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.

Antimatter

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.

Decays

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.

Quarks:

up (u) down (d)

1964 Gell-Mann and, independently, Zweig introduce the idea of quarks, the building blocks of composite particles, to explain the classification of particles observed in experiments.

1968 Physicists at the Stanford Linear Accelerator Center (SLAC) observe the first evidence for quarks inside the proton. Friedman, Kendall and Taylor receive the 1990 Nobel Prize.

strange (s)

1951 First observation of kaons (particles containing strange quarks) in cosmic-ray experiments.

1956 Gell-Mann, of the California Institute of Technology, explains the relative longevity of kaons with the concept of strangeness and receives Nobel Prize in 1969.

1964 At Brookhaven National Laboratory (BNL), Cronin and Fitch find that kaons violate the matter-antimatter symmetry (CP violation). They receive the 1980 Nobel Prize.

charm (c)

1972 Kobayashi and Maskawa predict the existence of a fourth quark (named the charm quark) and two additional quarks (known as bottom and top quarks) to explain the non-symmetric (CP-violating) behavior of kaons (particles containing a strange quark). They receive a share of the 2008 Nobel Prize.

1974 Physicists at SLAC and BNL discover independently a new particle that contains a new kind of quark, called the charm quark. Richter (SLAC) and Ting (BNL) receive the 1976 Nobel Prize.

bottom (b)

1977 Led by Lederman, a group of scientists at Fermilab discover the upsilon, a particle containing a bottom quark and an anti-bottom quark.

top (t)

1995 The CDF and DZero collaborations at Fermilab announce the discovery of the top quark, an elementary particle as heavy as a gold atom.

Leptons:

electron (e)

1897 Using a cathode tube, Thomson discovers the electron at the Cavendish laboratory in England. He receives the Nobel Prize in 1906.

electron neutrino (ν_e)

1956 Experimenters led by Cowan and Reines at the Savannah River plant detect the first neutrino. Reines shares the 1995 Nobel Prize.

1960s to 1990s Experimental groups led by Davis and Koshiba are among the first to observe neutrinos produced by the sun and by a supernova. Their measurements of solar neutrinos lead to the discovery that neutrinos have mass. Davis and Koshiba receive a share of the 2002 Nobel Prize.

muon (μ)

1937 Neddermeyer and Anderson discover the muon in a cosmic-ray experiment.

muon neutrino (ν_μ)

1962 Scientists at BNL discover the muon neutrino. Lederman, Schwartz and Steinberger receive the 1988 Nobel Prize.

tau (τ)

1976 Experimenters at SLAC discover the tau lepton, the first observation of a third-generation particle. Perl shares the 1995 Nobel Prize.

tau neutrino (ν_τ)

2000 Fermilab announces first direct evidence for the interaction of a tau neutrino in a detector. Indirect indications for the existence of this particle existed for more than two decades.

Force carriers:

photon (γ)

1905 Based on Planck's introduction of quanta of energy, Einstein describes the photoelectric effect using light particles called photons. They are carriers of the electromagnetic force. Planck receives the 1918 Nobel Prize, and Einstein is honored in 1921.

gluon (g)

1973 Gross, Politzer and Wilczek predict that the strong nuclear force, mediated by gluons, behaves like a rubber band: it becomes weaker as the distance between two quarks decreases. They share the 2004 Nobel Prize.

1979 At the Deutsches Elektronen-Synchrotron (DESY) in Germany, scientists report evidence for the gluon, the carrier of the strong force.

electroweak bosons (W, Z)

1983 Physicists at the European research laboratory CERN observe W and Z bosons, the only force carriers with mass. Rubbia and van der Meer receive the 1984 Nobel Prize.

Higgs boson (H)

1964 Higgs and other theorists use the concept of spontaneous symmetry breaking to explain why the W and Z bosons have mass while the photon has no mass. The explanation predicts the existence of at least one additional particle, now known as the Higgs boson.

Antimatter:

Every particle has its own antiparticle. Two major discoveries helped physicists to establish this fundamental principle:

positron (e⁺)

1931^- Examining^-cosmic-ray data, Anderson discovers the positively charged electron – later named the positron. He receives the 1936 Nobel Prize.

antiprotron (p^-)

1955^- Using an accelerator^-at Berkeley University, Segre and Chamberlain discover the antiproton. They receive the 1959 Nobel Prize. (Later, physicists learn that a proton contains quarks and an antiproton consists of antiquarks.)

Theory:

The theory of the Standard Model is intimately connected to the numerous discoveries in quantum physics in the first half of the 20th century. Here are the major theoretical breakthroughs of the second half of the 20th century that were honored with Nobel Prizes.

1965 Tomonaga, Schwinger and Feynman receive the Nobel Prize for formulating the theory of quantum electrodynamics, the most precisely tested theory in physics.

1969 Gell-Mann receives the Nobel Prize for his contributions to the classification of elementary particles and their interactions

1979 Glashow, Salam and Weinberg receive the Nobel Prize for the unification of the electromagnetic and weak interactions into the electroweak theory

1999 ‘t Hooft and Veltman receive the Nobel Prize for their quantum formulation of the electroweak theory.

2004 Gross, Politzer and Wilczek receive the Nobel Prize for their contributions to establishing the theory of quantum chromodynamics, the theoretical framework that correctly describes the strong nuclear force.

2008 Nambu receives half of the Nobel Prize for introducing the concept of spontaneous symmetry breaking into particle physics, which led, for example, to the explanation of why some particles have mass and others have not (Higgs mechanism). Kobayashi and Maskawa share the second half of the Nobel Prize for predicting the existence of additional quarks (charm, bottom and top quarks) and how their presence leads to a broken symmetry that explains why matter and antimatter behave differently.