Abstract
Measurements are presented of the single top quark production in proton-proton collisions at the LHC at a centre-of-mass energy of 8 TeV, using data collected with the CMS experiment during 2012. The first analysis, in the t-channel single top production mode, considers decay channels where the W comes from the top decays into electron-neutrino or muon-neutrino. It makes use of a template fit exploiting the pseudorapidity distribution of the recoil jet and the reconstructed top quark mass, using background estimates determined from control samples in data. The measurement of top/antitop cross section ratio is also presented.The second analysis, measuring the associated production cross section of single top quark and W boson, considers final states in which the associated W boson, as well as the one originating from the top quark, decay leptonically. Multivariate methods are used to separate the signal from the topologically similar top pair production background. Multivariate techniques are also adopted in the search for s-channel single top quark production finally reported, in order to discriminate the very small signal from the huge background processes.
Highlights
It should pass a tight threshold on the track counting high-purity (TCHP) btagging algorithm [9]
In conclusion the analysis reported the following cross section and charge ratio measurements: σt−ch. = 83.6 ± 2.3 ± 7.4 pb Rt−ch. = σt−ch.(t)/σt−ch.(t) =
The analysis considers the dilepton decay channels, so the final state is composed of two oppositely charged muons or electrons, a b-jet and two neutrinos
Summary
The top quark, discovered at the Tevatron proton antiproton collider in 1995 [1, 2], can be produced via strong interaction in association with the top anti-quark and via electroweak interaction as single top. In addition to testing the SM prediction at the electroweak scale, the single top production allows to probe directly the Wtb vertex, potentially detecting non-SM couplings (t and tW channels) and is sensitive to models of new physics involving non-SM mediators (s channel) In this contribution the precision measurement of the t-channel inclusive cross section and top over anti-top production charge ratio, the first observation of the tW associated production and the search for the s-channel single top production are presented. The systematic uncertainties of the measurement (divided into ’instrumental’ as jet energy scale and resolution, b-tagging efficiencies, etc., and ’theory’ ones, as signal generator, background models and normalisation, renormalisation and factorisation scales, PDFs, etc.) are determined by use of pseudo-experiments which take into account the effect of the systematic sources on the |η j | distribution and on the event yield of the different processes.
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