Standard Model of Particle Physics

The Standard Model:

• Classifies and describes all known subatomic particles.

• Can describe the 3 non-gravitational fundamental interactions (electromagnetism, the strong and weak nuclear forces).

The Standard Model is remarkably accurate to describe elementary particles and forces. However, the Standard Model does not include gravity. To understand gravity, we use general relativity, which is applicable on macroscopic scales. 

The particle content of the Standard Model:

Fermions: Fermions are one of the two families of subatomic particles. Fermions have a half-integer value spin and obey the Pauli-exclusion principle. This is a theoretical limit on their number per volume. There cannot be two fermions occupying the same quantum state, at the same time. No two fermions can occupy the same quantum state at the same time. The Pauli-exclusion principle is named after Wolfgang Pauli who formulated the principle in 1925 for electrons. Fermions are named after Enrico Fermi who worked to create the first nuclear reactor. He is considered by many to be the architect of the nuclear age. 

• Quarks

Quarks are fermions, as they have a half integer spin. There are 6 flavors of quarks: up, down, charm, strange, top and bottom. Quarks are bound by gluons into composite particles called hadrons. Quarks are never observed in isolation, however, only bound together. This is a phenomenon known as color confinement. The theory of this interaction is called quantum chromodynamics. Quarks are the only particles of the Standard Model that interact through all four of the fundamental interactions. Two examples of a hadron are the baryon and the meson. 

The baryon is composed of 3 quarks. These are the familiar particles like the proton and neutron that makes up the atomic nucleus. The proton is composed of 2 up quarks and 1 down quark, while the neutron is 1 up quark and 2 down quarks. This is a central idea for the Standard Model. 

A meson is composed of a quark-antiquark pair. An antiquark will have a bar over it's symbol. 

The quark model was first proposed in 1964 by Murray Gell-Mann and George Zweig. 

• The electron was discovered by J. J. Thomson.

• The muon was discovered by Carl D. Anderson.

• The tau was discovered by Martin Lewis Perl.

Gauge bosons: Gauge bosons (or vector bosons) have an integer value spin of 1. These are the photon, the gluon and the W and Z boson. These are the force carriers of the electromagnetic, strong and weak nuclear interactions. 

• Photon

Albert Einstein predicted that light exists in tiny particles in 1905 and they are now known as photons. The idea was that light was quantized and broken into discrete packets or quanta. Each of these packets of light had their own wavelength, energy and frequency. This was the explain a phenomenon known as the photoelectric effect: a beam of light will hit a metal surface and electrons will be released. This was one of Einstein's great papers of 1905 and was building off of Max Planck's work 5 years earlier on the quantization of the full spectrum of thermal radiation and led to the development of quantum mechanics in the subsequent decades. 

The photon is the quanta of all forms of electromagnetic radiation, including light. It is the force mediating particle for electromagnetism and is described by a theory called quantum electrodynamics.

Atoms can emit photons of light. In the model of the atom proposed by Niels Bohr in 1913, the energy of an electron is determined by the distance that it's orbital shell is from the atomic nucleus. When an electron jumps through empty space to a lower energy shell below it the electron falls into that lower energy shell. Thus, the energy that the electron lost is emitted as a photon or particle of light. 

The experimental evidence for photons came with the experiment of Arthur Compton in 1923. Compton demonstrated that photons have a kind of momentum. A beam of X-rays were shot at a block of graphite at a specific frequency. The scattered radiation had a lower frequency. Compton explained the drop in frequency to be related to the fact that light is made of particles. Photons, also, cannot have any mass. This is because they travel at the speed of light and have a finite energy content. This is a consequence of special relativity: anything with mass cannot reach light speed. 

A Feynman diagram  is a pictorial representation of the interactions of subatomic particles and were first proposed in 1948 by Richard Feynman. In a Feynman diagram, fermions are straight lines and bosons are represented as wavy lines. Time will be represented by one axis and space as another. A Feynman diagram can visually represent how two approaching electrons can exchange a photon and then move apart from one another. It can also represent how an electron and an antielectron (positron) can annihilate and produce a photon in the process. The particles will continue to move away, however, as a muon and an antimuon. 

The Electroweak theory:

Steven Weinberg, Sheldon Glashow and Abdus Salam, in 1967 and 1968 noticed an interesting symmetry between the photon and the W boson. The new theory of the W boson was different than the ones of the past. This unification of electromagnetism and the weak force was known as the Electroweak interaction.

Scalar boson - has no intrinsic angular momentum.

• Higgs boson

The Higgs boson is a scalar boson, theorized to exist by Peter Higgs in 1964. Peter Higgs was born in England and is very well known for his theory of the origin of mass of elementary particles. According to the theory of Peter Higgs, particles acquire mass by interacting with an energetic field known as the Higgs field. Higgs predicted a the existence of the particle of this field: the Higgs boson. The Higgs boson explains why all of the elementary particles, except, of course, for the photon and gluon, have mass. 

The necessity for the Higgs results from the Electroweak theory. The symmetry requirements of the Electroweak interaction require that the photon and the weak bosons have no mass. This is a significant obstacle for developing a consistent theory of particle physics. This Higgs mechanism is where elementary particles acquire their mass.

This is the reason the search for the Higgs is so critical, is that: according to the gauge theory of the Standard Model, the Electroweak theory, both the photon and W and Z boson particles are massless. Although the photon, is, indeed, massless, the W and Z bosons have mass. So, either the approach to the Electroweak theory of the Standard Model is incorrect, or, there is a field, or some other mechanism, by which these particles are interacting with, thus, acquiring their mass: perhaps the Higgs. 

The Higgs boson is a different type of particle. The discovery of the Higgs is a confirmation that the Standard Model is a consistent theory of elementary particle physics. Prior to the discovery of the Higgs, the origin of particle masses was not known for certain. If there was just mass from the beginning with no mechanism responsible for mass generation, then the theory itself would be inconsistent. It would make nonsensical predictions. The Higgs bosons tells us how the Higgs mechanism is implemented. 

The 1964 PRL symmetry breaking papers were authored by Peter Higgs, Francois Englert, Robert Brout and others. These were related papers that suggested potential mechanisms that would allow mass to arise in local gauge theories. These theorists are credited with the discovery of the Higgs field and the Higgs boson.