Search for flavour-changing neutral current top quark decays $t\to Hq$ in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

G. Aad 1 M.K. Ayoub 2 A. Bassalat 2 C. Bécot 2 S. Binet 2 C. Bourdarios 2 D. Delgove 2 J-B. de Vivie De Regie 2 L. Duflot 2 M. Escalier 2 L. Fayard 2 D. Fournier 2 E.L. Gkougkousis 2 J-F. Grivaz 2 T. Guillemin 2 F. Hariri 2 S. Henrot-Versille 2 J. Hrivnac 2 L. Iconomidou-Fayard 2 M. Kado 2 A. Lounis 2 N. Makovec 2 N. Morange 2 C. Nellist 2 P. Petroff 2 L. Poggioli 2 P. Puzo 2 A. Renaud 2 D. Rousseau 2 G. Rybkin 2 A.C. Schaffer 2 E. Scifo 2 L. Serin 2 S. Simion 2 R. Tanaka 2 D. Zerwas 2 Zhongkai Zhang 2 Y. Zhao 2 Z. Barnovska 3 N. Berger 3 M. Delmastro 3 L. Di Ciaccio 3 S. Elles 3 T. Hryn’ova 3 S. Jézéquel 3 I. Koletsou 3 R. Lafaye 3 J. Leveque 3 P. Mastrandrea G. Sauvage 3 E. Sauvan 1 O. Simard 3 T. Todorov 3 E. Yatsenko 3 I. Wingerter-Seez 3 L. Alio 1 M. Barbero Y. Coadou 1 C. Diaconu 1 S. Diglio 1 F. Djama 1 O. Duccu 4, 1 L. Feligioni 1 J. Gao 1 G.D. Hallewell 1 F. Hubaut 1 S.J. Kahn 1 E. B. F. G. Knoops 1 E. Le Guirriec 1 J. Liu 1 K. Liu 1 D. Madaffari 1 K. Mochizuki 1 E. Monnier 1 S. Muanza 1 Y. Nagai 1 E. Nagy 1 P. Pralavorio 1 A. Rozanov 1 T. Serre 1 M. Talby 1 R.E. Ticse Torres 1 E. Tiouchichine 1 S. Tisserant 1 J. Toth 1 F. Touchard 1 L. Vacavant 1 S. Albrand 5 J. Collot 5 S. Crépé-Renaudin 5 P.A. Delsart 5 C. Gabaldon 5 M.H. Genest 5 J.Y. Hostachy 5 F. Ledroit-Guillon 5 A. Lleres 5 A. Lucotte 5 F. Malek 5 C. Monini 5 J. Stark 5 B. Trocmé 5 M. Wu 5 D. Boumediene 6 E. Busato 6 D. Calvet 6 S. Calvet 6 J. Donini 6 E. Dubreuil 6 G. Gilles 6 Ph. Gris 6 H. Liao 6 R. Madar 6 D. Pallin 6 S.M. Romano Saez 6 C. Santoni 6 D. Simon 6 T. Theveneaux-Pelzer 6 F. Vazeille 6
Abstract : A search for flavour-changing neutral current decays of a top quark to an up-type quark ($q=u, c$) and the Standard Model Higgs boson, where the Higgs boson decays to $b\bar{b}$, is presented. The analysis searches for top quark pair events in which one top quark decays to $Wb$, with the $W$ boson decaying leptonically, and the other top quark decays to $Hq$. The search is based on $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3 fb$^{-1}$. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of $b$-quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by $t\bar{t} \to WbWb$ decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the $t\to Hc$ and $t\to Hu$ branching ratios respectively. The combination of this search with other ATLAS searches in the $H \to \gamma\gamma$ and $H\to WW^*, \tau\tau$ decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the $t\to Hc$ and $t\to Hu$ branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the $|\lambda_{tcH}|$ and $|\lambda_{tuH}|$ couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds on $tqH$ interactions measured so far.
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Article dans une revue
Journal of High Energy Physics, Springer Verlag (Germany), 2015, 12, pp.061. 〈10.1007/JHEP12(2015)061〉
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Soumis le : mardi 22 septembre 2015 - 09:53:24
Dernière modification le : mardi 22 mai 2018 - 21:48:10

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G. Aad, M.K. Ayoub, A. Bassalat, C. Bécot, S. Binet, et al.. Search for flavour-changing neutral current top quark decays $t\to Hq$ in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector. Journal of High Energy Physics, Springer Verlag (Germany), 2015, 12, pp.061. 〈10.1007/JHEP12(2015)061〉. 〈in2p3-01202985〉

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