Mass
Despite being extremely common, the bare mass of the down quark is not well determined, but probably lies between 4.1 and 5.7 MeV/c2.. Lattice QCD calculations give a more precise value: 4.79±0.16 MeV/c2.
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 mass–energy equivalence). For example, the effective mass of down quarks in a proton is around 330 MeV/c2. Because the bare mass of down quarks is so light, it cannot be straightforwardly calculated because relativistic effects have to be taken into account.
Read more about this topic: Down Quark
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