Final answer:
Mesons are composed of a quark and an antiquark; they do have real antiquarks, but these are of different flavors than the corresponding quarks. Mesons with the same flavor of quark and antiquark can annihilate each other, while those with different flavors have longer half-lives due to weak force decays.
Step-by-step explanation:
The question 'Why do mesons have no real antiquark?' seems to stem from a misunderstanding. By definition, mesons are hadrons which are composed of a quark and an antiquark. The composition includes different types of quarks such as up (u), down (d), charm (c), strange (s), top (t), and bottom (b). For instance, a positive pion (π+) is made of an up quark and an anti-down quark (ud), while its antiparticle, the negative pion (π-), is made up of an anti-up quark and a down quark (¯ud).
It is important to clarify that mesons do have real antiquarks; it's just that their antiquarks are of a different 'flavor' than the quarks. When mesons like the pion are said to be their own antiparticles, it refers to neutral mesons composed of a quark and its respective antiquark of the same flavor. For instance, the neutral pion (π⁰) has either an up quark and an anti-up quark, or a down quark and an anti-down quark as its constituents.
The baryon number is conserved in these particles and remains zero (+(1/3) ¯(1/3) = 0). They generally have a short half-life because quarks of matter and antiquarks of antimatter can annihilate each other if they are of the same flavor. However, mesons composed of different flavors of quarks and antiquarks can have relatively longer half-lives due to the weak force causing decay by changing one flavor into another.