Down quarks are the second lightest quark , the first being up quarks .
When found in mesons ( particles made of one quark and one antiquark ) or baryons ( particles made of three quarks ) , the ` effective mass ' ( or ` dressed ' mass ) of quarks becomes greater because of the binding energy caused by the gluon field between each quarks ( see massâ `` energy equivalence ) . For example , the effective mass of down quarks in a proton is around .