Standard Model

556px-Standard_Model_of_Elementary_Particles.svgThe picture to the left can be found in Wikipedia’s article about the Standard Model.

The Elementary Particles of Matter

Physicists have found 12 building blocks that are the fundamental constituents of matter. (The first three columns in the chart.) Our world is made of just three of these building blocks: the up quark, the down quark and the electron, in the first column.  That’s enough to make protons and neutrons and to form atoms and molecules. The electron neutrino, (an electron without electrical charge) observed in the decay of other particles, completes the first set of four building blocks.

This first generation of quarks and leptons gets replicated for a total of six quarks and six leptons. The mass of these particles increases from left to right. Like all quarks, the sixth quark, named top, is much smaller than a proton (in fact, no one knows how small quarks are), but the top is as heavy as a gold atom!

Although there are reasons to believe that there are no more sets of quarks and leptons, theorists speculate that there may be other types of building blocks, which may partly account for the dark matter implied by astrophysical observations. This poorly understood matter exerts gravitational forces and manipulates galaxies, therefore we know that there is something else in the universe, but we cannot see it, hence “dark energy” or “dark matter.”  The next generation of accelerators and telescopes will probably help us to identify its fabric.

Elementary Forces and their Transmission

Scientists distinguish four elementary types of forces acting among particles: strong, weak, electromagnetic and gravitational force.

  • The strong force is responsible for quarks “sticking” together to form protons, neutrons and related particles.
  • The electromagnetic force binds electrons to atomic nuclei (clusters of protons and neutrons) to form atoms.
  • The weak force facilitates the decay of heavy particles into smaller siblings.
  • The gravitational force acts between massive objects. Although it plays no role at the microscopic level, it is the dominant force in our everyday life and throughout the universe.

Particles transmit forces among each other by exchanging force-carrying particles called bosons. These force mediators carry discrete amounts of energy, called quanta, from one particle to another. You could think of the energy transfer due to boson exchange as something like the passing of a basketball between two players.

Each force has its own characteristic bosons:

  • The gluon mediates the strong force; it “glues” quarks together.
  • The photon carries the electromagnetic force; it also transmits light.
  • The W and Z bosons represent the weak force; they introduce different types of decays.

Physicists expect that the gravitational force may also be associated with a boson particle. Named the graviton or Higgs boson, this hypothetical boson is extremely hard to observe since, at the subatomic level, the gravitational force is many orders of magnitude weaker than the other three elementary forces.


Although it is a staple of science fiction, antimatter is as real as matter. For every particle, physicists have discovered a corresponding antiparticle, which looks and behaves in almost the same way. Antiparticles, though, have the opposite properties of their corresponding particles. An antiproton, for example, has a negative electric charge while a proton is positively charged.

Physicists at CERN (1995) and Fermilab (1996) created the first anti-atoms. To learn more about the properties of the “Mirror World,” they carefully added a positron (the antiparticle of an electron) to an antiproton. The result: antihydrogen.

Storing antimatter is a difficult task. As soon as an antiparticle and a particle meet, they annihilate, disappearing in a flash of energy. Using electromagnetic force fields, physicists are able to store antimatter inside vacuum vessels for a limited amount of time.


The Standard Model


The Standard Model explains the complex interplay between force carriers and building blocks. Physicists call the theoretical framework that describes the interactions between elementary building blocks (quarks and leptons) and the force carriers (bosons) the Standard Model. Gravity is not yet part of this framework, and a central question of 21st century particle physics is the search for a quantum formulation of gravity that could be included in the Standard Model.

Though still called a model, the Standard Model is a fundamental and well-tested physics theory. Physicists use it to explain and calculate a vast variety of particle interactions and quantum phenomena. High-precision experiments have repeatedly verified subtle effects predicted by the Standard Model.

So far, the biggest success of the Standard Model is the unification of the electromagnetic and the weak forces into the so-called electroweak force. The consolidation is a milestone comparable to the unification of the electric and the magnetic forces into a single electromagnetic theory by J.C. Maxwell in the 19th century. Physicists think it is possible to describe all forces with a Grand Unified Theory.

One essential ingredient of the Standard Model, however, still eludes experimental verification: the Higgs field. It interacts with other particles to give them mass. The Higgs field gives rise to a new force carrier, called the Higgs boson, which has not been observed. Failure to find it would call into question the Standard Model. Experimenters at Fermilab hope to find evidence for the Higgs boson and make further discoveries in the next few years.

The Standard Model is the name given to the current theory of fundamental particles and how they interact. This theory includes:

  • Strong interactions due to the color charges of quarks and gluons.

  • A combined theory of weak and electromagnetic interaction, known as electroweak theory, that introduces W and Z bosons as the carrier particles of weak processes, and photons as mediators to electromagnetic interactions.

The theory does not include the effects of gravitational interactions. These effects are tiny under high-energy Physics situations, and can be neglected in describing the experiments. Eventually, we seek a theory that also includes a correct quantum version of gravitational interactions, but this is not yet achieved.

The Standard Model was the model of particle physics in the 1970’s . It incorporated all that was known at that time and has since then successfully predicted the outcome of a large variety of experiments. Today, the Standard Model is a well established theory applicable over a wide range of conditions, but we know that there are limitations.

Matter Constituents as Understood in the 1930s

Matter is made of atoms, atoms are made of a nucleus plus electrons, and electrical forces between the nucleus and the electrons explain the structure and stability of the atom.

The nucleus is made of protons and neutrons, but what force holds them together in the nucleus?

Investigation of this questions led to the discovery of many more types of matter, and antimatter, and to the modern theory of matter — known as the Standard Model.

The attempt to understand and classify the many particles discovered led to the recognition that protons and neutrons are not fundamental particles but are made of smaller objects called quarks.

Matter Constituents as Understood Today

There are two major classes of matter — hadrons and leptons — distinguished by their behaviors, that is by the types of interaction in which they participate.

Particles that participate in all known types of interaction — strong, electromagnetic, weak and gravitational are called hadrons. Observable hadrons are composite objects made from quarks and antiquarks and gluons.

Particles that do not participate in strong interactions, but do participate in the other types are charged leptons –the most common of these being the electron.

Neutral leptons, i.e. those without electric charge are even more elusive — they participate only in weak and gravitational interactions, and thus are rather difficult to detect. These particles are called neutrinos.

In the Standard Model, quarks and leptons are fundamental particles. As far as we know today there is no evidence that refutes this assumption, nor any evidence for a size or structure for any of these particles. All we can say from experiment is that any structure is at a size smaller than 10-18 meters.


The notion of a constituent or building block gets a new twist here, too. Normally if something can fall apart into certain other objects, then we say that these objects were constituents of the first.

With fundamental particles a new possibility occurs: an object which is fundamental — that is, has no constituent parts — can nevertheless be unstable –that is it can decay radioactively, disappearing itself and producing two or more other fundamental particles which fly apart.

Furthermore objects can be composite — hadrons made from quarks — but never can be broken apart into their constituent parts! Modern theory says quarks cannot exist in isolation but are only to be found inside hadrons.

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