Rima El Kosseifi, "Search for chargino and neutralino pair production in final state with one lepton, two b-jets

Amphi (CPPM)



Abstract :
The R-parity conserving SUSY is one of the most motivated theories beyond the Standard Model.
During the LHC 8 TeV Run1, many searches were dedicated to this model. Among those, the search for
the lightest chargino and next-to-lightest neutralino (C1, N2) pair production in final states with one
lepton, two b-jets consistent with a Higgs boson and missing transverse energy. This search had neg-
ative outcome resulting in excluding C1, N2 masses up to 250 GeV for a massless lightest neutralino
(N1) at 95% C.L.. The Run1 search used 20.3/fb of 8 TeV ATLAS data collected in 2012. In this thesis,
this search with ATLAS at LHC 13 TeV Run2 is presented, it uses 36.1/fb ATLAS data collected in 2015
and 2016. This Run 2 search takes advantage of the sensitivity enhancement as the gain in the signal
cross section is higher than the gain in the cross section of the main background, ttbar. Therefore,
it provides an unprecedented sensitivity to high mass C1, N2 production. However, no evidence of
new physics is observed and tighter new limits are placed on the C1, N2 production, with significant
improvements over previous searches: C1, N2 masses up to 680 GeV for a massless neutralino N1 are
excluded at 95% C.L. The identification of jets containing B-hadrons, called b-tagging, plays a significant
role in this and many other analyses at the LHC, in particular in the Higgs measurements, as h→bb dominates
the total width (58%). The ATLAS b-tagging algorithms rely on B-hadron properties (long lifetime, high mass)
and they are based on jets-associated tracks properties. In this thesis, a study on the optimisation of
b-jet identification algorithms in ATLAS is presented as well as studies of the robustness of b-tagging
performance with various conditions of the pixel detector, being the most important sub-detector for
b-tagging. As most SUSY searches at the LHC, this analysis is carried out using Simplified Model of SUSY, i.e.
the mass spectrum is chosen by hand and the considered SUSY decays are forced (branching fractions
of 100%). This way, the experimental sensitivity to the signal can be factorized out of the actual
interpretation of the search result within realistic SUSY models. Suspect3 is one of the theory tools
utilized to map the search result into realistic SUSY models, it is a SUSY spectrum calculator which
calculates SUSY mass spectra and couplings. This thesis presents the post Higgs discovery SuSpect3
version we have been working on. This version takes advantage of the very important new Higgs
boson mass input to constrain other basic model parameter typically most important one of the soft
SUSY-breaking parameters, the top trilinear coupling, currently given as an input. The same idea
also applies to the SUSY particles mass limits resulting from the SUSY searches at the LHC. This new
Suspect3 version will facilitate generating SUSY spectra compatible with the experimental constraints,
and is also more natural given the new fundamental input now available with the Higgs boson mass,
which is moreover determined with a good accuracy.
Le jury est composé de:
Cristinel Diaconu, CPPM, Président
Mossadek Talby, CPPM, Examinateur
Steve Muanza, CPPM, Directeur de thèse
Jean-Loïc Kneur, LCC Montpellier, co-Directeur de thèse
Iacopo Vivarelli, University of Sussex, Rapporteur
Barbara Clerbaux, Université Libre de Bruxelles, Rapporteur
Sara Strandberg, University of Stockholm, Examinateur