Meson
Unstable hadrons composed of a quark and an antiquark.
Mesons are a class of hadronic subatomic particles consisting of an equal number of quarks and antiquarks—typically one of each—held together by the strong interaction. They have a measurable physical size, with a diameter of about one femtometre, which is roughly 0.6 times that of a proton or neutron. All mesons are unstable; even the longest-lived decay within a few tenths of a nanosecond. Heavier mesons decay into lighter ones, ultimately producing stable electrons, neutrinos, and photons. Outside atomic nuclei, mesons appear only fleetingly as products of very high-energy collisions between quark-based particles, such as cosmic rays or baryonic matter. They are also routinely created artificially in particle accelerators when protons, antiprotons, or other particles collide. While heavier mesons were produced momentarily in the Big Bang, they no longer play a role in nature today, though they are regularly generated in accelerator experiments to study heavier quarks. Mesons belong to the hadron family, which also includes baryons—particles composed of odd numbers of valence quarks (at least three). Some experiments suggest the existence of exotic mesons with four or more quarks. Because quarks have spin, conventional two-quark mesons are bosons, whereas baryons are fermions. Each meson has a corresponding antiparticle, or antimeson, in which quarks are swapped for their antiquarks. For example, a positive pion consists of an up quark and a down antiquark, while its antiparticle, the negative pion, contains an up antiquark and a down quark. Mesons participate in both the weak and strong interactions; those with net electric charge also interact electromagnetically. They are classified by quark content, total angular momentum, parity, and properties like C-parity and G-parity. Lower-mass mesons are more stable and easier to study than heavier ones. The lightest mesons are less massive than the lightest baryons, making them more readily produced in experiments. However, some mesons are very heavy: the J/Psi meson, containing the charm quark and first observed in 1974, is about three times as massive as a proton, while the upsilon meson, containing the bottom quark and discovered in 1977, is roughly ten times as massive. The concept of the meson was first predicted in 1934 by Hideki Yukawa, who proposed it as the carrier of the nuclear force holding atomic n
- type
- Hadronic subatomic particle
- composition
- Equal number of quarks and antiquarks (usually one of each)
- size
- Diameter roughly one femtometre (10⁻¹⁵ m)
- stability
- All unstable; longest-lived lasts a few tenths of a nanosecond
- spin
- Bosons (integer spin) due to quark-antiquark pair
- interactions
- Strong and weak interactions; electrically charged mesons also participate in electromagnetic interaction
Lore & Background
Mesons are hadronic subatomic particles composed of one quark and one antiquark, bound by the strong interaction. They have a physical size of roughly one femtometre in diameter, about 0.6 times the size of a proton or neutron. All mesons are unstable; the longest-lived last only a few tenths of a nanosecond, decaying into lighter mesons and ultimately into stable electrons, neutrinos, and photons. Outside the nucleus, mesons appear only as short-lived products of very high-energy collisions between quark-containing particles, such as cosmic rays and baryonic matter, and are routinely produced artificially in cyclotrons or particle accelerators. Heavier mesons were created momentarily in the Big Bang but are not thought to play a role in nature today, though they are regularly created in accelerator experiments. Mesons are bosons, unlike baryons which are fermions, due to their quark number difference. Each meson has a corresponding antiparticle, where quarks are replaced by antiquarks and vice versa. Mesons participate in the weak and strong interactions; those with net electric charge also interact electromagnetically. They are classified by quark content, angular momentum, parity, and other properties. The lightest mesons are less massive than the lightest baryons, making them easier to produce and study, though some mesons can be very massive—for instance, the J/Psi meson is about three times as massive as a proton, and the upsilon meson about ten times as massive.
Reader's Guide
Although the actual carrier of the strong force is now believed to be the gluon, mesons such as the pion are still used as approximate models for the nuclear force between protons and neutrons. Mesons are routinely produced artificially in particle accelerators and appear in nature only as short-lived products of high-energy collisions, such as cosmic rays. They are classified by quark content, spin, parity, and other properties, and heavier mesons decay to lighter ones, ultimately to stable electrons, neutrinos, and photons. The lightest mesons are less massive than the lightest baryons, making them easier to produce and study in experiments.
Did You Know?
- Mesons have a diameter of roughly one femtometre, about 0.6 times the size of a proton or neutron.
- The first true meson discovered was the pion, identified in 1947 by Cecil Powell and others using photographic films placed in the Andes mountains.
- The J/Psi meson, containing the charm quark, is about three times as massive as a proton.
- Mesons are bosons because they consist of a quark and an antiquark, each with spin 1/2, resulting in integer total spin.
Frequently Asked Questions
What is a meson in particle physics?
A meson is a hadronic subatomic particle built from an equal number of quarks and antiquarks, most commonly a single quark–antiquark pair held together by the strong force. It sits alongside baryons (like protons and neutrons) in the broader hadron family, but its quark content is fundamentally different.
What is a meson made of?
In its simplest form a meson is one quark bound to one antiquark by the strong interaction. Because the pair carries integer spin, every meson is classified as a boson rather than a fermion.
How long does a meson last before decaying?
All mesons are unstable; even the longest-lived species survive for only a few tenths of a nanosecond before falling apart. This extreme transience is one of the key reasons mesons are studied in high-energy collisions rather than in stable matter.
How big is a meson compared to a proton?
A meson's diameter is roughly one femtometre (10⁻¹⁵ m), which works out to about 60 % of a proton's or neutron's size. The compactness reflects how tightly the strong force squeezes the quark–antiquark pair together.
Which fundamental forces act on mesons?
Mesons take part in both the strong and weak nuclear interactions, and any meson that carries an electric charge also couples to the electromagnetic force. Together these three quantum interactions fully describe how mesons are created, scatter, and decay.
More in Particle And Nuclear Physics 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record