Quarks and Antiquarks

Quarks are fundamental particles that make up hadrons. At A level, OCR uses three types (flavours) of quark: up (u), down (d) and strange (s). Three heavier flavours (charm, top and bottom) also exist but are not needed here.Quarks have fractional charges, measured in units of the elementary charge e (1.60 × 10⁻¹⁹ C):up u: +²⁄₃e    down d: −¹⁄₃e    strange s: −¹⁄₃eEach quark has an antiquark with the same mass and the opposite charge. Antiquarks are written with a bar over the symbol:anti-up ū: −²⁄₃e    anti-down d̄: +¹⁄₃e    anti-strange s̄: +¹⁄₃eQuarks are never observed on their own. They are always bound together by the strong nuclear force inside hadrons, in combinations whose total charge is a whole number of e.The charge of a hadron is found by adding the charges of its quarks.Worked example: find the charge of a particle made of an up quark and an anti-down quark (u d̄).
charge = (+²⁄₃e) + (+¹⁄₃e) = +1eSimilarly, a particle made of u u s has charge (+²⁄₃) + (+²⁄₃) + (−¹⁄₃) = +1e.