Search for New Physics in tt ̅ Final States with Additional Heavy-Flavor Jets with the ATLAS Detector

Abstract

A Nani, por sus risas y susánimos que hacen fácil escribir una tesis y hacen maravilloso el estar juntos. ContentsIntroduction 1 IntroductionThe discovery of the Higgs boson in July 2012 by the ATLAS and CMS experiments was a milestone for high-energy physics, as the last missing piece of the Standard Model (SM) was found. Nonetheless, the mass of the Higgs boson, comparable to the electroweak energy scale, is still a puzzle difficult to explain. Radiative corrections are expected to raise the Higgs boson mass by 16 orders of magnitude, from the electroweak scale to the Planck scale. This issue, arising from the huge difference between the electroweak and the Planck scale is known as the hierarchy problem. This dissertation presents three analyses that probe the stability of the Higgs boson mass from different perspectives.The largest contribution to the radiative corrections arises from the coupling to the top quark. The large mass of the top quark and its coupling of order unity to the Higgs boson makes it a very special particle, or the only "natural" one. Since its discovery at Tevatron, the top quark has been studied extensively and its properties have been measured in detail. However, a measurement of the top-Higgs Yukawa coupling is not yet available. The top Yukawa coupling is the only coupling to the Higgs boson that can be accessed directly, in particular through the measurement of the production cross section of a Higgs boson in association with a top-antitop pair, ttH. Its production cross section is two orders of magnitude below the dominant gluon fusion process, and no evidence for this process has been observed yet. The dominant decay of the Higgs boson with a mass of 125 GeV is through a pair of b-quarks, producing a final state of tt with additional heavy-flavor jets. The first of the analyses aims to study the ttH process and to measure its production rate, from which the top Yukawa coupling can be extracted. The corroboration of the SM nature of the coupling would confirm that the Higgs boson mass is subject to large corrections from the top quark, and a mechanism to restore the observed Higgs mass has to be present.One of the proposed solutions to the hierarchy problem is the introduction of supersymmetry. The introduction of new partners for the SM particles, with spin differing by 1/2, would cancel the radiative contributions to the Higgs mass, giving an explanation for its value at the electroweak scale. At the same time, supersymmetric models can provide a good candidate for dark matter. Bosonic top-quark partners have been extensively searched for at the LHC, and although a wide range of the allowed masses for supersymmetric partners' masses was excluded, some low-mass regions remain uncovered. A search for bosonic top partners is presented targeting one of the "gaps" where supersymmetric particles have not been excluded.Although supersymmetry is a very elegant way of addressing the hierarchy problem, it is definitely not the only one. Non-supersymmetric extensions of the SM provide d...