ 400 GeV/c$^2$. In case no signal is observed, a $b^\prime$ quark with a mass less than 485 GeV/c$^2$ can be excluded at the 95\% confidence level.