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Edexcel A Level Physics:復(fù)習(xí)筆記8.11 The Standard Model

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年10月12日 下午3:20

Baryons & Mesons

  • All particles of?matter?are made up of either?quarks?and/or?leptons
    • The?standard?model?of particle physics categorises quarks and leptons by charge and mass

8-11-standard-model-edexcel_edexcel-al-physics-rn

Quarks and leptons form the standard model of particle physics. The first generation of particles make up all ordinary matter

 

  • Hadrons?are made up of?quarks?and interact with the?strong nuclear force
  • Baryons?and?mesons?are types of hadron
    • Baryons?consist of?3 quarks
    • Mesons?consist of a?quark-antiquark pair

     

  • The most common baryons are protons and neutrons
  • The most common mesons are pions and kaons

11.2.4-Baryons-and-Mesons

Hadrons may be either a baryon or a meson. Both baryons and mesons interact with the strong nuclear force

 

Bold-Textr

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

 

  • Quarks have never been discovered on their own, always in pairs or groups of three
  • 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. 

      11.2.4-Wrong-quark-composition

     

  • The anti–particle of a meson is still a quark and anti–quark pair. The difference being the quark becomes the anti–quark and vice versa

Worked Example

The baryon Δ++?was discovered in a particle accelerator using accelerated positive pions on hydrogen targets.

Which of the following is the quark combination of this particle?Worked-Example-Baryons-Mesons-e1615291775993

11.2.4-Worked-example-Baryons-and-mesons-e1615291800307

Exam Tip

  • Remembering quark combinations is useful for the exam
  • However, as long as you can remember the charges for each quark, it is easy to figure out the combination by making sure the combination of quarks adds up to the total charge of the particle (just like in the worked example!)

Leptons & Photons

Leptons

  • Leptons?are a group of?fundamental?(elementary) particles
    • This means they are not made up of any other particles (no quarks)

     

  • Leptons interact with other particles via the?weak,?electromagnetic?or?gravitational?interactions
    • Unlike hadrons (baryons & mesons), leptons?do?not?interact via the strong nuclear force

     

  • The most common leptons are:
    • The electron, e–
    • The electron neutrino, ve
    • The muon, μ–
    • The muon neutrino, vμ

     

2.2.4Leptons-Antileptons

The most common leptons are the electron and muon, along with their associated neutrinos

 

  • The muon is similar to the electron but is slightly?heavier
  • Electrons and muons?both?have a charge of -1e?and a mass of 0.0005u
  • Neutrinos are the most abundant leptons in the universe and have?no charge?and?negligible?mass (almost 0)
  • Although quarks are fundamental particles too, they are not classed as leptons

Worked Example

Circle all the anti-leptons in the following decay equation.

WE-Leptons-question-image

11.2.6-Worked-example-leptons-e1615296390352

Photons

  • Photons are a group of particles which mediate the?electromagnetic interaction
    • They are?uncharged
    • They have zero?mass
  • They are sometimes called "exchange bosons" because they mediate one of the fundamental forces (electromagnetism)
    • For example, the electrostatic repulsion between two electrons is understood in terms of?exchanging photons

Exam Tip

In some topics, you may need to use the?energy?of a photon. This is given by the equation?E?=?hf?=?