Role In Electroweak Symmetry Breaking Research Articles

Machine learning techniques in searches for tt̄h in the h → bb̄ decay channel

Study of the production of pairs of top quarks in association with a Higgs boson is one of the primary goals of the Large Hadron Collider over the next decade, as measurements of this process may help us to understand whether the uniquely large mass of the top quark plays a special role in electroweak symmetry breaking. Higgs bosons decay predominantly to bb̄, yielding signatures for the signal that are similar to tt̄ + jets with heavy flavor. Though particularly challenging to study due to the similar kinematics between signal and background events, such final states (tt̄ bb̄) are an important channel for studying the top quark Yukawa coupling. This paper presents a systematic study of machine learning (ML) methods for detecting tt̄h in the h → bb̄ decay channel. Among the eight ML methods tested, we show that two models, extreme gradient boosted trees and neural network models, outperform alternative methods. We further study the effectiveness of ML algorithms by investigating the impact of feature set and data size, as well as the structure of the models. While extended feature set and larger training sets expectedly lead to improvement of performance, shallow models deliver comparable or better performance than their deeper counterparts. Our study suggests that ensembles of trees and neurons, not necessarily deep, work effectively for the problem of tt̄h detection.

The shape of spins

After the discovery of a Higgs-like particle at the LHC, the determination of its spin quantum numbers across different channels will be the next step in arriving at a more precise understanding of the new state and its role in electroweak symmetry breaking. Event shape observables have been shown to provide extremely sensitive observables for the discrimination of the scalar Higgs boson's CP quantum numbers as a consequence of the different radiation patterns of Higgs production via gluon fusion vs. weak boson fusion in the $pp\to X+2j$ selection. We show that a similar strategy serves to constrain the spin quantum numbers of the discovered particle as a function of the involved couplings. We also discuss the prospects of applying a similar strategy to future discoveries of Higgs-like particles.

High energyWWscattering at the LHC with the matrix element method

Perhaps the most important question in particle physics today is whether the boson with mass near 125 GeV discovered at the Large Hadron Collider (LHC) is the Higgs Boson of the Standard Model. Since a particularly important property of the Standard Model Higgs is its role in unitarizing longitudinal WW scattering, we study the ability of the LHC to probe this process in the case of same-sign W pair production. We find that the use of the Matrix Element Method increases the significance with which the Higgs sector can be probed in this channel. In particular, it allows one to distinguish between a light and heavy SM Higgs in this channel alone with a high degree of significance, as well as to set important limits in the parameter space of the Two Higgs Doublet Model and the Strongly-Interacting Light Higgs Model with less than 200/fb at the 14-TeV LHC, thus providing crucial information about the putative Higgs boson's role in electroweak symmetry breaking.

WWscattering in the era of post-Higgs-boson discovery

More evidences have now been collected at the Large Hadron Collider suggesting the new 125 - 126 GeV boson is likely the long sought Higgs boson in the standard model. One pressing question continued being asked by theorists is whether this Higgs boson is a lone player responsible for the full electroweak symmetry breaking. Current data still allow room for additional Higgs bosons or some other UV physics that may play a partial role in electroweak symmetry breaking as well. We use the WW scattering to investigate such a possibility, using the two-Higgs-doublet model as a prototype. The WW scattering becomes strong when the extra Higgs bosons are very heavy. We study the sensitivity of the partially strong WW scattering signals at the 13 TeV Large Hadron Collider.

Dynamical electroweak symmetry breaking with superheavy quarks and2+1composite Higgs model

Recently, a new class of models describing the quark mass hierarchy has been introduced. In this class, while the t quark plays a minor role in electroweak symmetry breaking (EWSB), it is crucial in providing the quark mass hierarchy. In this paper, we analyze the dynamics of a particular model in this class, in which the b' and t' quarks of the fourth family are mostly responsible for dynamical EWSB. The low energy effective theory in this model is derived. It has a clear signature, a 2 + 1 structure of composite Higgs doublets: two nearly degenerate \Phi_{b'} and \Phi_{t'}, and a heavier top-Higgs resonance \Phi_t \sim \bar{t}_{R}(t,b)_L. The properties of these composites are described in detail, and it is shown that the model satisfies the electroweak precision data constraints. The signatures of these composites at the Large Hadron Collider are briefly discussed.

THE "TOP PRIORITY" AT THE LHC

The LHC (Large Hadron Collider) will be a top-quark factory. With 80 million pairs of top quarks and an additional 34 million single tops produced annually at the designed high luminosity, the properties of this particle will be studied to a great accuracy. The fact that the top quark is the heaviest elementary particle in the Standard Model with a mass right at the electroweak scale makes it tempting to contemplate its role in electroweak symmetry breaking, as well as its potential as a window to unknown new physics at the TeV scale. We summarize the expectations for top-quark physics at the LHC, and outline new physics scenarios in which the top quark is crucially involved.

Searches for physics beyond the Standard Model in $t\bar{t}$ events

The top quark is currently only observed at the Tevatron, where it is mainly produced in $t\bar{t}$ pairs. Due to the very high mass of the top quark compared to the other quarks and the gauge bosons, it is expected to play a special role in electroweak symmetry breaking. Therefore it might be especially sensitive to new physics. Measurements of various production and decay quantities of the top quark could lead to discoveries of physics beyond the standard model. Several such measurements were performed by the CDF collaboration during Run1 of the Tevatron. These measurements and first results from CDF in Run2 are presented. PACS: 14.65.Ha Properties of the Top Quark – 12.60.-i Models beyond the Standard Model

Mass enhancement and critical behavior in technicolor theories.

Quark and lepton masses in technicolor theories can be enhanced if the high-energy, extended technicolor (ETC) interactions play an important role in electroweak symmetry breaking. This happens when the ETC coupling and the technicolor gauge coupling at high energies lie close to a certain critical line. The enhancement has been associated with the existence of composite scalars made mainly of technifermions, with masses small compared to the ETC scale. The initial study of these states was carried out with the technicolor gauge coupling neglected. In this paper we investigate the properties of such scalars including the technicolor gauge interactions. We find that for realistic values of the gauge coupling, the scalars will not be narrow resonances. Mass and width estimates are made and some comments about the phenomenology of these states are included.

Aspects of dynamical electroweak symmetry breaking

In technicolor models it has recently been suggested that dynamics at scales much higher than the electroweak scale can play a significant role in electroweak symmetry breaking, and that these effects can lead to a heavy top quark while alleviating the flavor changing neutral current problems endemic to extended technicolor theories. In this paper we formulate the conditions necessary for this hierarchy to occur. We show that, if extended technicolor interactions are to play a role in electroweak symmetry breaking and in particular if they are to enhance fermion masses above their values in the usual weak extended technicolor scenario, then the chiral symmetry breaking phase transition must be second order. We also show that the effective low-energy theory below the ETC scale must include both technicolor and a light composite scalar with large couplings to ordinary fermions and technifermions.