CIE A Level Physics复习笔记11.2.2 Quark Composition

• Protons and neutrons are not fundamental particles. They are each made up of three quarks
• Protons are made up of two up quarks and a down quark
• Neutrons are made up of two down quarks and an up quark

Protons and neutrons are made up of three quarks

• You will be expected to remember these quark combinations for exam questions

• Step 1: Calculate number of protons:
  • The number of protons is from the proton number = 26 protons
• Step 2: Calculate number of neutrons:
  • The number of neutrons = nucleon number - proton number = 56 - 26 = 30 neutrons
• Step 3: Up quarks in a proton:
  • Protons are made up of uud quarks = 2 up quarks
• Step 4: Up quarks in a neutron:
  • Neutrons are made up of udd quarks = 1 up quark
• Step 5: Total number of up quarks:
  • 26 protons x 2 up quarks = 52 up quarks
  • 30 neutrons x 1 up quark = 30 up quarks
  • 52 + 30 = 82 up quarks

• Hadrons are the group of subatomic particles that are made up of quarks
• These may be either a:
  • Baryon (3 quarks)
  • Meson (quark and anti-quark pair)

Hadrons may be either a baryon or a meson

• Quarks have never been discovered on their own, always in pairs or groups of three
• Anti-hadrons can be either
  • Anti-baryons (3 anti-quarks)
  • Anti-meson (quark and anti-quark pair)

Anti-hadrons may be either an anti-baryon or an anti-meson

• Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.
• This means quarks in a baryon are either all quarks or all anti-quarks. Combination of quarks and anti-quarks don’t exist in a baryon
  • e.g.
• The anti-particle of a meson is still a quark-antiquark pair. The difference being the quark becomes the anti-quark and vice versa

• Beta decay happens via the weak interaction
  • This is one of the four fundamental forces and it’s responsible for radioactive decays

Quark Composition: β^- decay

• Recall that β^- decay is when a neutron turns into a proton emitting an electron and anti-electron neutrino
• More specifically, a neutron turns into a proton because a down quark turning into an up quark

Beta minus decay is when a down quark turns into an up quark

Quark Composition: β⁺ decay

• Recall that β⁺ decay is when a proton turns into a neutron emitting an positron and an electron neutrino
• More specifically, a proton turns into a neutron because an up quark turns into a down quark

Beta minus decay is when an up quark turns into a down quark