# Particle Interaction Equations(Edexcel International A Level Physics)

Author

Ashika

Expertise

## 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:

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 is

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

#### Exam Tip

Note:

• 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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