Search for single top tW associated production in the dilepton decay channel in pp collisions at sqrt s =7 TeV

This note describes the first study of the single top W-associated production (tW) carried out with the CMS experiment in 7 TeV proton-proton collisions using an integrated luminosity of 2.1~fb$^{-1}$, recorded during the first data-taking period of 2011. The search is performed in the dileptonic final states with a selection based on kinematical properties and b-tag information. The contribution of the Z+jets processes to the background is estimated with a data driven method. Two ${\rm t\bar{t}}$ dominated control regions are defined to constrain the ${\rm t\bar{t}}$ normalization in the signal region. An excess of events over the expected background is observed with a significance of 2.7$\sigma$, while the expected significance of the tW signal is $1.8\pm0.9\sigma$. Assigning the excess to events from tW production the extracted cross section is in agreement with the standard model expectation.