Recherche de nouveaux quarks lourds avec l'expérience ATLAS a u LHC - Mise en oeuvre d'algorithmes d'identification de jets issus de quarks b

The hypothesis of a fourth generation of fermions - the matter particles described in the Standard Model (SM) of particle physics - is one of the simplest model of new physics still not excluded and accessible at the start of the Large Hadron Collider (LHC) - the world most powerful hadron collider since 2009. We search for the pair production of up-type t' quarks decaying to a W boson and a b-quark. The search is optimized for the high quark mass regime, for which the production can be distinguished from the top background by exploiting kinematic features of the decay products arising from the proton-proton collisions occurring at the center of the ATLAS detector. We present a novel search strategy reconstructing explicitly very high-pT W bosons from their collimated decay products. The analysis benefits from the commissioning of algorithms intended to identify jets stemming from the fragmentation of b-quarks. These algorithms are based on the precise reconstruction of the trajectory of charged particles, vertices of primary interaction and secondary vertices in jets. The b-tagging ability allows for ATLAS to improve the (re)discovery of the SM, and the sensibility to new physics. It will hence play an important role in the future of the LHC, the reason why we study the expected performance with an upgrade of the ATLAS pixel detector, called IBL and currently under construction. Our search of t' quark, using 4.7 fb^−1 of the 7 TeV data collected in 2011, has resulted in the world most stringent limit, excluding t' masses below 656 GeV, with also an interpretation in the framework of vector-like quarks.