Search For Single Top Quark Production Research Articles

Search for s-channel single top-quark production in pp collisions at 8 TeV with the CMS experiment at the LHC

A search for single top-quark production in the s channel in proton-proton collisions at a centre-of-mass energy of s=8 TeV by the CMS detector at the LHC is presented. Leptonic decay modes of the top quark with an electron or muon in the final state are considered. The signal is extracted by performing a maximum-likelihood fit to the distribution of a multivariate discriminant defined using Boosted Decision Trees to separate the expected signal contribution from the background processes. Data collected in 2012, corresponding to an integrated luminosity of 19.3/fb, lead to an upper limit on the cross section times branching ratio of 11.5 pb at 95% confidence level.

Search for single top-quark production via flavour-changing neutral currents at 8TeV with the ATLAS detector.

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton–proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb^{-1} are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the t rightarrow Wb branching fraction is set. The observed 95 % CL limit is sigma _{qg rightarrow t} {,times ,} mathcal {B}(t rightarrow Wb)< {3.4},mathrm{pb} and the expected 95 % CL limit is sigma _{qg rightarrow t} times mathcal {B}(t rightarrow Wb)< {2.9},mathrm{pb}. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics kappa _{ugt}/Lambda < 5.8 times 10^{-3}, mathrm{TeV}^{-1} and kappa _{cgt}/Lambda < 13 times 10^{-3}, mathrm{TeV} and on the branching fractions mathcal {B}(t rightarrow ug) < {4.0 times 10^{-5}} and mathcal {B}(t rightarrow cg) < {20 times 10^{-5}}.

10.1007/s11450-008-2019-2

A separation of a signal of various physics processes from an overwhelming background is one of the most important problems in contemporary high-energy physics. The application of various multivariate statistical methods, such as the neural-network method, has become one of the popular steps toward optimizing relevant analyses. The choice of optimum variables that would disclose distinctions between a signal and a background is one of the important elements in the application of neural networks. A universal method for determining an optimum set of such kinematical variables is described in the present article. The method is based on an analysis of Feynman diagrams contributing to signal and background processes. This method was successfully implemented in searches for single top-quark production with the D0 detector (Tevatron, Fermilab) in analyzing Run I and Run II data. Brief recommendations concerning an optimum implementation of the neural-network method in physics analysis are given on the basis of experience gained in searches for single top-quark production with the D0 detector.

Method of “optimum observables” and implementation of neural networks in physics investigations

A separation of a signal of various physics processes from an overwhelming background is one of the most important problems in contemporary high-energy physics. The application of various multivariate statistical methods, such as the neural-network method, has become one of the popular steps toward optimizing relevant analyses. The choice of optimum variables that would disclose distinctions between a signal and a background is one of the important elements in the application of neural networks. A universal method for determining an optimum set of such kinematical variables is described in the present article. The method is based on an analysis of Feynman diagrams contributing to signal and background processes. This method was successfully implemented in searches for single top-quark production with the D0 detector (Tevatron, Fermilab) in analyzing Run I and Run II data. Brief recommendations concerning an optimum implementation of the neural-network method in physics analysis are given on the basis of experience gained in searches for single top-quark production with the D0 detector.

Searches for physics beyond the standard model at HERA

This article summarizes the results of searches for new physics beyond the standard model at HERA performed by the ZEUS and H1 experiments. Recent results are presented on direct searches for electron–quark resonant states such as leptoquarks or squarks in R-parity violating SUSY, on indirect searches for new physics leading to eeqq contact interactions and on searches for single top-quark production through anomalous flavour-changing coupling in neutral-current processes. Most of the results shown are based on the full statistics collected before the luminosity upgrade of HERA and some of them were also updated with newer results made available after the workshop.