Particle Interaction Equations (Edexcel International A Level Physics)

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Particle Interaction Equations

  • All particle interactions must obey a set of conservation laws. These are conservation of:
    • Charge, Q
    • Baryon number, B
    • Lepton Number, L
    • Energy (or mass-energy)
    • Momentum

  • Quantum numbers such as Q, B and L can only take discrete values (ie. 0, +1, –1, 1/2)
  • To know whether a particle interaction can occur, check whether each quantum number is equal on both sides of the equation
    • If even one of them, apart from strangeness in weak interactions, is not conserved then the interaction cannot occur

Worked example

A reaction that is proposed to create antiprotons in a laboratory is shown below: 

p presubscript 1 presuperscript 1 plus p presubscript 1 presuperscript 1 rightwards arrow p presubscript 1 presuperscript 1 plus p presubscript 1 presuperscript 1 plus pi presubscript 1 presuperscript 0 superscript plus plus scriptbase p with bar on top end scriptbase presubscript negative 1 end presubscript presuperscript negative 1 end presuperscript

Determine whether this reaction is permitted. 

Step 1: Determine conservation of charge Q

    • There are two protons on the left hand side
    • There are two protons on the right hand side, with a positively charged pion (Q = +1) and an antiproton (Q = –1)
    • Therefore charge is conserved, because:  

1 + 1 = 1 + 1 + 1 + (– 1)

Step 2: Determine conservation of baryon number, B

    • There are two baryons on the left hand side, each with a baryon number B = +1 (protons are baryons) 
    • On the right hand side
      • Two protons each with baryon number +1
      • One pion, with a baryon number 0 (it is a meson)
      • One anti-proton with a baryon number –1 
    • Therefore baryon number is not conserved, because:  

1 + 1 = 1 + 1 + 0 + (– 1)

Step 3: Conclude whether this reaction is permitted

    • This reaction is not permitted
    • Because baryon number is not conserved

Worked example

The equation for β decay is2.3.3 Beta Minus Equation

Using the quark model of beta decay, prove that the charge is conserved in this equation.

Worked example - beta decay quarks, downloadable AS & A Level Physics revision notes

Exam Tip


  • Quantum numbers for any exotic particles will be given in the question
  • Additional information can always be deduced using the information provided

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