Prospects of measuring the CKM matrix element [formula omitted] at the LHC

Abstract

We study the prospects of measuring the CKM matrix element |Vts| at the LHC with the top quarks produced in the processes pp→tt¯X and pp→t/t¯X, and the subsequent decays t→W+s and t¯→W−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 |Vtb| 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% accuracy for the signal (t→Ws) at a background (t→Wb) rejection by a factor 103 (required due to the anticipated value of the ratio |Vts|2/|Vtb|2≃1.6×10−3) can be achieved at the LHC@14 TeV with an integrated luminosity of 10 fb−1.