Conservation laws and the standard model Edexcel International A Level Physics revision
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In plain words
Physicists smash particles together and watch what comes out. Not everything is allowed. Just as charge can never appear from nowhere, there are two other tallies that must come out the same before and after. Check all three and you can tell at once whether a reaction can happen.
5 things to know
- The standard model sorts particles into baryons (three quarks, such as protons and neutrons), mesons (a quark and an antiquark, such as pions), leptons (fundamental particles, such as electrons and neutrinos) and photons.
- Quarks and leptons come in matching families. That symmetry led physicists to predict the top quark years before it was found.
- Baryon number: +1 for a baryon, −1 for an antibaryon, 0 for mesons and leptons (each quark carries ⅓). Lepton number: +1 for a lepton, −1 for an antilepton, 0 for everything else.
- An interaction can only happen if charge, baryon number and lepton number are each the same before and after. Energy and momentum must be conserved as well.
- An antiparticle has the same mass as its particle, and the opposite charge, baryon number and lepton number.
Worked example
Check whether β⁻ decay, in which a neutron becomes a proton, an electron and an antineutrino, is allowed.
- Charge: 0 before; (+1) + (−1) + 0 = 0 after. Conserved.
- Baryon number: 1 before; 1 + 0 + 0 = 1 after. Conserved.
- Lepton number: 0 before; 0 + (+1) + (−1) = 0 after. Conserved, thanks to the antineutrino. The decay is allowed.
Tips and tricks
- Check charge, baryon number and lepton number in turn. One that fails is enough to rule the interaction out.
- Without the antineutrino, lepton number would not balance in β⁻ decay.
It lands in your notebook with its questions as flashcards.
Conservation laws and the standard model: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the baryon number of a meson?
A quark (+⅓) and an antiquark (−⅓).
What is the lepton number of an electron?
It is a lepton.
What is the lepton number of a positron?
It is an antilepton.
Which of these must be the same before and after any particle interaction?
Mass can turn into energy, and particles can be created, but charge is always conserved.
Which particle was predicted from the symmetry of the standard model before it was found?
The families of quarks and leptons had to match.
What is the baryon number of a single quark?
Three quarks make a baryon with baryon number 1.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 8 marks. Write your answers on paper, then check them.
Conservation laws and the standard model
Edexcel International A Level Physics WPH · 8 marks · papermunch.org
Name ______________________________ Date ______________
Use conservation laws to decide whether two protons can collide to give two protons and a neutron, and nothing else.[3]
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No. Charge is conserved (2 before and 2 after), but baryon number is not: 2 before and 3 after.
Deduce the charge and baryon number of an antiproton.[2]
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Charge −e and baryon number −1. (Its mass is the same as a proton's.)
Two protons collide to give a proton, a neutron and a positive pion (a meson with charge +e). Show that this is allowed by the conservation laws.[3]
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Charge: 2 before; 1 + 0 + 1 = 2 after. Baryon number: 2 before; 1 + 1 + 0 = 2 after. Lepton number: 0 before and after. All three are conserved.
Answers: Conservation laws and the standard model
- 1. No. Charge is conserved (2 before and 2 after), but baryon number is not: 2 before and 3 after.
- 2. Charge −e and baryon number −1. (Its mass is the same as a proton's.)
- 3. Charge: 2 before; 1 + 0 + 1 = 2 after. Baryon number: 2 before; 1 + 1 + 0 = 2 after. Lepton number: 0 before and after. All three are conserved.