Quark Transformation in β⁻ Decay

Steps:

• β⁻ decay converts a neutron (udd quarks) to a proton (uud quarks), increasing atomic number by 1.
• This requires one down quark (d) in the neutron to transform into an up quark (u) via the weak interaction.
• The process emits an electron (e⁻) and an antineutrino (ν̄_e) to conserve charge, lepton number, and energy.
• The quark change d → u + W⁻ boson, where W⁻ decays to e⁻ + ν̄_e.

Why A is correct:

• Matches the weak interaction rule: d → u + e⁻ + ν̄_e, conserving quantum numbers like charge (+2/3 for u vs. -1/3 for d, balanced by e⁻).

Why the others are wrong:

• B: Describes β⁺ decay, not β⁻ (wrong quark change and particles: u → d + e⁺ + ν_e).
• C: Wrong quark change (u → d decreases atomic number, opposite of β⁻).
• D: Wrong particles (positron and neutrino indicate β⁺, not β⁻; neutrino should be antineutrino).

Final answer: A