Physics 9702 · Paper 2

Beta Decay at the Quark Level

Earlier we said beta decay turns a neutron into a proton. Now that we know both are made of quarks, we can see the exact change. It is one quark.

One quark changes

A neutron is udd and a proton is uud. Only one quark is different. To turn a neutron into a proton, a single down quark changes into an up quark.

Beta-minus decay in detail: inside the neutron (udd) a single down quark turns into an up quark, making a proton (uud). The change d → u also releases an electron (the β⁻ particle) and an antineutrino.

d \rightarrow u + {}^{\ \ 0}_{-1}\beta + \bar{\nu}

That is beta-minus decay at the quark level: $d \rightarrow u$ , releasing an electron and an antineutrino. Check the charge: a down ( $-\tfrac{1}{3}$ ) becomes an up ( $+\tfrac{2}{3}$ ), a rise of $+1$ . The electron takes away that $-1$ , so the charge still balances.
Beta-plus decay is the reverse. A proton turns into a neutron, so an up quark becomes a down quark, $u \rightarrow d$ , releasing a positron and a neutrino. (9702/23/O/N/24 Q7(c)) (9702/12/F/M/25 Q39)
The quantities still balance across the change: charge, and the count of quarks. This is the same event as the nuclear equation, seen in detail.

Beta-minus is one quark changing: $d \rightarrow u$ (neutron udd → proton uud), plus an electron and an antineutrino. Beta-plus is the reverse, $u \rightarrow d$ , plus a positron and a neutrino.

What causes a quark to change

A quark changing flavour is not something gravity or the electric force can do. It takes a fourth fundamental force, the weak nuclear force. This force is what causes beta decay. It is called weak compared with the strong nuclear force that holds the nucleus together.

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Beta Decay at the Quark Level

Earlier we said beta decay turns a neutron into a proton. Now that we know both are made of quarks, we can see the exact change. It is one quark.

One quark changes

A neutron is udd and a proton is uud. Only one quark is different. To turn a neutron into a proton, a single down quark changes into an up quark.

d \rightarrow u + {}^{\ \ 0}_{-1}\beta + \bar{\nu}

That is beta-minus decay at the quark level: $d \rightarrow u$ , releasing an electron and an antineutrino. Check the charge: a down ( $-\tfrac{1}{3}$ ) becomes an up ( $+\tfrac{2}{3}$ ), a rise of $+1$ . The electron takes away that $-1$ , so the charge still balances.
Beta-plus decay is the reverse. A proton turns into a neutron, so an up quark becomes a down quark, $u \rightarrow d$ , releasing a positron and a neutrino. (9702/23/O/N/24 Q7(c)) (9702/12/F/M/25 Q39)
The quantities still balance across the change: charge, and the count of quarks. This is the same event as the nuclear equation, seen in detail.

Beta-minus is one quark changing: $d \rightarrow u$ (neutron udd → proton uud), plus an electron and an antineutrino. Beta-plus is the reverse, $u \rightarrow d$ , plus a positron and a neutrino.

What causes a quark to change