The Standard Model of Particle Physics

All matter in the universe is composed of different kinds of particles. So far, the Standard Model is research’s best attempt to explain the existence, properties, and behavior of these particles.

The most fundamental particles of matter can be divided into two basic groups—quarks and leptons. Each group contains six particles, segregated into three pairs or "generations", according to their mass and stability. First-generation quarks and leptons make up all stable matter, while second and third-generation particles only exist briefly in high-energy environments before decaying into more stable particles.

Figure 1. The particles in the Standard Model. Image Credit: Wikipedia.

Quarks

Quarks are a kind of fermions, particles with a spin of ½. The spin value of a particle represents the quantum state of their angular momentum and determines how the particles interact with each other. Fermions obey Fermi-Dirac statistics, which implies that only two fermions may exist in the same state within a given system. This property is what makes electrons form a wide range of chemical elements.

Quarks exhibit electrical charge and color charge. They exist exclusively bound together into larger particles called hadrons. Color charge has nothing to do with the color that humans experience in the macroscopic world; rather, color is a property of fundamental particles which is only apparent on a sub-nuclear scale. The theory of Quantum Chromodynamics explains the strong nuclear force in terms of the interacting color charge of quarks and gluons.

Quarks always bind together to form color-neutral hadrons. A wide variety of hadrons exist, as there are several variations on the way quarks combine. Two-quark hadrons, formed from a quark and antiquark, are called mesons. Three-quark hadrons are called baryons. They include the protons and neutrons which make up all stable atomic nuclei.

Leptons

Leptons are the second class of fermions in the Standard Model. They do not possess any color charge. Electrons, which are part of all atoms and are responsible for chemical interactions, fall into this category. The muon and tau are heavier, less stable cousins of the electron, which only exist for extremely short periods of time before decaying into more stable particles.

Neutrinos are chargeless leptons with tiny masses that are several orders of magnitude lighter than electrons. All three generations of neutrino are stable and exist in huge quantities throughout the universe; however, because of the lack of interactions with other matter, neutrinos are incredibly difficult to detect.

Bosons

Another statistical model that fundamental particles follow is Bose-Einstein statistics. Particles that fall under this category, called bosons, have a spin of 0 or 1. These particles tend to exist in the lowest possible energy state, regardless of how many there are within the system. There is, therefore, no limit on the number of bosons that can exist within a certain volume. This allows the formation of Bose-Einstein condensates which are behind many quantum phenomena, such as superconductivity.

The Standard Model also includes force carrier particles, or gauge bosons, which mediate the exchange of the four fundamental forces between other particles. This group includes the photon, W and Z bosons, and the gluon.

Gauge Bosons

The four fundamental forces which govern every interaction in the universe are gravity, electromagnetic force, weak nuclear force, and strong nuclear force. According to the Standard Model, each of these is mediated by the exchange of force-carrying bosons or gauge bosons. These bosons all have a spin of 1 but vary in their other properties.

Table 1. Force-mediating bosons in the Standard Model

Higgs Boson

Proposed by Peter Higgs, Robert Brout, Francois Englert, Gerald Guralnik, Carl Hagen and Tom Kibble, the Higgs boson is a zero-spin particle proposed as an addition to the Standard Model. The Higgs theory explains the process by which other elementary particles acquire mass, including the reasons why photons and gluons are massless while other gauge bosons, W and Z bosons, are massive.

Conceptualizations about the Higgs particle were entirely hypothetical until contemporary researches stepped in. High-energy experiments with the Large Hadron Collider (LHC) at CERN have shown the existence of a "Higgs-like" particle. The ATLAS and CMS experiments at the LHC recently confirmed the discovery of the Higgs particle.

Figure 2. Colliding protons release a Higgs boson, which then decays into a stream of hadrons and electrons. Image Credit: TACC

Discovery of Standard Model Particles

The Standard Model has had considerable success in predicting the existence of particles. The charm, bottom and top quarks, tau neutrino, gluon, and W and Z bosons, were all predicted theoretically by the Standard Model even before they were observed experimentally.

Most of the discoveries of the particles in the Standard Model have earned the researchers a Nobel Prize in Physics. For example:

• Burton Richter and Samuel Ting in 1976, for the discovery of the charm quark in 1974
• Richard Taylor, Jerome Friedman and Henry Kendall in 1990 for the discovery of the up and down quarks in experiments at SLAC in the late 1960s
• Martin Perl in 1995, for the discovery of the tau lepton in 1975, also at SLAC
• Peter Higgs and Francois Englert, for the discovery of the Higgs Particle through the ATLAS at CMS experiments in 2013

Looking Beyond the Standard Model

While the Standard Model has been successful in predicting the existence and behavior of fundamental particles, there are a few areas where it is not accurate:

• It does not agree with general relativity at very small scales, implying that researchers do not have a good description of how matter behaved in the very early universe or in black holes.
• It also has no way to account for dark matter or dark energy, which appears to make up the majority of the mass in the observable universe.
• Many physicists consider the model to be inelegant and arbitrary because it depends on several "fiddle factor" constants that are not related to each other, and any expansion of the theory would require the addition of several more.

The ultimate aim of modern theoretical physics is a Grand Unification Theory or "Theory of Everything", which would remove the disparities between Quantum Field Theory, the Standard Model (which explain electromagnetism and the strong and weak forces), and gravity according to General Relativity.

Several routes to this goal have been proposed, but it is not yet clear which, if any, will result in a full unification of physics. Nevertheless, researchers have laid out specific properties of all versions of the Grand Unification Theory, including relation to proton decay, relation to neutrino masses, and occurrence in string phenomenon. These properties are expected to guide future directions and propositions about the Grand Unification Theory.

Potential Routes to a Grand Unification Theory

• Supersymmetry (effectively an extension of the Standard Model)
• String Theory
• M-Theory
• Loop Quantum Gravity

Sources and Further Reading

• "The Standard Package" - CERN
• "Missing Higgs" - CERN
• "Limits of the Standard Model" - ArXiv.org
• “GUT” – NCAT Lab
• “The Higgs boson” - CERN

This article was updated on 3^rd January, 2019.