Meson


In particle physics, a meson is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre, which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few tenths of a nanosecond. Heavier mesons decay to lighter mesons and ultimately to stable electrons, neutrinos and photons.
Outside the nucleus, mesons appear in nature only as short-lived products of very high-energy collisions between particles made of quarks, such as cosmic rays and baryonic matter. Mesons are routinely produced artificially in cyclotrons or other particle accelerators in the collisions of protons, antiprotons, or other particles.
Higher-energy mesons were created momentarily in the Big Bang, but are not thought to play a role in nature today. However, such heavy mesons are regularly created in particle accelerator experiments that explore the nature of the heavier quarks that compose the heavier mesons.
Mesons are part of the hadron particle family, which are defined simply as particles composed of two or more quarks. The other members of the hadron family are the baryons: subatomic particles composed of odd numbers of valence quarks, and some experiments show evidence of exotic mesons, which do not have the conventional valence quark content of two quarks, but four or more.
Because quarks have a spin, the difference in quark number between mesons and baryons results in conventional two-quark mesons being bosons, whereas baryons are fermions.
Each type of meson has a corresponding antiparticle in which quarks are replaced by their corresponding antiquarks and vice versa. For example, a positive pion is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion, is made of one up antiquark and one down quark.
Because mesons are composed of quarks, they participate in both the weak interaction and strong interaction. Mesons with net electric charge also participate in the electromagnetic interaction. Mesons are classified according to their quark content, total angular momentum, parity and various other properties, such as C-parity and G-parity. Although no meson is stable, those of lower mass are nonetheless more stable than the more massive, and hence are easier to observe and study in particle accelerators or in cosmic ray experiments. The lightest group of mesons is less massive than the lightest group of baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher-energy phenomena more readily than do baryons. But mesons can be quite massive: for example, the J/Psi meson containing the charm quark, first seen 1974, is about three times as massive as a proton, and the upsilon meson containing the bottom quark, first seen in 1977, is about ten times as massive as a proton.

History

From theoretical considerations, in 1934 Hideki Yukawa predicted the existence and the approximate mass of the "meson" as the carrier of the nuclear force that holds atomic nuclei together. If there were no nuclear force, all nuclei with two or more protons would fly apart due to electromagnetic repulsion. Yukawa called his carrier particle the meson, from μέσος mesos, the Greek word for "intermediate", because its predicted mass was between that of the electron and that of the proton, which has about 1,836 times the mass of the electron. Yukawa or Carl David Anderson, who discovered the muon, had originally named the particle the "mesotron", but he was corrected by the physicist Werner Heisenberg. Heisenberg pointed out that there is no "tr" in the Greek word "mesos".
The first candidate for Yukawa's meson, in modern terminology known as the muon, was discovered in 1936 by Carl David Anderson and others in the decay products of cosmic ray interactions. The "mu meson" had about the right mass to be Yukawa's carrier of the strong nuclear force, but over the course of the next decade, it became evident that it was not the right particle. It was eventually found that the "mu meson" did not participate in the strong nuclear interaction at all, but rather behaved like a heavy version of the electron, and was eventually classed as a lepton like the electron, rather than a meson. In making this choice, physicists decided that properties other than particle mass should control their classification.
There were years of delays in the subatomic particle research during World War II, with most physicists working in applied projects for wartime necessities. When the war ended in August 1945, many physicists gradually returned to peacetime research. The first true meson to be discovered was what would later be called the "pi meson". During 1939–1942, Debendra Mohan Bose and Bibha Chowdhuri exposed Ilford half-tone photographic plates in the high altitude mountainous regions of Darjeeling, and observed long curved ionizing tracks that appeared to be different from the tracks of alpha particles or protons. In a series of articles published in Nature, they identified a cosmic particle having an average mass close to 200 times the mass of electron. This discovery was made in 1947 with improved full-tone photographic emulsion plates, by Cecil Powell, Hugh Muirhead, César Lattes, and Giuseppe Occhialini, who were investigating cosmic ray products at the University of Bristol in England, based on photographic films placed in the Andes mountains. Some of those mesons had about the same mass as the already-known mu "meson", yet seemed to decay into it, leading physicist Robert Marshak to hypothesize in 1947 that it was actually a new and different meson. Over the next few years, more experiments showed that the pion was indeed involved in strong interactions. The pion is also used as force carrier to model the nuclear force in atomic nuclei. This is an approximation, as the actual carrier of the strong force is believed to be the gluon, which is explicitly used to model strong interaction between quarks. Other mesons, such as the virtual rho mesons are used to model this force as well, but to a lesser extent. Following the discovery of the pion, Yukawa was awarded the 1949 Nobel Prize in Physics for his predictions.
For a while in the past, the word meson was sometimes used to mean any force carrier, such as "the Z meson", which is involved in mediating the weak interaction. However, this use has fallen out of favor, and mesons are now defined as particles composed of pairs of quarks and antiquarks.

Overview

Spin, orbital angular momentum, and total angular momentum

is a vector quantity that represents the "intrinsic" angular momentum of a particle. It comes in increments of Planck constant|.
Quarks are fermions—specifically in this case, particles having spin . Because spin projections vary in increments of 1, a single quark has a spin vector of length, and has two spin projections, either or. Two quarks can have their spins aligned, in which case the two spin vectors add to make a vector of length, with three possible spin projections and, and their combination is called a vector meson or spin-1 triplet. If two quarks have oppositely aligned spins, the spin vectors add up to make a vector of length and only one spin projection, called a scalar meson or spin-0 singlet. Because mesons are made of one quark and one antiquark, they are found in triplet and singlet spin states. The latter are called scalar mesons or pseudoscalar mesons, depending on their parity.
There is another quantity of quantized angular momentum, called the orbital angular momentum, that is the angular momentum due to quarks orbiting each other, and also comes in increments of 1 . The total angular momentum of a particle is the combination of the two intrinsic angular momentums and the orbital angular momentum. It can take any value from up to, in increments of 1.
Spin |Angular momentum operator#Orbital angular momentum|#Parity|Total angular momentum|
00−00
01+11
02−22
03+33
10−11
11+2, 02, 0
12−3, 13, 1
13+4, 24, 2

Particle physicists are most interested in mesons with no orbital angular momentum, therefore the two groups of mesons most studied are the = 1; = 0 and = 0; = 0, which corresponds to = 1 and = 0, although they are not the only ones. It is also possible to obtain = 1 particles from = 0 and = 1. How to distinguish between the = 1, = 0 and = 0, = 1 mesons is an active area of research in meson spectroscopy.

P-parity

-parity is left-right parity, or spatial parity, and was the first of several "parities" discovered, and so is often called just "parity". If the universe were reflected in a mirror, most laws of physics would be identical—things would behave the same way regardless of what we call "left" and what we call "right". This concept of mirror reflection is called parity. Gravity, the electromagnetic force, and the strong interaction all behave in the same way regardless of whether or not the universe is reflected in a mirror, and thus are said to conserve parity. However, the weak interaction does distinguish "left" from "right", a phenomenon called parity violation.
Based on this, one might think that, if the wavefunction for each particle were simultaneously mirror-reversed, then the new set of wavefunctions would perfectly satisfy the laws of physics. It turns out that this is not quite true: In order for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by −1, in addition to being mirror-reversed. Such particle types are said to have negative or odd parity, whereas the other particles are said to have positive or even parity.
For mesons, parity is related to the orbital angular momentum by the relation:
where the is a result of the parity of the corresponding spherical harmonic of the wavefunction. The "+1" comes from the fact that, according to the Dirac equation, a quark and an antiquark have opposite intrinsic parities. Therefore, the intrinsic parity of a meson is the product of the intrinsic parities of the quark and antiquark. As these are different, their product is −1, and so it contributes the "+1" that appears in the exponent.
As a consequence, all mesons with no orbital angular momentum have odd parity.