Quark Flavor Change in Beta-Minus Decay

Steps:

• Neutron (udd) decays to proton (uud) via β⁻ emission, requiring one d quark to become a u quark.
• The weak interaction mediates this: d → u + W⁻ virtual boson.
• W⁻ decays to e⁻ + \bar{ν}_e (electron and antineutrino).
• Overall process: d → u + e⁻ + \bar{ν}_e, conserving charge, lepton number, and baryon number.

Why A is correct:

• Describes the exact quark transition and emitted particles in neutron β⁻ decay, per the weak force rules.

Why the others are wrong:

• B: ū (antiquark) would violate baryon number; ν (not \bar{ν}) mismatches lepton conservation.
• C: u → d decreases up quarks, yielding ddd (not proton); wrong for β⁻.
• D: u → d with e⁺ + ν describes β⁺ decay (proton → neutron), opposite process.

Final answer: A