In beta decay, what happens to a neutron?

• A. A proton turns into a neutron.
• B. A neutron turns into a proton.
• C. A neutron decays into a photon and neutrino.
• D. A neutron splits into two neutrons.

Answer: (B)

In beta decay, a neutron converts into a proton through the weak interaction. One down quark inside the neutron turns into an up quark, changing udd into uud, which raises the charge by +1 while keeping the baryon number the same. This transformation is accompanied by the emission of an electron and an electron antineutrino, carried away by the mediating W- boson. So the net result is a neutron becoming a proton with these light particles emitted. The other descriptions don’t fit: turning a proton into a neutron describes the opposite process; decaying into a photon and neutrino would violate the typical particle-emission pattern of beta decay; splitting into two neutrons would violate fundamental conservation laws.