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A meson is a particle made of one quark and one antiquark — and the lightest meson, the pion, is the main source of the neutrinos we study. Mesons round out the family of particles built from quarks, and they sit at the junction between the quark world and neutrino physics, because their decays are how most neutrinos are made.
Quark plus antiquark
A meson is a composite particle: one quark bound to one antiquark by the strong nuclear force. This makes it a hadron, the family of strongly interacting particles — but a different kind from the baryon, which is made of three quarks. Because a meson pairs a quark with an antiquark, it is always unstable: the two can annihilate, so mesons decay quickly into lighter particles. Although they are built from spin-½ quarks, mesons have whole-number spin, which technically makes them bosons — though composite ones, not fundamental force carriers.
A family of mesons
There are many mesons, distinguished by which quarks they contain. The lightest and most important is the pion, made of the lightest quarks. Heavier mesons include the kaon, which contains a strange quark, and still heavier ones containing charm or bottom quarks. The pion was originally predicted by Hideki Yukawa in the 1930s as the carrier of the force binding atomic nuclei — the first meson conceived, and the reason the whole category exists.
Why mesons make neutrinos
The crucial link to neutrino physics is decay. Charged mesons decay through the weak force, and when they do, they produce neutrinos. A charged pion decays into a muon and a muon neutrino; kaons decay in similar ways. This is the dominant source of neutrinos in two major settings: the atmospheric neutrinos created when cosmic rays strike the air and produce showers of pions, and the accelerator neutrino beams made by firing protons at a target to create pions that decay in flight. Nearly every neutrino from these sources traces back to a decaying meson.
Kaons and the matter-antimatter puzzle
Mesons also have a deeper connection to one of the questions neutrinos may help answer. CP violation — the subtle difference in how matter and antimatter behave — was first discovered in the 1960s in the decays of neutral kaons. That discovery established that nature does distinguish matter from antimatter, a prerequisite for explaining why the Universe is made of matter. The amount of CP violation in kaons turned out to be too small to account for the matter excess, which is part of why physicists now look to the neutrino sector for more.
Mesons versus neutrinos
For clarity: a neutrino is not a meson. The neutrino is a fundamental lepton, with no quarks and no internal structure, feeling only the weak force. A meson is a composite hadron made of quarks, feeling the strong force. They belong to different categories entirely — but mesons are the parents of most neutrinos, linked by the weak-force decays that bring neutrinos into being.
In short
A meson is a composite particle made of a quark and an antiquark, unstable and quick to decay. The lightest meson, the pion, together with the kaon, produces most of the neutrinos physicists detect, and kaon decays revealed the first CP violation. Though neutrinos are not mesons, these quark-antiquark particles are where most neutrinos are born.