The Quark Model (OCR A Level Physics)

Quark Model of Hadrons

• 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

Quark Model of the Proton and Neutron

• 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

Charges of 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.