At hadron colliders top quarks are predominantly produced in pairs via the flavour-conserving strong interaction, but single top-quark production can occur via charged-current electroweak processes involving a Wtb vertex. In the Standard Model of particle physics, at leading-order in QCD perturbation theory, three sub-processes contribute to single top-quark production: an exchange of a virtual W boson either in the t-channel or in the s-channel, or the associated production of a top-quark with an on-shell W boson.
In proton–proton collisions, the t-channel exchange is the dominant production process of single top-quarks. Furthermore, as a consequence of the vector–axial-vector form of the Wtb vertex in the Standard Model, the produced top-quarks are predicted to be highly polarised. Due to its short lifetime, the top quark decays before hadronisation and its spin orientation is directly accessible through the angular distributions of its decay products. Within the Standard Model the top quark decays through the electroweak interaction, and almost exclusively into a W boson and a b-quark. The W boson, which is produced as a real particle in the top-quark decay, also possesses a polarisation (or helicity state). The W boson polarisation can be accessed via angular distributions of its decay products, as well. Measuring the top-quark and W boson polarisations in t-channel single top-quark production provides a powerful and unique probe for studying the Wtb vertex in both top-quark production and decay. Physics beyond the Standard Model resulting in corrections to the Wtb vertex (anomalous couplings) would affect the measurement of the top-quark and W boson polarisations.
At the LHC run-1, the main physics analyses carried out at the LPSC to study single top-quark production in proton–proton collisions were the following:
- Cross-section and polarisation measurements in t-channel production
- Cross-section measurement in associated W+t production
- Search for s-channel production
For the LHC run-2, the physics analyses will be focused on studies of the Wtb top-quark couplings through the combination of cross-section and polarisation measurements, and their interpretation in terms of anomalous couplings and effective quantum field theory of the Standard Model.
Group members
- Annick Lleres, DR-CNRS (2008)
Past members
- Carolina Gabaldon, Post-doc Labex Enigmass (2013-2016)
- Reinhard Schwienhorst, Visitor Labex Enigmass (2014-2015)
- Arnaud Lucotte, DR-CNRS (2006-2014)
- Benoit Clément, MCF-UJF (2007-2012)
- Julien Donini, CDD-IN2P3 (2007-2011)
PhD students
- Caterina Monini (2011-2014)
- Xiaohu Sun (2010-2013)
- Thomas Delemontex (2009-2012)
- Jin Wang (2009-2012)
- Carole Weydert (2008-2011)
- Florent Chevallier (2004-2007)
Publications
- Evidence for the associated production of a W boson and a top quark in ATLAS at sqrt(s)=7 TeV, Phys. Lett. B 716 (2012) 142
- Measurement of the t-channel single top-quark production cross section in pp collisions at sqrt(s)=7 TeV with the ATLAS detector, Phys. Lett. B 717 (2012) 330
- Search for s-channel single top-quark production in proton-proton collisions at sqrt(s)=8 TeV with the ATLAS detector, Phys. Lett. B 740 (2015) 118
- Measurement of the production cross-section of a single top quark in association with a W boson at 8 TeV with the ATLAS experiment, JHEP01 (2016) 064
- Probing the Wtb vertex structure with t-channel single top-quark events in pp collisions at sqrt(s)=8 TeV with the ATLAS detector, JHEP04 (2017) 124
Theses
- Single top s-channel cross section measurement with the ATLAS detector, Caterina Monini, Université de Grenoble, 12 septembre 2014 (supervision Annick Lleres/Arnaud Lucotte), CERN-THESIS-2014-189
- Top polarization measurement in single top quark production with the ATLAS detector, Xiaohu Sun, Université de Grenoble, 1 octobre 2013 (supervision Annick Lleres), tel-00873541
- Mesure de la section efficace de production single top W+t dans le canal dilepton auprès de l'expérience ATLAS, Thomas Delemontex, Université de Grenoble, 5 octobre 2012 (supervision Arnaud Lucotte), tel-00793216
- Mesure de la section efficace de production du single top en voie-t en utilisant des arbres de décision avec ATLAS à sqrt(s)=7 TeV, Jin Wang, Université Joseph Fourier Grenoble, 29 juin 2012 (supervision Julien Donini/Annick Lleres/Cunfeng Feng), tel-00716925
- Recherche d'un boson de Higgs chargé avec le détecteur ATLAS: de la théorie à l'expérience, Carole Weydert, Université de Grenoble, 5 septembre 2011 (supervision Benoit Clément/Michael Klasen), tel-00629349
- Préparation de l'expérience ATLAS: étalonnage électronique du calorimètre électromagnétique, mesure de la polarisation des bosons W dans la décroissance des quarks top, Julien Labbé, Université Joseph Fourier de Grenoble, 1 juillet 2009 (supervision Benoit Clément/Benjamin Trocmé), tel-00420986
- Mesure de la section efficace de production de quarks top en paires dans le canal lepton+jets à D0 et à ATLAS et interprétation en terme de bosons de Higgs chargé dans ATLAS, Florent Chevallier, Université Joseph Fourier de Grenoble, 10 mai 2007 (supervision Arnaud Lucotte/Sabine Crépé), tel-00181009
