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Particle Physics - The Standard Model

The Standard Model of Particle Physics is a theory, not a law, that is used to explain the existence of all the elementary particles that have been observed and the forces that hold atoms together or lead to their decay.

It was developed in stages during the second half of the twentieth century by scientists from around the World, as new elementary particles were discovered, to classify their properties and to describe the fundamental forces acting between them.

  • It extended the familiar atomic structure of matter consisting of protons, neutrons and electrons, and showed that they are made up from even smaller elementary particles.
  • It explains three of the four known fundamental forces of nature including the electromagnetic force and the weak and strong nuclear forces, but not the gravitational force for which no suitable explanation has yet been developed.

Family Tree of Elementary Particles

The Structure of Matter

Since early times, atoms were thought to be nature's smallest building blocks of matter. In 460 B.C. Greek philosopher Democritus described these particles as "indivisible" with the name "atom" being derived from the Greek "a" meaning not and "tomos" meaning cut.

The first indication that the atom was not the smallest particle came in 1897 when J.J. Thompson discovered that atoms consisted of a nucleus surrounded by electrons which he thought were randomly dispersed in what he described as a "plum pudding model".

In 1911 Rutherford refined this model with his belief that the electrons moved around the nucleus in distinct orbits producing the classical image, the so called "planetary model", still associated with nuclear physics though the science has moved on.

The model was further developed in 1932 when Chadwick discovered the neutron showing that the nucleus itself was divisible, consisting of sub-atomic particles - protons and neutrons.

Around this time a series of previously unknown particles were discovered in cosmic rays and also in the laboratory. Investigations showed that some of these particles were elementary or fundamental particles from which the protons and neutrons were composed. Others were composite particles made up from these elementary particles as well as other more familiar particles.

Existing electromagnetic theory was not able to explain the behaviour of these new elementary particles which was governed by some as yet unknown forces.

The Standard Model was developed to describe the properties of these fundamental partcles and the forces governing the interactions between them. It explains how the entire Universe is built from just twelve matter particles called fermions and how the forces between them are mediated by just six force carrying particles called bosons, plus gravity which is unfortunately still without explanation and so not shown in the diagram.

See the Timeline of Particle Physics Theories, Predictions and Discoveries which outlines the history and more detailed background of these developments.

The diagram of the Standard Model below shows the 18 fundamental particles noted above together with their key properties.

Click on the individual particles in the diagram for more background information.


The Standard Model

MATTER (Fermions)

FORCES (Bosons)

Three Generations of Matter (Fermions)








Link to Up Quark Information Link to Charm Quark Information Link to Top Quark Information
Link to Down Quark Information Link to Strange Quark Information Link to Bottom Quark Information


Link to Electron Information Link to Muon Information Link to Tau Information
Link to Electron Neutrino Information Link to Muon Neutrino Information Link to Tau Neutrino Information


Link to Gluon Information
Link to Photon Information
Link to Z Boson Information
Link to W Bosons Information


Link to Higgs Boson Information


Fundamental Particles

Fundamental Particles (also called elementary particles) are the smallest building blocks of the universe. The key characteristic of fundamental particles is that they have no internal structure. In other words, they are not made up of anything else. They come in two separate classes, matter particles called fermions and force particles called bosons, each with its distinct behaviour.

  • Fermions form the constituents of all matter and obey the Pauli exclusion principle which means that each particle in a quantum group must be in a different quantum or wave state.
  • Bosons are the force carriers whose exchange holds matter (the fermions) together. They do not obey the exclusion principle and can exist simultaneously in large numbers in a single wave.

Fundamental Forces

The fundamental interactions or forces that govern the behavior of elementary particles are:

  • The strong nuclear force, also called the colour force, which holds the nucleus together. Carried by gluons
  • This is the strongest of the forces but acts over a very short range of about 10-15m, which is the average diameter of a medium sized nucleus. This force is attractive and only applies to quarks and particles composed of quarks.

  • The weak nuclear force which causes beta decay. Carried by Z0, W+, and W- bosons.
  • All particles, both quarks and leptons, experience this force which is weak at 10-6 the magnitude of the strong force and has the shortest range of 10-18m, which is 0.1% of the diameter of a proton.

    Whereas the other fundamental forces act through attraction / repulsion mechanisms, the weak force is, by contrast, responsible for transmutations – changing one element into another – and conversions between mass and energy at the nuclear level.

  • The electromagnetic force which causes interactions between electrically charged particles. Carried by photons
  • This is the second strongest force with a strength of 1/137 relative to the strong force but with an infinite range extending between stars across the Galaxy. It can carry either positive or negative charges which give it properties of attraction and repulsion.

  • The gravitational force which causes attraction between masses.
  • This is the weakest of the forces with a strength of 6 × 10-39 in comparison to the strength of the strong force but like the electromagnetic force it has an infinite range. It is always attractive.

  • The Higgs force which causes particles to have inertia. Carried by the Higgs boson
  • It arises from the ubiquitous, constant background, scalar Higgs field which causes a coulpling force on particles, restricting their movement, which depends on their mass. The Higgs boson is not a force-exchanging vector (gauge) boson like the other bosons of the Standard Model.


All matter is made up of fermions which come in two families each containing six different particles commonly called flavours often with whimsical names. These families are:

  • Quarks which do not exist independently, but must bind together in composite particles. They come in two types, those with a positive charge of 2/3 and those with a negative charge of -1/3. They are held together in triplets by the strong force.
  • Leptons (from the Greek leptós, "fine, small, thin") which exist independently and do not bind into larger groups. They also come in two types, those carrying a negative charge, and their associated neutral partner, a neutrino which carries no charge. Neither particle is affected by the strong force.
    • The charged leptons, the electron, the muon and the tau are affected by the electromagnetic force and can combine with other particles to form various composite particles such as atoms. The heavier muons and taus rapidly decay into electrons and neutrinos.
    • On the other hand the neutral leptons (neutrinos), are affected by the weak force and rarely interact with anything. Interactions with matter are extremely rare and they pass practically unhindered through everything from planets to people, even passing through over a light year of lead before interacting with any of the particles in the lead. This makes them extremely difficult to detect and they are often called "ghost" particles.

    See more about quarks and leptons, who discovered them and how they did it.

All fermions have 1/2 integer spin.

Fermions come in three generations. The first generation makes up 99.9% of all visible matter in the Universe. The second and third generations are heavier versions of the first with the third generation being over 1000 times heavier. These heavier fermions were created at very high energies which existed at the beginning of the Universe or they have been duplicated recently in high energy particle accelerators. Heavy particles, are typically unstable with a very short lifetime, quickly decaying into lighter forms of matter with the heaviest particles decaying the fastest. This explains the abundance of the lighter particles and the relative scarcity of their heavier ancestors.

Each generation is thus made up from pairs of related quarks and similar pairs of related leptons.


All quarks and leptons have associated antiparticles with the same mass but with opposite physical properties such as electric charge or spin. For example, the antiparticle of the electron is the antielectron, more commonly called a positron which carries a positive charge. Particle–antiparticle pairs can annihilate each other, emitting radiant energy.


Bosons are the fundamental exchange particles that carry force and mediate the interactions between matter particles. They are the glue for fermions. There are five different gauge bosons and one scalar boson. They all have integer or zero spin but have different combinations of mass and charge giving them different characteristic properties. See diagram above.

The essential characteristics of bosons are:

  • Their integral spin. This means that they do not obey the Pauli Exclusion Principle so that many such particles can congregate together in the same quantum state.
  • This freedom of association, coupled with their mobility also endows the bosons with their vital ability to act as quantum force carriers in particle interactions.

Within this general class of bosons there are several sub-classes associated with different forces;

  • Gauge bosons with integral (non-zero) spin, are vector force carriers. They include photons (electromagnetic force), gluons (strong force), and the W+, W- and Z0 bosons (weak force) which are all fundamental particles.
  • Photons and gluons are massless. All other bosons have mass.

  • Scalar bosons such as the Higgs boson are also fundamental force carrier particles, but with zero spin. Since the Higgs force carrier does not have a spin component, it is not a vector but a scalar quantity. Its associated Higgs field influences the mass of fundamental particles.
  • Mesons, such as the pion and kaon, which are composite particles comprising a quark and an antiquark, whose opposite spins cancel eachother out leaving the meson with a net zero spin are also bosons. These bosons are however not involved in transmitting forces.

See more about bosons.

A place is reserved in the Standard Model for a possible sixth boson, the graviton, which up to now has not yet been identified.

Composite Particles

Composite particles are subatomic particles which are made up from more than one quark. They are called hadrons (from the Greek hadros - "bulky").

  • Hadrons come in two different families, baryons and mesons, which comprise all the quark-based particles. They interact with the strong nuclear force which keeps the protons in tne nucleus from flying apart.

    • Baryons - (from the Greek barys - "heavy") are hadrons which are composed of 3 quarks. They include the sub-atomic particles, the neutron and the proton.
      • The neutron consists of one up quark with a charge of +2/3 and two down quarks each with a charge of -1/3 giving it a net total charge of 0.
      • The proton consists of two up quarks each with a charge of +2/3 and one down quark with a charge of -1/3 giving it a total charge of 1.

      Though the nuclear strong force has a very short range, dropping to zero just beyond the edge of the nucleus, it extends far enough from each baryon so as to bind the neutrons and protons together in the nucleus against the repulsive electrical force between the positively charged protons. See also isotopes.

    • Mesons - (from the Greek mesos - "middle") consist of only two quarks forming a quark and antiquark pair. They carry the strong force. Since the quark's spin of 1/2 is neutralised by the antiquark's opposite -1/2 spin, the meson has zero spin which is the same as a boson.
    • See more about mesons

Properties of Fundamental Particles and Particle Systems

Properties such as mass, charge and spin are used to describe the key characteristics or states, of the fundamental particles shown in the diagram of the Standard Model above. These properties are also used to describe composite particles as well as groups of particles in a closed system.

Since these particles are extremely small as well as being unfamiliar, the following definitions and examples should help to provide scale and to put their properties into context.

  • Mass

  • Each fundamental particle has its own unique characteristic mass and some are massless. Mass is not a quantised property.

    The familiar unit of mass is the kilogram and in terms of these units the mass of an electron is 9.109 X 10 -31 Kg. For comparison, the mass of one proton which is a larger composite particle is 1.672 x 10-27 Kg which is 1836 times heavier than the electron and the W± the Z0 bosons are respectively 86 and 97 times heavier than the proton.

    Particle physicists however find it more convenient to use Einstein's energy equivalent to represent particle mass. On this basis the electron mass is quoted in terms of eV/c2 where eV is the particle's energy in electron volts and c is the velocity of light. Based on these units, the mass of the electron is 0.511 MeV/c2 while the mass of the proton is 938.3 MeV/c2.

    We might expect the proton which consists of 2 up and 1 down quarks to have a mass of only 9.2 MeV/c2, however the proton mass is in fact 100 times more than the mass of the three quarks due to containment or binding energy derived from the gluon field confining the quarks inside the particle.

    Just to make life more confusing, for even more convenience, many published texts leave out the c2 factor from the units and simply, though incorrectly, quote the mass in eV.

    For reference:

    • The conversion factor between the mass units is 1 MeV/c2 = 1.79x10-30 Kg.
    • 1 electronVolt (eV) is the energy gained by an electron when accelerated by an electrical field of one Volt. (1 eV = 1.6 x 10-19 Joules).

  • Charge

  • Particles may carry zero or multiple units of electric charge which may be positive (e) or negative (-e). Charge is a quantised quantity and fractional charges are not allowed, however quarks are quoted as having an electric charge of +1/3 or -2/3. But quarks do not exist in isolation. They only exist in triplets which in combination always have an integer or zero charge.

    • The basic elementary charge is the electric charge carried by a single proton (e) or electon (-e). This charge is a fundamental physical constant whose magnitude is 1.6022 x 10-19 Coulombs.
    • The Coulomb is the total charge transported by a constant current of one ampere in one second.

  • Colour Charge

  • Quarks also possess another property called the colour charge which is analogous to the electric charge. It has nothing to do with colour but is just a confusing name for another "degree of freedom" or property of the strong force holding quarks together as well as the atomic nucleus. It has three possible values, red green and blue and the combination of all three is white (that is neutral). See more about the colour charge

  • Spin

  • Spin is the intrinsic angular momentum of a particle or body with units Planck's Constant h divided by 2 pi (h bar) equal to Planck's constant h divided by 2π. It is a quantised quantity which means only discrete values of spin or angular momentum are possible.

    Particles with 1/2 integer spin are fermions and obey Pauli's exclusion principle. Particles with integral or zero spin are bosons and do not obey the exclusion principle.

    See more about spin.

    • Planck’s Constant is equal to 6.626176 x 10-34 joule-seconds and is defined by the relationship E = hf where E is the energy of one photon (the smallest possible energy "packet") of an electromagnetic wave and f is the frequency of that wave.






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