Nihal BRAHIMI, Recherche du boson de Higgs dans le canal ttH(H->bb) et mesure de la section efficace de production de ttbb avec le détecteur ATLAS au LHC / Search for the Higgs boson in the ttH(H->bb) channel and measurement of the production cross-section of ttbb with the ATLAS detector at the LHC.

jeudi 31 oct. 2019, 14:00 → 16:00 Europe/Paris

Amphi (CPPM)

Since the discovery of the Higgs boson at a mass around 125 GeV by both ATLAS and CMS collaborations in July 2012, it became crucial to measure 

its properties, such as its couplings to fermions, and unravel any deviations from the Standard Model (SM) predictions. The top quark Yukawa coupling

is the strongest in the fermionic sector and one of the best probes to shed light on phenomena beyond the SM. The Higgs boson production in association with a pair of top quarks and its subsequent decay into a pair of bottom quarks, ttH(H->bb), grants direct access to this coupling in addition to the bottom quark Yukawa coupling. Although the ttH production was recently established by both ATLAS and CMS combining several channels including ttH(H->bb), the latter has never been observed directly. The complexity of the ttH(H->bb) final state with a large number of b-jets, and the overwhelming background of the top pair production with additional jets (tt+jets), especially b-jets (ttbb), are the major challenges of this channel. Thus, b-jet identification referred to as b-tagging, and the control of the ttbb 

background are key ingredients for this analysis. This thesis presents the search for ttH(H->bb) performed with 36.1 /fb  of LHC data collected by the ATLAS detector in 2015and 2016. Novel techniques to improve the search sensitivity with full Run 2 data are also developed. Particularly, detailed studies are presented including a differential cross-section measurement of the main ttbb background. The optimization of the b-tagging algorithms in view of the High Luminosity LHC upgrade, after the replacement of the ATLAS inner detector, is also presented.

 

Membres du jury :

Aurelio JUSTE (Université Autonome de Barcelone, Espagne)

Jessica LEVEQUE (LAPP Annecy, France)

Andrea COCCARO (Université de Gênes, Italie)

Cristinel DIACONU ( CPPM - Marseille)

Chafik BENCHOUK (USTHB, Alger, Algérie)

Laurent VACAVANT (CPPM- Marseille)

Georges AAD ( CPPM - Marseille)