Measurement of the top quark mass using the invariant mass of lepton pairs in soft muonb-tagged events

We present the first measurement of the mass of the top quark in a sample of $t\overline{t}\to \ell \overline{\nu }b\overline{b}q\overline{q}$ events (where $\ell =e,\mu$) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muon $b$ tagging). The $p\overline{p}$ collision data used correspond to an integrated luminosity of $2{\mathrm{fb}}^{-1}$ and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from the $W$ boson of the $t\to Wb$ decay and the muon from a semileptonic $b$ decay. We fit template histograms, derived from simulation of $t\overline{t}$ events and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of $180.5\pm 12.0(\mathrm{stat})\pm 3.6(\mathrm{syst})\mathrm{GeV}/c^2$, consistent with the current world average value.