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Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre (10 −15 m), [1] which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few tenths of a nanosecond.
Mesons named with the letter "f" are scalar mesons (as opposed to a pseudo-scalar meson), and mesons named with the letter "a" are axial-vector mesons (as opposed to an ordinary vector meson) a.k.a. an isoscalar vector meson, while the letters "b" and "h" refer to axial-vector mesons with positive parity, negative C-parity, and quantum numbers I G of 1 + and 0 − respectively.
The intrinsic parity of the pion is P = −1 (since the pion is a bound state of a quark and an antiquark, which have opposite parities, with zero angular momentum), and parity is a multiplicative quantum number. Therefore, assuming the parent particle has zero spin, the two-pion and the three-pion final states have different parities (P = +1 ...
The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's short lifetime. The combination of a bottom antiquark and a bottom quark is not a B meson, but rather bottomonium, which is something else entirely. Each B meson has an antiparticle that is composed of a bottom quark and an up (B − ...
However, the electroweak interaction – which can transform one flavour of quark into another – causes a small but significant amount of "mixing" of the eigenstates (with mixing angle θ P = −11.5°), [11] so that the actual quark composition is a linear combination of these formulae. That is:
In the quark model, an up quark and an anti-down quark make up a π +, whereas a down quark and an anti-up quark make up the π −, and these are the antiparticles of one another. The neutral pion π 0 is a combination of an up quark with an anti-up quark, or a down quark with an anti-down quark.
The quark composition of the ω meson can be thought of as a mix between u u, d d and s s states, but it is very nearly a pure symmetric u u-d d state. This can be shown by deconstructing the wave function of the ω into its component parts. We see that the ω and ϕ mesons are mixtures of the SU(3) wave functions as follows. [7]
Ordinary mesons are made up of a valence quark and a valence antiquark. Because mesons have integer spin (0 or 1) and are not themselves elementary particles, they are classified as "composite" bosons, although being made of elementary fermions. Examples of mesons include the pion, kaon, and the J/ψ.