Antineutron quark composition

Steps:

• Neutron consists of one up quark and two down quarks: udd.
• Antineutron, as the antiparticle, has corresponding antiquarks: one anti-up (ū) and two anti-down (d̄, denoted ā).
• Standard notation uses ū for anti-up and ā for anti-down.
• Choice A matches ū d̄ d̄ exactly.

Why A is correct:

• By the Dirac equation and particle-antiparticle symmetry in quantum field theory, the antineutron's quark content is the charge-conjugate of the neutron's udd, yielding ū d̄ d̄.

Why the others are wrong:

• B: ū ū d̄ represents the antiproton, not antineutron.
• C: ū d d mixes antiquarks and quarks, violating antiparticle purity.
• D: ū d ū has two anti-up quarks and one down quark, mismatching the required two anti-down.

Final answer: A