AQA A Level Physics

Revision Notes

2.2.5 Quarks & Antiquarks

Properties of Quarks & Antiquarks

Quarks

  • The three most common flavours of quarks are: up, down and strange
  • The majority of hadrons are made up of different combinations of these quarks

2.2.5Quarks

The three main quarks that make up most hadrons

  • Each quark has a charge, baryon number and strangeness
  • These will be provided on the data sheet

Quarks Table provided on the datasheet

2.2.5Quarks-Table

  • The charge of a hadron is determined by the sum of the charges of its quarks
  • 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:

  • Equivalently, the baryon number and strangeness of a hadron is determined by the sum of the baryon numbers and strangeness of its quarks

Anti-quarks

  • The equivalent antiparticle of the quark is the anti-quark

2.2.5Anti-quarks

The three main anti-quarks that make up most hadrons

  • These are identical to quarks except with opposite relative charges, baryons numbers and strangeness

Anti-quarks Table

2.2.5Anti-quarks-Table

  • Quarks have a baryon number of +1/3
  • Anti-quarks having a baryon number of –1/3
  • Strange quarks have a strangeness of –1
  • Anti-strange quarks have a strangeness of +1
    • This is unique to the strange quark

Worked Example

A K particle has a strangeness of –1. Determine the quark structure of this particle.

Quarks & Antiquarks Worked Example

Exam Tip

You will not be expected to remember the charge of each quark as this information is provided on the datasheet. So, instead of memorising the charges of anti-quarks too, just remember they are identical but with opposite signs.

Quark Combinations

Baryons

  • 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

 

2.2.5Proton-and-Neutron-Quarks

Protons and neutrons are made up of three quarks

Mesons

  • Pions and kaons are made up of a quark and anti-quark pair
  • Pions are either:
    • π+ made up of an up quark and an anti-down quark
    • π made up on an anti-up quark and a down quark
    • π0 made up of an up quark and anti-up quark or down quark and anti-down quark

Pion Quark Combination, downloadable AS & A Level Physics revision notes

Pions consist of up and down quarks, and their respective antiparticles, depending on their sign

  • Kaons are either:
    • K+ made up of an up quark and an anti-strange quark
    • K made up on an anti-up quark and a strange quark
    • K0 made up of an down quark and anti-strange quark or anti-down quark and strange quark

Kaon Quark Combination, downloadable AS & A Level Physics revision notes

Kaons consist of up, down and strange quarks, and their respective antiparticles, depending on their sign

Worked Example

Worked example - quarks in an element, downloadable AS & A Level Physics revision notes

Step 1: Calculate the number of protons

             The number of protons is from the proton number = 26 protons

Step 2: Calculate the number of neutrons

    Number of neutrons = nucleon number – proton number

    Number of neutrons = 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 × 2 up quarks = 52 up quarks

    30 neutrons × 1 up quark = 30 up quarks

    Total number of up quarks = 52 + 30 = 82 up quarks

Exam Tip

The quark combinations for protons, neutrons, pions and kaons will be expected to be memorised for the exam.

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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