Higgs and BSM Studies at the LHC

Higgs and BSM Studies at the LHC: History Edit

Subjects: Others

Introduction

The theory behind particle physics, called for historic reasons the standard model (SM), is based on local gauge symmetries In certain cases, broken symmetries were introduced in order to explain the fundamental experimental observations, the most important ones being parity violation and the Brout–Englert–Higgs (BEH) mechanism of spontaneous symmetry breaking. Left-handed currents could take care of parity violation observed in weak interactions, and using the BEH mechanism, the standard model could account for the masses of the elementary fermions and bosons. In spite of the excellent agreement between the experimental data and the predictions and fitting of the standard model, several mysteries stayed unsolved in particle physics. Numerous extensions for the standard model were proposed to solve those, the most popular one being supersymmetry, a broken fermion-boson symmetry. It helps to interpret the dark matter of cosmology within particle physics and also to solve the hierarchy problem, the quadratic divergence of the calculated mass of the Higgs boson in the standard model. Other problems unsolved in the framework of the standard model are the lack of antimatter in the Universe and baryogenesis, as well as neutrino oscillations. In this review, we shall try to summarize these concepts and the corresponding experimental evidence in high-energy physics.

Fundamental Particles in the Standard Model

According to the standard model, the world consists of two kinds of particles, fermions and bosons, different by their spins, intrinsic angular momenta, measured in units of ℏ, the reduced Planck constant. The fermions in general have half-integer spins:

S = \frac{1}{2}, \frac{3}{2}, \dots

, whereas the spins of the bosons are integer:

S = 0, 1, 2, \dots

The elementary (fundamental) fermions of the standard model are the leptons and the quarks of three families (see Table 1) with

S = \frac{1}{2}

; those are also called matter particles, as our matter consists of the fermions of the first family. The elementary bosons have integer spins; those mediating the three interactions, the photon, the eight gluons, and the three weak bosons have

S = 1

, whereas the Higgs boson is a scalar particle with zero spin. The LEP experiments have shown by measuring the decay width of the Z boson that only those three families exist with light enough neutrinos to allow for the Z →

ν \bar{ν}

decay process.

Table 1. Leptons and quarks, the three families of basic fermions.

T_{3}

is the third component of the weak isospin; index L stands for the left polarization of the weak isospin doublets; and the apostrophe indicates the mixed quark states.

	Family 1	Family 2	Family 3	Charge	$T_{3}$
Leptons	${(\begin{matrix} ν_{e} \\ e \end{matrix})}_{L}$	${(\begin{matrix} ν_{μ} \\ μ \end{matrix})}_{L}$	${(\begin{matrix} ν_{τ} \\ τ \end{matrix})}_{L}$	$\begin{matrix} 0 \\ - 1 \end{matrix}$	$\begin{matrix} + \frac{1}{2} \\ - \frac{1}{2} \end{matrix}$
Quarks	${(\begin{matrix} u \\ d^{'} \end{matrix})}_{L}$	${(\begin{matrix} c \\ s^{'} \end{matrix})}_{L}$	${(\begin{matrix} t \\ b^{'} \end{matrix})}_{L}$	$\begin{matrix} + \frac{2}{3} \\ - \frac{1}{3} \end{matrix}$	$\begin{matrix} + \frac{1}{2} \\ - \frac{1}{2} \end{matrix}$

The term elementary means that those particles are point-like and structureless, with no excited state. The fundamental fermions have three families, consisting of a pair of quarks and a pair of leptons each. All fermions have antiparticles of opposite charges, but similar other properties. The leptons can propagate freely, but the quarks are confined in composite particles, the hadrons. They are either composite fermions, bound states of three quarks, the baryons (like the proton and neutron), and three antiquarks, antibaryons (like the antiproton), or bosons composed of a quark and an antiquark, the mesons (like the pion). The quarks in the hadrons are bound together by the strong interaction, and because of the two possible compositions, its three-state source is called color charge; in analogy with human sight, they are called the colorless states.