A meson is a subatomic particle made up of an up, down, charm, strange and bottom quark and an anti-up quark that is a hadron, or particle composed of quarks and antiquarks, and thus includes mesons as well as baryons. The most stable mesons are pions (made of a quark and an antiquark), because they are tightly bound because of the Pauli exclusion principle. They are produced in the decay of nuclei and are the most abundant particles in our universe. The other two families of mesons are the more weakly bound kaons and the more loosely bound eta mesons. Mesons exist only in the confined environment of a nucleus or in the vacuum. They can be created in particle colliders and are the building block for baryons. Mesons decay quickly, but can have strange lifetime below a few minutes due to quark-antiquark annihilation (see quark mixing). Fast moving mesons decay into pions by emitting virtual quark-antiquark pairs. This is why pions and kaons can be created in particle accelerators by colliding high energy quarks with protons: the heavier quark-antiquark pairs are created, and the lighter ones can be created by the subsequent decaying of the heavier mesons. The heavy quarks (charm, strange, bottom) are less massive than quarks of the first two generations and are less tightly bound, so they decay more quickly and hence tend to be shorter lived than the first generation quarks.
Thus, matter can be defined as the sum of all known elementary fermions (quarks, antiquarks, leptons and antileptons) that, in principle, may be associated with protons and neutrons, in different combinations, and different completely different particles, such as neutrinos, neutrino-antineutrinos, neutrinos-antineutrinos, neutrinos-neutrino-antineutrinos, neutrinos-neutrinos-neutrino-antineutrinos, etcetera. This even includes some exotic particles whose particles may not yet be known. The mass of a particle is defined in terms of its baryon or lepton number (and charge); so matter is a sum of baryons, leptons and mesons. d2c66b5586