A proton is made up of two up [[quark]]s and one down quark. According to the [[Standard Model]] of particle physics, quarks are [[fundamental particle]]s, meaning that they cannot be split into smaller particles.<ref>Krane: ''Modern Physics'', 2nd edition</ref> | A proton is made up of two up [[quark]]s and one down quark. According to the [[Standard Model]] of particle physics, quarks are [[fundamental particle]]s, meaning that they cannot be split into smaller particles.<ref>Krane: ''Modern Physics'', 2nd edition</ref> |