Discovery of the Higgs Boson

Aleandro Nisati, Vivek Sharma

• Publisher: WSPC
• Year: 2016
• eBook ISBN: 9789814425469
• Topic: Biological Sciences
• Subtopic: Science Research & Methodology
• Index: Biological Sciences

The recent observation of the Higgs boson has been hailed as the scientific discovery of the century and led to the 2013 Nobel Prize in physics. This book describes the detailed science behind the decades-long search for this elusive particle at the Large

Chapter 1

The Higgs boson in the Standard Model

Abdelhak Djouadi* and Massimiliano Grazzini^†

*Laboratoire Physique Théorique
U. Paris Sud, 91405V Orsay, France
^†Physik-Institut, Universität Zürich
CH-8057 Zürich, Switzerland

The major goal of the Large Hadron Collider is to probe the electroweak symmetry breaking mechanism and the generation of the elementary particle masses. In the Standard Model this mechanism leads to the existence of a scalar Higgs boson with unique properties. We review the physics of the Standard Model Higgs boson, discuss its main search channels at hadron colliders and the corresponding theoretical predictions. We also summarize the strategies to study its basic properties.

1.Introduction

Establishing the precise mechanism of the spontaneous breaking of the electroweak gauge symmetry has been the central focus in high energy physics for many decades and is certainly the primary goal of the LHC. In the Standard Model (SM),^1–6 the theory that describes the electromagnetic, weak and strong interactions, explicit mass terms for the electroweak gauge bosons and the fermions are not allowed by the SU(2)_L × U(1)_Y gauge symmetry. Electroweak symmetry breaking (EWSB) is achieved via the Brout–Englert–Higgs mechanism,^7–9 in which a scalar field, a doublet under weak isospin, acquires a non-zero vacuum expectation value, thus providing masses for the electroweak gauge bosons and for the fermions. One of the four degrees of freedom of the original scalar field corresponds to a physical scalar particle: the Higgs boson.

Clearly, the discovery of this last missing piece of the SM has profound importance. In fact, despite of its phenomenal success in explaining the precision data, the SM could not be considered to be completely established until the Higgs boson was observed experimentally. The discovery in July 2012 of a scalar resonance compatible with the SM Higgs boson by the ATLAS and CMS collaborations^10,11 is, in this respect, historic as it crowns the SM as the correct theory of fundamental particles and interactions among them, at least up to the Fermi energy scale.

In addition, the fundamental properties of the Higgs particle such as its mass, spin and other quantum numbers, as well as its couplings to various matter and gauge particles and its self-couplings must be determined in the most precise way. These studies are important in order to achieve further clarity into the dynamics of the EWSB mechanism. The many important questions which one would like answered by probing the Higgs boson properties are: does the dynamics involve new strong interactions and is the Higgs a composite field? If elementary Higgs particles indeed exist in nature, how many fields are there and in which gauge representations do they appear? Does the EWSB sector involve sizable CP violation? Theoretical realizations span a wide range of scenarios and a complete discussion of Higgs physics thus touches upon almost all the issues under active investigation in theoretical and experimental particle physics. Nevertheless, in this chapter, only the phenomenology of the Higgs sector in the SM will be discussed (for a detailed review, see e.g. Ref. [12]). After summarizing the EWSB mechanism in the SM in Sec. 2 and the pre-LHC theoretical and experimental constraints on the Higgs boson mass in Sec. 3, its decay modes, production cross sections and detection channels at hadron colliders will be described in Secs. 4, 5 and 6, respectively. The theoretical predictions for differential distributions and the Monte Carlo generators used in the experimental analyses are briefly reviewed in Secs. 7 and 8. The strategies to measure the fundamental properties of the Higgs boson are finally summarized in Sec. 9.

2.Electroweak symmetry breaking and mass generation

The Standard Model is based on a very powerful principle: gauge symmetry. The fields corresponding to the particles, as well as the particle interactions, are invariant with respect to local transformations of an internal symmetry group. The model is a generalization of quantum electrodynamics (QED), the relativistic quantum theory of electromagnetism which describes the interaction of electrically charged particles through the exchange of photons. The QED Lagrangian is invariant under phase transformations on the charged fermionic fields collectively denoted by ψ,

where x_μ = (t,

) is the spacetime four-vector and Q is the fermion electric charge. These transformations are called gauge or local transformations as the parameter θ depends on x_μ. The photon field mediating the interaction and described by the four-vector A_μ =(A₀,

), transforms as

where ∂_μ is the derivative with respect to x_μ. The gauge transformation group is noted U(1)_Q for the group of unitary matrices of dimension one and conserves the quantum number that is the electric charge Q.

In fact, the interaction of charged fermions via the exchange of photons can be induced in a minimal way in the Lagrangian density of the free fermion and photon systems, by substituting the usual derivative ∂_μ by the so called covariant derivative D_μ

∂_μ − iQA_μ.

In the SM, the symmetry...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Dedication
• Contents
• Preface
• 1. The Higgs boson in the Standard Model
• 2. Searches for the Standard Model Higgs boson at the LEP collider
• 3. Searches for the Standard Model Higgs boson at the Tevatron collider
• 4. Overview of the Large Hadron Collider and of the ATLAS and CMS experiments
• 5. Higgs boson observation and measurements of its properties in the H → ZZ → 4l decay mode
• 6. Observation of the diphoton decay of the Higgs boson and measurements of its properties
• 7. Observation of the Higgs boson in the H → W+W−(*) → l+ νl−ν final state
• 8. Industrial Applications
• 9. Evidence for Higgs boson decays to τ leptons
• 10. Search for the Higgs boson in the bb final state at the LHC
• 11. Higgs boson search in the ZZ → llνν and llqq final states
• 12. Higgs combination and properties of the Higgs boson
• 13. Summary and outlook
• Appendix A. Statistical methods
• Appendix B. Multivariate analysis techniques
• Acknowledgments
• About the Authors