composite particle made up of an even number of elementary particles, most commonly one particle and one anti-particle.
This can be contrasted with mesons, which have an odd number of elementary particles, as mentioned at baryon vs meson vs lepton.
Conceptually the simplest mesons. All of them have neutral color charge:
- charged: down + anti-up or up + anti-down, therefore with net electrical charge electron charge
- neutral: down + anti-down or up + anti-up, therefore with net electrical charge 0
One strange quark bound with one up quark or a down quark. 6 combinations exist, 4 if we consider antiparticles the same as particles.
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A meson is a hadron with integer spin and zero baryon number. In the ordinary valence quark model, it contains one quark and one antiquark. Their spins and relative orbital angular momentum combine to determine total angular momentum and parity.
Ground-state pions and kaons are examples of pseudoscalar mesons, with spin-parity . A meson need not be electrically neutral: the constituent quark charges can sum to a nonzero value. The valence label is a classification of its quantum numbers, not a claim that its exact interacting state consists of only two particles at all times.