Measurement of the tt¯ cross section in pp¯ collisions at s=1.96 TeV using dilepton events with a lepton plus track selection

Le résumé fourni par la source

This paper reports a measurement of the cross section for the pair production of top quarks in $p\overline{p}$ collisions at $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ at the Fermilab Tevatron. The data were collected from the CDF run II detector in a set of runs with a total integrated luminosity of $1.1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. The cross section is measured in the dilepton channel, the subset of $t\overline{t}$ events in which both top quarks decay through $t\ensuremath{\rightarrow}Wb\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\nu}b$, where $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, or $\ensuremath{\tau}$. The lepton pair is reconstructed as one identified electron or muon and one isolated track. The use of an isolated track to identify the second lepton increases the $t\overline{t}$ acceptance, particularly for the case in which one $W$ decays as $W\ensuremath{\rightarrow}\ensuremath{\tau}\ensuremath{\nu}$. The purity of the sample may be further improved at the cost of a reduction in the number of signal events, by requiring an identified $b$ jet. We present the results of measurements performed with and without the request of an identified $b$ jet. The former is the first published CDF result for which a $b$-jet requirement is added to the dilepton selection. In the CDF data there are 129 pretag $\mathrm{\text{lepton}}+\mathrm{\text{track}}$ candidate events, of which 69 are tagged. With the tagging information, the sample is divided into tagged and untagged subsamples, and a combined cross section is calculated by maximizing a likelihood. The result is ${\ensuremath{\sigma}}_{t\overline{t}}=9.6\ifmmode\pm\else\textpm\fi{}1.2(\mathrm{stat}{)}_{\ensuremath{-}0.5}^{+0.6}(\mathrm{sys})\ifmmode\pm\else\textpm\fi{}0.6(\mathrm{lum})\text{ }\text{ }\mathrm{pb}$, assuming a branching ratio of $\mathrm{BR}(W\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\nu})=10.8%$ and a top mass of ${m}_{t}=175\text{ }\text{ }\mathrm{GeV}/{c}^{2}$.

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