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What are the fundamental particles and how do we probe them?

Quarks and leptons, hadrons (baryons and mesons), particles and antiparticles, the use of accelerators to create particles, and conservation laws in particle interactions.

A focused answer to the Edexcel 9PH0 particle physics content, covering quarks and leptons, hadrons (baryons and mesons), antiparticles, the use of accelerators to create particles, and conservation laws in interactions.

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  1. What this dot point is asking
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What this dot point is asking

Edexcel wants you to classify particles into quarks and leptons, describe hadrons as baryons and mesons built from quarks, understand particles and antiparticles and annihilation, explain how accelerators create new particles from kinetic energy, and apply conservation laws (charge, baryon number, lepton number, energy, momentum) to interactions.

The answer

Quarks and leptons

Quarks are never found alone; they are always confined inside hadrons. Leptons can exist freely, the electron being the familiar example and the neutrino being nearly massless and almost non-interacting.

Hadrons: baryons and mesons

The proton and neutron are the everyday baryons. Mesons are unstable and short-lived, produced copiously in high-energy collisions. The total charge of a hadron is the sum of its quark charges, and its baryon number is set by its quark content.

Particles and antiparticles

Every particle has a corresponding antiparticle with the same mass and rest energy but opposite charge (and opposite baryon or lepton number). The positron is the antielectron. When a particle meets its antiparticle they annihilate, their entire rest mass converting to energy as photons (pair production is the reverse: a high-energy photon creates a particle-antiparticle pair). The minimum photon energy for pair production is twice the rest energy of the particle created.

Accelerators and conservation laws

Accelerators (linear accelerators and synchrotrons) use electric fields to speed charged particles to near light speed and magnetic fields to steer them. In collisions, the enormous kinetic energy converts into the rest mass of new particles via E=mc2E = mc^2, which is how heavy and exotic particles are created and studied. Whatever the interaction, certain quantities are always conserved:

Examples in context

The Large Hadron Collider accelerates protons to near light speed and collides them, converting kinetic energy into new particles such as the Higgs boson. PET medical scanners detect the back-to-back photon pairs from positron-electron annihilation inside the body. Cosmic-ray muons reaching the ground are leptons created high in the atmosphere. Beta decay, governed by lepton-number conservation, underlies radiocarbon dating and the energy generation in stars.

Try this

Q1. State the quark composition of a proton. [1 mark]

  • Cue. Two up quarks and one down quark (uud).

Q2. A particle is made of a quark and an antiquark. State its classification and its baryon number. [2 marks]

  • Cue. It is a meson; its baryon number is 00.

Q3. Explain why a single photon cannot be produced when an electron and positron at rest annihilate. [2 marks]

  • Cue. The total momentum before is zero; one photon would carry momentum, so two photons moving in opposite directions are needed to conserve momentum and energy.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20184 marksA proton is made of two up quarks and one down quark. The up quark has charge +23e+\frac{2}{3}e and the down quark −13e-\frac{1}{3}e. Show that this gives the correct charge for a proton, and state the quark composition of a neutron.
Show worked answer →

Proton charge: 2×(+23e)+1×(−13e)=+43e−13e=+1e2 \times (+\frac{2}{3}e) + 1 \times (-\frac{1}{3}e) = +\frac{4}{3}e - \frac{1}{3}e = +1e, the correct proton charge.

A neutron is two down quarks and one up quark (udd): 2×(−13e)+(+23e)=−23e+23e=02 \times (-\frac{1}{3}e) + (+\frac{2}{3}e) = -\frac{2}{3}e + \frac{2}{3}e = 0, as required for a neutral neutron.

Markers reward the charge sum giving +1e+1e for the proton and the udd composition giving zero for the neutron.

Edexcel 20215 marksAn electron and a positron, each of rest energy 0.510.51 MeV, annihilate at rest. Calculate the total energy released and explain why two photons, rather than one, are produced.
Show worked answer →

Total energy released equals the combined rest energies: E=2×0.51=1.02E = 2 \times 0.51 = 1.02 MeV.

In joules: E=1.02×106×1.6×10−19=1.6×10−13E = 1.02 \times 10^{6} \times 1.6 \times 10^{-19} = 1.6 \times 10^{-13} J.

Two photons are produced because the initial total momentum is zero (both particles at rest); a single photon would carry momentum, violating conservation of momentum. Two photons travelling in opposite directions conserve both energy and momentum.

Markers reward the total rest energy 1.021.02 MeV, the joule conversion, and the momentum-conservation reason for two photons.

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