6.8.3 The Quark Model

• Hadrons are a 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

• 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

Types of Quark

• Quarks are fundamental particles that make up other subatomic particles such as protons and neutrons
• Protons and neutrons are in a category of particles called hadrons
  • Hadrons are defined as any particle made up of quarks

• Fundamental means that quarks are not made up of any other particles. Another example is electrons
• Quarks have never been observed on their own, they’re either in pairs or groups of three
• There are six flavours (types) of quarks that exist:

The six flavours of quarks

You only need to know about up, down and strange, as well as their antiquarks.

Charges of Quark

• The charge of a hadron is determined by the sum of the charges of its quarks
• Each flavour of quark has a certain relative charge:

Each flavour of quark has a charge of either +⅔e or -⅓e

You only need to know about up, down and strange, as well as their antiquarks.

• For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:

+⅔e + ⅔e - ⅓e = +1e

• The equivalent antiparticle of the quark is the anti-quark
• These are identical to quarks except with opposite relative charges

Each flavour of anti-quark has a charge of either -⅔e or +⅓e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks

You only need to know about up, down and strange, as well as their antiquarks.

• The correct answer is D because: 
  • Each answer option has the following quarks and charges:  
    • A:  = = 0
    • B:  = = +1
    • C:  = 
    • D:  = 
  • We can see that the correct answer is D the combination anti-up, anti-up, anti-strange give a charge of -1