Searches for flavour violation and flavour changing neutral currents in top quark final states

N. M. Köhler
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Abstract

The remarkably large integrated luminosity collected by the ATLAS detector at the highest protonproton collision energy provided by the Large Hadron Collider (LHC) allows to probe the presence of new physics thatmight breakwell established symmetries or enhance extremely rare processes in the StandardModel (SM) of particle physics. Two such significant examples are lepton universality and Flavour Changing Neutral Currents (FCNC). Recent measurements involving B-meson decays sparked renewed interest in testing lepton universality between tau and light leptons because of observed deviations at the four-standard-deviation level. By selecting events with two opposite sign leptons (muon pairs and electron-muon pairs) and at least two b-tagged jets, a highly pure sample of top-quark pair decays is assembled and used to extract a large unbiased sample of W bosons decaying to leptons down to low transverse momenta. A fit to the two dimensional distribution of the transverse momentum and the transverse impact parameter of the lepton is then used to differentiate between leptons originating directly from the W boson and those resulting from the W-boson-to-tau-lepton decay chain. This results in a precise measurement of the ratio between the probability for the W boson decay to tau to the probability for its decay to muon. On the other hand, SM FCNC involving the top-quark decay to another up-type quark and a neutral boson are so small that any measurable branching ratio for such a decay is an indication of new physics. Two searches for FCNC couplings are presented. First, the search for the FCNC coupling between the top quark and the Higgs boson uses events with one electron or muon, missing energy and three or four jets to search for both the production of a single top-quark in association with a Higgs boson or the production of a top-quark pair with one top quark decaying to a Higgs boson and an up or a charm quark. The results are interpreted also in terms of effective field theory couplings describing the tqH vertex. Finally, 81 fb−1 of integrated luminosity are used to search for FCNC via the coupling of a top quark, a photon and an up or charm quark in events with one photon, one lepton (electron or muon), one b-tagged jet and missing transverse momentum.
在顶夸克最终态中寻找味道违背和改变味道的中性电流
在大型强子对撞机(LHC)提供的最高质子-质子碰撞能量下,ATLAS探测器收集到的非常大的综合光度允许探测新物理的存在,这些新物理可能会打破已经建立的对称性,或者增强粒子物理标准模型(SM)中极其罕见的过程。两个这样重要的例子是轻子普适性和风味变化中性电流(FCNC)。由于在四标准差水平上观察到的偏差,最近涉及b介子衰变的测量引发了对测试tau和轻子之间轻子普适性的新兴趣。通过选择具有两个相反符号的轻子(介子对和电子-介子对)和至少两个b标记喷流的事件,组装了一个高度纯的顶夸克对衰变样本,并用于提取一个大的无偏W玻色子衰变到轻子的低横向动量样本。然后利用横向动量的二维分布和轻子的横向冲击参数的拟合来区分直接由W玻色子产生的轻子和由W玻色子到τ轻子衰变链产生的轻子。这使得W玻色子衰变为tau的概率与衰变为μ子的概率之间的比值得到了精确的测量。另一方面,涉及顶夸克衰变到另一个上型夸克和中性玻色子的SM FCNC是如此之小,以至于这种衰变的任何可测量的分支比都是新物理学的标志。给出了FCNC耦合的两种搜索方法。首先,寻找顶夸克和希格斯玻色子之间的FCNC耦合,使用带有一个电子或μ子、缺失能量和三个或四个喷流的事件来寻找与希格斯玻色子相关的单个顶夸克的产生,或由一个顶夸克衰变成希格斯玻色子和一个up或粲夸克的顶夸克对的产生。这些结果也用描述tqH顶点的有效场论耦合来解释。最后,利用81 fb−1的积分光度,通过一个顶夸克、一个光子和一个up夸克或粲夸克的耦合,在一个光子、一个轻子(电子或介子)、一个b标记射流和缺失横向动量的事件中寻找FCNC。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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