Mesons

Type of physical science: Elementary particle (high-energy) physics

Field of study: Systematics (particle physics)

Mesons are a class of elementary particles of integral spin (bosons) that succumb to the strong nuclear reaction. Over two hundred distinct mesons have been detected experimentally, from the lightest, pions, to the very heavy upsilon mesons, which are about one hundred times heavier than pions.

Overview

Over two hundred varieties of mesons can be experimentally accounted for in modern elementary particle physics. Before the 1950s, mesons were designated "middleweights" in the particle mass family to differentiate them from leptons ("lightweights") and hadrons ("heavyweights"). In 1935, Hideki Yukawa proposed that such particles as mesons existed and that their mass was somewhere between that of an electron (a lepton) and a proton or neutron (the original hadrons). In units of megaelectronvolts per square centimeter, the electron has a mass of 0.511, while pi mesons, or pions, are about 140 and protons and neutrons are about 940. One megaelectronvolt per square centimeter is equivalent to 1.77 x 10^-30 kilograms and is the most convenient mass-energy unit for designating the important basic property of inertial mass. (Energy, E, and mass, m, are expressed in the equation E = mc², where c is the speed of light in a vacuum.)

As more and more particles were discovered by an increasing number of particle accelerators and their higher-energy regimes of operation, designations by weight or mass fell by the wayside. Some leptons are relatively heavy, heavier than many mesons; several mesons, particularly the upsilon mesons, are themselves extremely massive, on the order of ten times the mass of a proton. Mesons and baryons are both hadrons, which are heavy particles that succumb to the strong nuclear interaction. The mesons are distinguished by having integral spin (0, 1), making them bosons, while the baryons have half-integral values (1/2, 3/2) and are thus designated as fermions. Bosons obey a set of rules called Bose-Einstein statistics, while fermions obey Fermi-Dirac statistics, which contains the Pauli exclusion principle.

The muon was historically considered a meson (hence its name, derived from "mu meson") because of its intermediate mass, but when the standard model of particle physics was codified in the 1970s, it was more properly classified as a lepton. The muon was found experimentally before the pion, and most physicists at the time thought it was the carrier of the strong interaction that had been predicted by Yukawa. When the pion was discovered soon after, it proved to be the predicted particle instead. Pions were the first mesons discovered and still can be viewed as the most primitive mesons.

With the advent of the quark theory in the mid-1960s, mesons came to be defined as quark-antiquark (qq̄) composites. These quark-antiquark combinations are held together by gluons, of which there are eight varieties. As a whole, mesons can be categorized by their quarks. There are six different types, or flavors, of quark: up (u), down (d), strange (s), charm (c), top (t), and bottom (b).

Pions are the lightest and perhaps the most primitive of all mesons. There are three types of pion, each with a different charge state and a different combination of up and down quarks and antiquarks: π⁺ (uđ, positively charged), π^- (dū, negatively charged), and π⁰ (uū or dđ, no charge). Pions play an important role in low-energy nuclear interactions and also help mediate the attraction between nucleons, which holds protons and neutrons together in the nucleus. Pions have zero spin and a bifurcated mass scheme: uncharged pions have a mass of 134.97 megaelectronvolts per square centimeter, while the two charged pions have masses of 139.47 megaelectronvolts per square centimeter. Charged pions are the longest-lived common mesons, with lifetimes of about 10^-8 second.

K mesons, or kaons, the next level up in mass from pions, also come in three charge states and are composed of a strange quark (or antiquark) and an up or down quark (or antiquark). Because each one has a strange quark or antiquark, they are sometimes classified as strange mesons. The mass of a kaon is about 3.7 times that of a pion. Kaons are very interesting in that they exhibit a tremendous variety of excited states and result from decays of several of the other quark-flavored families. They are also surprisingly long-lived. Neutral kaons undergo the phenomenon of neutral particle oscillation, also known as flavor oscillation, which is when particles transform into their own antiparticles and back.

B mesons are mesons composed of an up, down, strange, or charmed quark and a bottom antiquark. The two chargeless B mesons—the one with the down quark and the one with the strange quark, known as the strange B meson—also exhibit flavor oscillation, though in the case of the strange B meson, the phenomenon was purely theoretical until 2006, when researchers at the Fermi National Accelerator Laboratory (Fermilab) in Illinois observed it for the first time. The strange B meson was at the center of another breakthrough in 2012, when scientists at the European Center for Nuclear Research (CERN) observed it decaying into two muons—a vanishingly rare occurrence—and were finally able to determine the rate at which this specific decay takes place: fewer than 4.5 times out of 1 billion.

Another set of mesons is designated as charmed, meaning they all contain one charm quark (or antiquark) and one up or down quark (or antiquark). Bottom mesons follow the same pattern, with one bottom and one up or down. Other categories include charmed, strange mesons (cs̄ or c̄s); bottom, strange mesons; and bottom, charmed mesons. Mesons made of a quark and its own antiquark are called quarkonia. The only quark that is not known to form quarkonium is the top quark, due to its significant mass. However, this nomenclature is usually only used for charmonium and bottomonium; quarkonia with up, down, and strange quarks are more commonly described as light unflavored mesons.

There seems to be no compelling reason why any even combination of quarks and antiquarks—three quarks and three antiquarks, for example—cannot combine to form "exotic" mesons, and the failure to identify such states has been somewhat disturbing to proponents of quark models. In fact, researchers have searched for tetraquarks, mesons composed of two quarks and two antiquarks, with promising but inconclusive results. In 2013, two independent groups, the BESIII Collaboration at China's Beijing Electron-Positron Collider and the Belle Collaboration at Japan's High Energy Accelerator Research Organization, reported evidence of a particle known as Z_c(3900), which may be the first experimentally observed tetraquark.

One distinct way of classifying mesons is by spin angular momentum states. Those mesons in which the quark-antiquark combination spins cancel to yield zero spin are called pseudoscalar mesons. Each quark or antiquark has a spin of 1/2. Mesons with spins of 1 are designated as vector mesons, and those with total angular momentum of 2, 3, or higher are called tensor mesons. These latter states can be arrived at by adding spin angular momentum to orbital angular momentum, which is similar to what is done in atomic physics. In other words, the quark-antiquark combinations sometimes can be viewed as orbiting each other in discrete integral angular momentum states.

Applications

Mesons have been used to probe nuclear matter and elementary particles such as the stabler baryons. Charged pions, the least massive (except for uncharged pions) and stablest of all the mesons, have the greatest strong-interaction range, on the order of a few femtometers (one femtometer equals 10^-15 meters). Kaons, the next largest set of mesons in both mass and stability, have interaction ranges that are four times smaller than those of pions, and kaons, too, can be used as diagnostic probes. Pion and kaon accelerators exist at several research facilities throughout the world.

Pions have also been used in the treatment of cancer patients. Because of their relatively low mass and well-known decay times, they can be used to implant a sizable amount of decay energy into the most dangerous parts of a localized tumor. Pions decay into muons and neutrinos, and muons in turn decay into electrons and neutrinos. The electrons move about in the tumor and damage its structure without damaging other tissue or organs. Other types of particles and radiation, such as x-rays and gamma rays, damage healthy tissue irreparably on their way to the tumor. High-energy electrons, which are also often used to damage tumors, usually deposit their energies near the surface of the tumor, not deep inside it.

Uncharged kaons exhibit a strange bifurcation that has been productive in physics theories. Most relations, equations, and laws in physics seem to conserve what physicists call parity. In other words, any basic physical process would look the same in the mirror as in the real world. However, kaon decays sometimes violate the conservation of parity, and the discovery of this fact has had marked importance in physics theories since the 1970s. One area of kaon studies focuses on whether any other sacrosanct rules are being violated in certain experiments, since, according to the famous charge-parity-time (CPT) symmetry concept, if parity is violated, then time reversal might be violated too.

Since pions, kaons, and certain other mesons are universal products of the decays of other particles, their detection signals the presence of nuclear interactions, cosmic-ray showers, and other high-energy events that are very important in analyzing the environment of the modern world. Many small nuclear reactors might be made feasible in the near future by a better understanding of pions and kaons.

Context

Pions, which are postulated to carry the nuclear strong force, were the first true mesons, both in theory and in practice, and they signaled the development of what has become known as elementary particle physics. Modern particle accelerators owe their existence to the discovery of pions, and even in the most exotic experiments performed in these same accelerators, pions invariably appear in the decay products.

A large percentage of particle-physics research is devoted to exploring the meson mass spectra at higher and higher energies. That so many mesons have been detected to date is a mystery of the first order in elementary particle physics. If the quark models of mesons are correct in stating that six quarks (and six antiquarks) in various combinations can explain all mesons, then it is a remarkable occurrence indeed.

If the history of physics of the twentieth and early twenty-first is any indication, several more families of mesons should be discovered by new families of accelerators and better cosmic-ray detective devices. Satellites orbiting the earth may be the most advanced form of mesonic detectors in the future, and what they detect might give scientists important insights into the processes that are taking place at the core of this galaxy and elsewhere in the universe.

Principal terms

BARYON: a type of fermion of 1/2 or 3/2 spin that participates in the strong interaction

BOSON: an elementary particle of integral spin that obeys Bose-Einstein statistics

FERMION: an elementary particle that obeys Fermi-Dirac statistics and has a spin of 1/2, 3/2, 5/2, or any other half-integer

HADRON: a type of composite particle that comprises two categories, baryons and mesons

LEPTON: a fermion of 1/2 spin that does not participate in the strong interaction

QUARK: a fermion of 1/2 spin and a fractional charge; quarks in varying combinations make up all hadrons

SPIN: the intrinsic angular momentum associated with all elementary particles, given in units of n/2, where n is any nonnegative integer

Bibliography

Fritzsch, Harald. Quarks: The Stuff of Matter. New York: Basic, 1983. Print. Chapters 3, 5, and 10 provide a popular account of developments in modern quark theory. The strong interactions are considered in chapter 3, and the way in which mesons are composed of quarks and antiquarks is explained in chapter 5. Can be read by high school students who have a basic knowledge of science.

Jamieson, Valerie. "Mesons May Explain Early Universe's Bias for Matter." New Scientist 9 July 2011: 15. Print.

Moskowitz, Clara. "Rare Subatomic Particle Discovery Pushes Limits of Current Physics." LiveScience. TechMedia, 5 Mar. 2012. Web. 10 Jan. 2014.

Particle Data Group. Review of Particle Properties. Physics Letters B 204 (1988): 1–486. Print. Every two years, the Particle Data Group, a committee of particle physicists from all over the world, updates all data available on elementary particles. About 50 percent of all elementary particles are mesons, and their place in theories and their experimental features are itemized clearly here. Although this study is detailed in sections, sizable parts can be read and understood by interested people or high school science students.

"Physicists Observe Subatomic Quick-Change Artist." Phys.org. Phys.org, 25 Sept. 2006. Web. 10 Jan. 2013.

Protopopescu, S. D., and N. P. Samios. "Light Hadron Spectroscopy: Experimental and Quark Model Interpretations." Annual Review of Nuclear and Particle Science 29 (1979): 333–39. Print. This is one of the most complete review articles written in the 1970s on light hadron, mainly meson, spectroscopy. It clearly outlines the experimental features and separates them from model-dependent features. It is somewhat specialized, but most of it can be read by undergraduate physics students.

Quigg, Chris. "Elementary Particles and Forces." Scientific American Apr. 1985: 84–95. Print. This very readable article, written by one of the leading researchers in theoretical particle physics, contains excellent diagrams that show how mesons fit into the present-day particle zoo. Shows how the strong interactions fit into the modern orthodox views of physics.

Serway, Raymond A., Clement J. Moses, and Curt A. Moyer. Modern Physics. 3rd ed. Belmont: Brooks, 2005. Print. This is a typical modern physics book used by undergraduates in science and engineering. It includes a lucid outline of elementary particle physics and a good section on the experimental and theoretical features of mesons.

Swanson, Eric. "Viewpoint: New Particle Hints at Four-Quark Matter." Physics 6.69 (2013): n. pag. Web. 10 Jan. 2014.

Thomson, Mark. Modern Particle Physics. New York: Cambridge UP, 2013. Print.

Weinberg, Steven. The Discovery of Subatomic Particles. New York: Scientific Amer., 1983. Print. This highly readable book by the winner of the 1979 Nobel Prize in Physics gives a good historical overview of the development of particle classifications into mesons, hadrons, and baryons and ends with a picture of the status of particle physics up to the early 1980s. For the informed reader.

Quarks and the Strong Interaction