The Standard Model


Human nature is to question. Just ask any pre-schooler what their favorite word is and you'll probably receive the response, "Why?" followed by "what", "how" and "when". Eventually these children move on from their post-toddler obsession with monosyllabic words and develop into adults, yet they always retain some of that curiosity that is so intrinsic to human nature. Physicists, on the other hand, can't stop asking those questions and are the people who never grew up. One puzzle that philosophers and physicists have pondered for centuries is the riddle, "What is matter?" The Greek philosopher Democritus was the first to propose that matter is comprised of tiny "indivisibles" which he called "atoms".

Democritus was on the right path, and far ahead of his time. Today we know that atoms are not the smallest building blocks of matter; rather, there exists a whole world of particles more fundamental than atoms. Although less poetic than Democritus, we would say, "there are quarks, leptons, gluons and space". Physicists, through experimentation and theory, have created the Standard Model of particle physics, which outlines what they believe to be the most basic building blocks of matter.


The history of physics is a long and involving tale, which will not be told here. This is simply a brief history of particle physics pertinent to the development of the standard model. For more information on the history of physics, please visit the American Physical Society's A Century of Physics timeline.

-Pre 1800 Up until 1800 not much work is done involving the theory of matter. The majority of the exploration falls under chemistry through the identification of elements

-1802 Dalton revives the study of matter with his Atomic theory, which states that atoms are the fundamental building blocks of nature and can only combine in whole number ratios

-1898 J. J. Thompson discovers that cathode rays are electrons, a fundamental particle

-1905 Einstein publishes his theory of the wave-particle duality of light. This forms a foundation for quantum mechanics

-1911 Rutherford discovers that the atom has a concentrated positive nucleus

-1913 Bohr furthers Rutherford's model of the atom to include electron orbits at discrete radii to account for distinct atomic spectra emission lines

-1919 The bending of starlight due to the curvature of space-time is observed, confirming Einstein's general relativity

-1923 Louis de Broglie proposes the wave-particle duality of matter

-1925 Heisenberg creates his uncertainty principle, which puts limits on the precision of experimentation

-1925-26 Schrodinger rescues the wave-particle duality of nature from confusion with the wave equation

-March 1926 Quantum mechanics is formulated

-1932 James Chadwick announces discovery of neutron

-1956-57 Tsung-Dao Lee and Chen Ning Yang propose parity non-conservation in certain sub-atomic processes, which is confirmed by experimentalist Chien-Shiung Wu

-1962 The first experimental observation of the muon neutrino occurs

-1967 Raymond Davis creates the first solar neutrino detector, finding only half of the predicted solar neutrino flux

-1967 Steven Weinberg, Sheldon Glashow (collaboration) and Abdus Salam (independent) create the electro-weak theory, unifying the electromagnetic and weak nuclear force (they win Nobel prizes in 1979)

-1964 Quarks are proposed by Murray Gell-Mann and George Zweig

-1969 Jerome Friedman, Henry Kendall, and Richard Taylor find the first evidence of quarks

-1970-73 Standard model of particle physics is developed

-1974 The charmed quark is observed

-1975 Evidence of the tau lepton is found

-1977 Experimenters find proof of the bottom quark

-1983 Carlo Rubbia discovers the W and Z bosons, mediators of the weak-force

-1994 Planning for LHC (Large Hadron Collider) at CERN begins

-1995 Evidence for the top quark, the final undiscovered quark, is found at Fermilab

-2000 The tau neutrino, the last piece to the standard model, (with the exceptopm of the higgs particle) is observed at Fermilab


Components of the Standard Model:


The standard model is divided into three sections: quarks, leptons and force carriers. The quarks and leptons, which in turn are divided into three generations, are members of a family of particles called fermions (particles with half integer spins). Both the quarks and leptons come in pairs. For example, quarks are grouped up and down, charm and strange, and top and bottom (And yes, those are their real names). Experimental evidence for the top quark was recently found here at Fermilab in 1995. Scientists have proven that quarks combine in triplets to form baryons or quark-antiquark pairs to form mesons, both types of elementary particles.

Leptons, which belong to a class of particles called fermions, also come in pairs. The electron, muon and tau particles each have an associated low mass, charge-less neutrino. The electron, like the proton and the neutron, is a stable particle and is present in almost all matter. The muon and tau particles are unstable and are found primarily in decay processes.

The intermediate vector bosons, or force carriers, make up the third section of the standard model. They transmit three of the four fundamental forces through which matter interacts. The gluon, like its namesake, is responsible for the most powerful force, the strong force, which binds together quarks inside protons and neutrons, and holds together particles inside an atomic nucleus. The photon is the electromagnetic force carrier that governs electron orbits and chemical processes. Lastly, the W and Z bosons are attributed to the weak force, playing a role in radioactive decay. The weak force is very important in observing neutrino reactions, because the neutrinos are impervious to the electromagnetic force (due to their lack of charge) and unaffected by the strong (which governs nuclear interactions), leaving only the weak force to characterize the neutrino.

The standard model is not a complete theory; in fact it is far from being so. Detectors at Fermilab and eventually at the LHC at CERN are looking for the elusive Higgs particle, which, if found, will either explain the standard model or force us to readjust our conception of matter. Also the standard model does not have a place for gravity, the fourth force, which does not play a significant part in atomic and subatomic processes because it is so weak on those scales. Physicists are searching for a grand unified theory that would unite all four of the forces, currently only those included in the standard model are united. The next twenty years should prove very exciting for this field of physics.

Last updated: 6/29/01 comments