Standard Model

The Standard Model is the complete catalogue of fundamental particles known to physicists at this time. It gives a complete account of the irreducible piece-parts or "fundamental particles" of which all matter and force are made, so far as those particles are known at this time. The Standard Model is not an all-embracing "theory of everything," however, because it does not describe the force of gravity.

The Standard Model was developed gradually starting in the late nineteenth century, when English physicist J. J. Thomson (1856–1940) discovered the electron. During the first half of the twentieth century, the basic concepts of quantum mechanics—the system of ideas and equations that describes particles and forces at small spatial scales—were developed. The Standard Model is the overall picture of physical reality produced by quantum mechanics after almost a century of further refinement and experimental testing.

After the electron, the now-familiar proton and neutron were discovered (in the 1920s and 1930s, respectively), along with less-familiar particles such as the positron. As years passed, physicists discovered dozens of particles, most of them short-lived, using particle accelerators to smash protons, electrons, whole atoms, and other particles into each other at high speeds. Theory after theory was produced to account for the increasingly complex list of "fundamental" particles thus discovered, and physicists eventually became convinced that this list was too messy, too "ugly," to be truly fundamental. In the 1960s, physicists proposed that many of these "fundamental" particles are in fact composite structures built up from a smaller family of more fundamental particles, the quarks. This view was confirmed by experiments, and the Standard Model settled into approximately its present form in the 1970s.

The quark greatly simplified the list of known particles, but did not succeed in reducing it to only one or a few "fundamental" particles: the Standard Model still requires dozens of unique and irreducible particles to account for all known experimental data. It does, however, account for that data; all experimental data so far collected about the small-scale behavior of particles and forces is accurately described by the Standard Model. In this sense, though still incomplete it is one of the most successful physical theories ever devised.

The Standard Model's catalogue of particles can be broken down into two basic categories (see Tables 1a and 1b): matter constituents (fermions) and force carriers (bosons), which are generated by fermions. Each of these

categories can be further broken down into two subcategories (as shown in the tables), that shall be described further below. All known physical phenomena other than gravity can be described in terms of the behavior of the fermions and bosons listed by the Standard Model.

Below, the structure of the Standard Model's roster of particles is reviewed in detail. First, however, the terms "matter constituent" and "force carrier" require some explanation. (1) A matter constituent (fermion) is a fundamental particle (i.e., a particle that cannot, so far as physicsts now know, be broken into smaller parts) that has mass and exerts forces on other fermions via baryon exchanges. Ordinary matter is constructed from quarks and other fermions described by the Standard Model. (2) A force carrier (boson) is a particle that mediates forces between fermions. That is, when two fermions exert a force on each other (e.g., electrical repulsion, in the case of two electrons), they are actually exchanging bosons (e.g., photons, in the case of electrical repulsion). One can roughly picture force mediation via particle exchange by imagining two people perched on wheeled stools who are throwing a series of baseballs to each other. Each ball thrown and caught causes the two ball-throwers to move apart with slightly increased velocity. Such an image cannot account for attractive forces mediated by particle exchange, but does lend the idea of force mediation a basic plausibility. Photons and gluons have zero mass, while the other bosons have nonzero mass.

Fermions

QUARKS. All matter is built up of fermions. There are two types of fermions, quarks and leptons, and three paired varietiesor "generations" of quarks. The properties of quarks, such as "flavor," have been given whimsical names by physicists because they do not correspond to anything in our everyday sensory world. The three generations of quarks, with their associated flavors, are (1) an "up" quark and a "down" quark, (2) a "charm" quark and a "strange" quark, and (3) a "top" quark and a "bottom" quark. Each of these six quark types may come in three "colors" (again, an arbitrary term denoting a physical property having nothing to do with visible color): red, blue, or green. There is, for example, an up, green quark; an up, blue quark; a down, red quark; and so on, for a total of 18 varieties (6 flavors × 3 colors = 18). Moreover, each of these 18 quark varieites is matched by a distinct antiquark having opposite electric charge and other complementary properties. There is thus a total of 36 distinct, fundamental quarks (18 quarks + 18 antiquarks = 36). Only 12—the four up and down quarks and antiquarks, in three colors each—are long-lived enough to be constituents of ordinary, stable matter.

Quarks, which have charges that are multiples of 1/3 the charge of an electron, have never been observed alone; they are always bound together into composite particles (hadrons) because the amount of energy required to preserve them in a singular or "naked" state is too great to be realized even in particle accelerators. Quarks experience all four fundamental forces: the strong force, the weak force, the electro-magnetic force, and gravitation. (Physicists believe that all these forces are, in fact, manifestations of a single underlying force, and have already proved this underlying unity for the weak and electromagnetic forces. The unified weak-electromagnetic force is termed the electroweak force.)

LEPTONS. Leptons are fermions that do not bind with each other into larger, composite particles such as protons and neutrons. An electron is a lepton. Unlike quarks, leptons can be observed in isolation.

There are three lepton generations, each composed of an electrically charged particle and an associated, uncharged

neutrino: (1) the electron and electron neutrino,
(2) the muon (pronounced MEW-on) and muon neutrino, and (3) the tau and tau neutrino. Each of these six particles has a corresponding antiparticle (e.g., the positron for the electron), giving a total of 12 leptons. Leptons do not experience the strong force, but they do experience the weak force and gravitation. The neutrino leptons have no electric charge and therefore do not experience the electromagnetic force; the electron, muon, and tau leptons all have a negative charge and do experience the electromagnetic force.

Note: Every flavor of quark comes in three "colors," not listed here. Also, every quark and lepton has a corresponding antiparticle that is denoted by a bar over the particle's letter. For example, the antiquark corresponding to an up quark would be denoted by ū .

Bosons

The three fundamental forces—electroweak, strong, and gravitational—are mediated between fermions by exchanges of bosons. The photon mediates the electroweak force and can also travel alone, as electromagnetic radiation. The W ^—, W ⁺ , and Z⁰ bosons also mediate the electroweak force. One boson, the gluon, mediates the strong force that binds quarks together into larger particles such as protons and neutrons and that binds protons and neutrons together into atomic nuclei. A sixth boson, the Higgs boson, is hypothesized based on the mathematics describing the Standard Model, and is thought to explain the possession of mass by many of the particles in the model. The Higgs boson is the only particle described by the Standard Model that has not yet been detected experimentally. Bosons do not have antiparticles.

Hadrons

Hadrons are particles built up out of quarks. There are about 260 different hadrons, including the proton and neutron. The quarks that constitute hadrons are held together by the strong force, and hadrons interact with each other via the strong force. The strong force was originally discovered because it was needed to explain how the protons and neutrons in the nucleus of an atom stick together, despite the electrical repulsion between the like-charged protons.

There are two types of hadrons: (1) baryons and antibaryons, each of which is made up of three quarks (the baryons) or antiquarks (the antibaryons), and (2) mesons, each of which is made up of one quark and one antiquark.

Except for detection of the Higgs boson, which particle physicists hope to achieve in the next decade or two, the Standard Model corresponds almost perfectly to experiment. However, it has several flaws and odd features that lead physicists to believe that it must eventually be absorbed into another, broader theory (possibly string theory).

First, it seems to list too many "fundamental" particles—several dozen of them. There are questions as to whether this really be the simplest possible account of physical reality.

Second, the Standard Model does not attempt to explain gravitation. (A gravitation-mediating boson, the graviton, is often listed along with the other bosons of the Standard Model, but is not actually predicted by the mathematics of the model.)

Third, the Standard Model relies on 19 numerical values that cannot be derived from its mathematics but must be determined by experiment. These values include the masses of various particles, the strengths of the strong and weak interactions, and more.

Again, most physicists argue that it does not seem plausible that a model with 19 adjustable parameters can really be the simplest possible. Thus, refinement of the Standard Model continues on several fronts, especially detection of the Higgs boson (or, possibly, bosons) and on the absorption of the Standard Model into a more comprehensive theory (possibly string theory) that explains gravitation as well as the strong, weak, and electromagnetic forces.