Top quark | EPFL Graph Search

The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_{t} is very close to unity; in the Standard Model of particle physics, it is the largest (strongest) coupling at the scale of the weak interactions and above. The top quark was discovered in 1995 by the CDF and DØ experiments at Fermilab. Like all other quarks, the top quark is a fermion with spin  1 /2 and participates in all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of + 2 /3 e. It has a mass of 172.76GeV/c2, which is close to the rhenium atom mass. The antiparticle of the top quark is the top antiquark (symbol: , sometimes called antitop quark or simply antitop), w

A Strong Sector at the LHC: Top Partners in Same-Sign Dileptons

Jan Mrazek, Andrea Wulzer

Heavy partners of the top quark are a common prediction of many models in which a new strongly-coupled sector is responsible for the breaking of the electroweak symmetry. In this paper, we investigate their experimental signature at the LHC, focusing on the particularly clean channel of same-sign dileptons. We show that, thanks to a strong interaction with the top quark which allows them to be singly produced at a sizable rate, the top partners will be discovered at the LHC if their mass is below 1.5 TeV, higher masses being possible in particularly favorable (but plausible) situations. Since the partners are expected to be lighter in both the Higgsless and composite-Higgs scenarios, then one of same-sign dileptons is found to be a very promising channel in which these models could be tested. We also discuss several experimental signatures which would allow, after the discovery of the excess, to attribute it uniquely to the top partners production and to measure the relevant physical parameters, i.e. the top partners' masses and couplings. We believe that our results constitute a valid starting point for a more detailed experimental study.

2010

Automation of One-Loop Computations for Scattering Amplitudes and Applications to Collider Phenomenology

Valentin Hirschi

This thesis presents the automation of loop computations for the calculation of Next-to-Leading Order contributions to theoretical predictions for particle colliders. We start with the general techniques for performing such predictions and how they can be expressed as perturbative expansions in the coupling constants controlling the strength of particle interactions. This leads to the discussion of the subtleties arising when considering higher order corrections, in particular the methods employed for isolating and canceling the infrared divergences occurring at intermediate steps of the computation. We then introduce the Passarino-Veltman and Ossola-Papadopolous-Pittau loop reduction algorithms. The latter is used by the computer code MadLoop that we wrote specifically for the automation of loop computations. The main part of the thesis focuses on the description of this program, starting with its original loop diagram generation algorithm and how it is embedded within the MadGraph5 environment. An initiation to the usage of the code is given, followed by a discussion of its optimizations where particular attention is paid to the implementation of the open-loop technique. Great details about the validation of MadLoop results against those of other codes are given in appendix B for specific kinematic configurations. We also list in the main text the total rates obtained for various processes. Quantitative information on the runtime speed and numerical stability performances are presented for many processes, each representative of a certain class of complexity. This serves as a comparison benchmark, and shows that realistic studies can be performed for any 2 → 3 and most 2 → 4 processes in the Standard Model. We finish by providing two examples of phenomenology study at the Large Hadron Collider using the tools we developed. The first treats the production of a scalar or pseudo-scalar in association with a top quark pair while the second addresses the tri-boson production channel Z W+ W− with MadSpin simulating the subsequent decay to leptons. We conclude with some insights on MadLoop prospects.

EPFL2013

Scalar and pseudoscalar Higgs production in association with a top-antitop pair

Valentin Hirschi

We present the calculation of scalar and pseudoscalar Higgs production in association with a top-antitop pair to the next-to-leading order (NLO) accuracy in QCD, interfaced with parton showers according to the MC@NLO formalism. We apply our results to the cases of light and very light Higgs boson production at the LHC, giving results for total rates as well as for sample differential distributions, relevant to the Higgs, to the top quarks, and to their decay products. This work constitutes the first phenomenological application of aMC@NLO, a fully automated approach to complete event generation at NLO in QCD. (C) 2011 Elsevier B.V. All rights reserved.

2011

Automation of One-Loop Computations for Scattering Amplitudes and Applications to Collider Phenomenology

Valentin Hirschi

EPFL2013