Baryons and Mesons

Hadrons are particles made of quarks. They are the only particles that feel the strong nuclear force, and they come in two families: baryons and mesons.A baryon is made of three quarks. The proton (uud) and the neutron (udd) are baryons. An antibaryon is made of three antiquarks, for example the antiproton (uud). Every baryon has baryon number B = +1 and every antibaryon has B = −1. The proton is the only stable baryon; a free neutron decays with a half-life of about 10 minutes.A meson is made of one quark and one antiquark, so its baryon number is 0. All mesons are unstable.The OCR specification uses two meson families, pions and kaons:pions: π⁺ (ud), π⁻ (du), π⁰ (a mixture of uu and dd)
kaons: K⁺ (us), K⁻ (su), K⁰ (ds)Pions contain only up and down quarks and antiquarks; every kaon contains a strange quark or a strange antiquark.Worked example: find the charge of the K⁺ meson. The up quark has charge +2/3 e and the anti-strange quark has charge +1/3 e (the opposite of the strange quark's −1/3 e).Q = +2/3 e + 1/3 e = +eThe same method works for any hadron: add the charges of its quarks and antiquarks. The charge of a hadron is always a whole number of e.