...as are top and bottom quarks (which some prefer to name truth and beauty)
First, it is worth noting that for every type of quark there is a corresponding antiquark. An antiproton consists of 2 up antiquarks and 1 down antiquark.
A Meson is composed of one quark and one antiquark:
From Wikipedia:
In particle physics, mesons (/ˈmiːzɒnz/ or /ˈmɛzɒnz/) are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a diameter roughly one femtometre,[citation needed] which is about 2⁄3 the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos. Uncharged mesons may decay to photons.
Mesons are not produced by radioactive decay, but appear in nature only as short-lived products of very high-energy interactions in matter, between particles made of quarks. In cosmic ray interactions, for example, such particles are ordinary protons and neutrons. Mesons are also frequently produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles.
In nature, the importance of lighter mesons is that they are the associated quantum-field particles that transmit the nuclear force, in the same way that photons are the particles that transmit the electromagnetic force. The higher energy (more massive) mesons were created momentarily in the Big Bang but are not thought to play a role in nature today. However, such particles are regularly created in experiments, in order to understand the nature of the heavier types of quark which compose the heavier mesons.
Flavourless mesons consist of a quark and an anti-quark of the same type, for example a charm quark and a charm antiquark. Flavoured quarks consist of a quark and a different type of antiquark, for example a charm quark and an antidown quark is a neutral kaon (meson).
The link provides various tables listing the kinds of mesons determined by the types of quark and antiquark constituents.
Hope this helps.
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