Abstract
This Letter presents a search at the LHC for s-channel single top-quark production in proton–proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3 fb−1. Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single top-quark production cross-section of 14.6 pb at the 95% confidence level. The fit gives a cross-section of σs=5.0±4.3 pb, consistent with the Standard Model expectation.
Highlights
At hadron colliders, top-quarks are produced predominantly in pairs via the strong interaction but can be produced singly through the electroweak interaction
At the Large Hadron Collider (LHC), the production cross-section of single tops in proton–proton collisions was measured both in the t-channel and in association with a W boson by the CMS [8,9,10,11] and ATLAS [12,13,14] Collaborations
The multivariate classifier is trained using simulated events selected with relaxed criteria: the threshold on the W boson transverse mass is lowered to 30 GeV and the selection on the transverse momentum of the b-jet associated with the best top-quark candidate is not applied
Summary
Top-quarks are produced predominantly in pairs (tt) via the strong interaction but can be produced singly through the electroweak interaction. At the Large Hadron Collider (LHC), the production cross-section of single tops in proton–proton (pp) collisions was measured both in the t-channel and in association with a W boson by the CMS [8,9,10,11] and ATLAS [12,13,14] Collaborations. For the s-channel process, the total inclusive cross-section for pp collisions at a centre-of-mass energy of 8 TeV is predicted to be σs = 5.61 ± 0.22 pb This assumes a top-quark mass of 172.5 GeV and uses the MSTW2008 [18] NNLO set of parton distribution functions (PDFs). The electron and muon channels are merged independently of the lepton charge in order to measure the combined production cross-section of top-quarks and top-antiquarks
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