Prospects of measuring the CKM matrix element vertical stroke Vts vertical stroke at the LHC

We study the prospects of measuring the CKM matrix element vertical stroke Vts vertical stroke at the LHC with the top quarks produced in the processes pp→t anti tX and pp→t/ anti tX, and the subsequent decays t→W+s and anti t→W- anti s. To reduce the jet activity in top quark decays, we insist on tagging the W± leptonically, W±→l±νl (l = e,μ,τ), and analyse the anticipated jet profiles in the signal process t→Ws and the dominant background from the decay t→Wb. To that end, we analyse the V0 (K0 and Λ) distributions in the s- and b-quark jets concentrating on the energy and transverse momentum distributions of these particles. The V0s emanating from the t→Wb branch have displaced decay vertexes from the interaction point due to the weak decays b→c→s and the b-quark jets are rich in charged leptons. Hence, the absence of secondary vertexes and of the energetic charged leptons in the jet provide additional (b-jet vs. s-jet) discrimination in top quark decays. These distributions are used to train a boosted decision tree (BDT), a technique used successfully in measuring the CKM matrix element vertical stroke Vtb vertical stroke in single top production at the Tevatron. Using the BDT classifier, and a variant of it called BDTD, which makes use of decorrelated variables, we calculate the BDT(D)-response functions corresponding to the signal (t→Ws) and background (t→Wb). Detailed simulations undertaken by us with the Monte Carlo generator PYTHIA are used to estimate the background rejection versus signal efficiency for three representative LHC energies √(s)=7 TeV, 10 TeV and 14 TeV, of which only the analysis for the √(s)=14 TeV case is shown in detail. We argue that a benchmark with 10% signal (t→Ws) efficiency and a background (t→Wb) rejection by a factor 103 (required due to the anticipated value of the ratio vertical stroke Vts vertical stroke 2/ vertical stroke Vtb vertical stroke 2≅1.6 x 10-3) can be achieved at the LHC rate at 14 TeV with an integrated luminosity of 10 fb-1. (orig.)