The Standard Model – Learn

Following the discovery of the proton and neutron, many other particles were discovered, including things like pions and kaons. The properties of these new particles were studied, and similiarities between them and known particles were detailed. This led to a creation of a framework devised to categorise all particles, and explain the similarities. This became known as the standard model, it has been provided below.

From this graphic, three categories of fundamental particles can be identified, quarks, leptons and bosons, these will addressed in detail later in this chapter. Additionally, new metrics for each particle have been included, mass, charge and spin, details of which are provided below.

Mass – The same as the conventional definition of mass, how much matter is contained in each particle. The masses provided vary greatly, from the relatively massive top quark to the tiny electron neutrino. It is important to note that the bosons (except the Higgs boson) are massless.

Charge – Earlier in this course, you were introduced to the base unit of charge, 1.602 x 10-19 C, the charge of an electron or proton. You will see that the electron has been given a charge of -1, which provides a frame of reference for the other charges given. It should be noted that particles must from with a whole integer (number) charge, not violating the quantisation of charge discovered by Millikan.

Spin – This is a complex fundamental property, associated with the intrinsic angular momentum of the particle. You needn’t understand it in any detail, although may be interested to know that it is inherently related to magnetism.

Quarks

To make sense of the common properties between new and existing particles, fundamental building blocks were proposed, called quarks. Each quark has specific individual properties which explained the newly observed particles. All quarks have a half integer spin (all particles with a total half integer spin are called fermions) and a charge of either +2/3 or -1/3.

Larger particles consisting of quarks are called hadrons, most commonly they bind together in groups of three to form protons and neutrons. Although other combinations do exist.

The image above shows the quark configuration of a proton and a neutron. As you can see, a proton is composed of two up quarks and one down quark. Considering that up quarks have a charge of +2/3 and down quarks have a charge of -1/3, this gives the proton a charge of +1 overall (equal in magnitude to the charge of the electron). Conversely, the two down quarks and singular up quark which make neutron, led to an overall charge of zero. Quarks in the nucleus are held together by gluons, which are the carrier of the strong force (more on this later on).

Leptons

Leptons are grouped underneath the quarks as they are also fermions, having a half integer spin. Some of the leptons may be recognisable, as they include electrons, neutrinos and muons, all of which have been previously discussed in this course. They differ from quarks as they have a whole integer charge, either 0 or 1. Leptons are already fundamental particles and cannot be broken down any further, although do vary in size greatly.

The composition of atoms

Describing the fundamental composition of an atom is a common examination question. It is important to include all quarks and leptons in involved. For example, to correctly describe the fundamental particles comprising a helium-3 atom, the composition of the nucleus must be considered, as well as any associated electrons. The nucleus of He-3 contains two protons (UUD) and one neutron (UDD), and it is orbit by two electrons. Hence, the composition is five up quarks, four down quarks and two electrons.

Antimatter particles

Every fermion in the standard model has a corresponding antimatter particle. For example, the corresponding antimatter particle for an electron is a positron, having the exact same properties but an opposite charge of +1. Antimatter quarks exist also, following the same rules. Antiquarks can form larger particles, like antiprotons, comprised of two up antiquarks and one down antiquarks. It is important to remember that when particle meets its corresponding antimatter particle, that they annihilate forming pure energy.

Quarks can sometimes form particles with unmatched antiquark, these are known as mesons. These are not fermions as this pair of quarks would not have a half integer spin. Examples of mesons include pions and kaons.

Bosons

There are four fundamental forces, electromagnetism (photon), gravity, the strong interaction (gluon) and weak interaction (W and Z boson). The bosons are the carriers of these forces. They are massless, and have a whole integer charge and spin. The Higgs boson is not specifically related to a force, but mass itself.

The fundamental forces

According to the Big Bang Theory, in the very early stages of the universe there was a single unified force. As the universe expanded and cooled, this singular force broke into the four fundamental forces known today. Electromagnetism and gravity have already been discussed at length in this course, while the strong and weak interaction will be somewhat unfamiliar.

Gravity – First quantified by Newton and later refined by Einstein, this is the weakest of the fundamental forces. It is a force of attraction between masses. While it is the weakest of the forces, it acts over an infinite range and plays a large roll in the formation of the universe. Each of the other fundamental forces have a boson associated with them, except gravity. It is theorised that it is yet to be discovered, and already has a name, the graviton.

Electromagnetism – The interaction between particles with charge and can be attractive or repulsive. While it is stronger than gravity, and operating over an infinite range, it is less apparent over large distances. This is because the universe contains many objects with large masses, though not large charge. The carrier of the electromagnetic force is the photon.

Weak interaction – When particle undergo decay, this is mediated by the weak interaction. For example, during beta negative decay a down quark is converted to an up quark (with an electron and an antineutrino being emitted), and this is reliant upon the W boson. There are two bosons from the standard model associated with the weak interaction, the W and Z boson. It is a stronger force than gravity, although weaker than electromagnetism, and only acts over a very small range.

Strong interaction – The strongest of the fundamental forces, it binds together subatomic particles and nuclei. As with the weak interaction, it only acts over a very small range. While it is usually an attractive force, it is repulsive at a minute distance. It is mediated by the gluon.

Particle accelerators

Many recent discoveries in physics have been as a result of particle accelerators. There are two general types, linear accelerators and circular accelerators, which function as the names suggest. Work is done on particles, usually electrons or protons, by very strong electric fields, enabling them to reach speeds approaching the speed of light. The path of the particles is then controlled through the use of powerful magnets.

In Melbourne, there is a large circular accelerator called the Australian Synchrotron. Electrons travelling at 0.99999998c follow a circular path, producing energetic electromagnetic radiation as the particles experience centripetal acceleration. These x-rays are then used to study the internal structure of many things. This function differs from the Large Hadron Collider in Switzerland, where beams of protons travelling at 0.99999999c are directed towards each other and the energies associated with the collisions are examined.

One of the most impressive recent discoveries in physics, was the identification of the Higgs Boson in 2012. The existence of the Higgs Boson was theorised some 50 years earlier to explain how particles with mass interact with other forces. The prediction of this particle from the standard model, validated the model itself and suggests that other particles, like the graviton, will be uncovered with time.