Particles and Antiparticles Revision Notes | A Level AQA Physics

1. The Mirror Universe: What Are Antiparticles?

Picture every particle you know — the electron, the proton, the neutron — and imagine each one has a mirror twin. That twin has the same mass, the same rest energy, but an opposite charge. That twin is the antiparticle. This is not science fiction; it is a fundamental prediction of relativistic quantum mechanics, confirmed experimentally since the positron was detected in 1932.

The idea matters enormously in particle physics because whenever a particle and its antiparticle meet, they annihilate — converting their combined rest energy entirely into photons. Equally, a photon with sufficient energy can spontaneously produce a particle-antiparticle pair. Understanding the properties that distinguish a particle from its antiparticle is therefore central to the whole of this topic.

The AQA specification requires you to know four specific antiparticle pairings by name. These are the foundation for everything that follows in the particles topic, so it is worth locking them in now before moving on to their properties.

Notice the notation: antiparticles are written with a bar above the symbol (e.g. $\bar{p}$ for the antiproton). The positron is a special case — it gets its own symbol, $e^+$, because the superscript charge already distinguishes it clearly from the electron $e^-$.

2. Comparing Particles and Antiparticles: Mass, Charge, and Rest Energy

Now that you know the names, the next step is to understand precisely how a particle and its antiparticle compare. There are three quantities the specification asks you to consider: mass, charge, and rest energy in MeV. Two of these are identical between a particle and its antiparticle; one is opposite.

The rule is clean and universal: mass is always the same, rest energy is always the same, and for charged particles, charge is equal in magnitude but opposite in sign. For neutral particles such as the neutron and neutrino, both the particle and its antiparticle carry zero charge — the opposite of zero is still zero.

The neutron and antineutron case is worth pausing on. Both have zero charge, so you might wonder how they differ at all. The answer lies in other quantum numbers — specifically baryon number and quark content — which you will meet later in this topic. For now, the key point is that they are genuinely distinct particles, not the same particle.

Rest energy in MeV is the energy equivalent of the particle's rest mass, calculated from Einstein's $E = mc^2$ and then converted into megaelectronvolts. Because mass is identical for a particle and its antiparticle, rest energy is identical too. The electron and positron both have a rest energy of 0.511 MeV — this value appears frequently in AQA exam questions, so it is worth memorising.

3. Rest Energy in MeV — Why This Unit and How to Use It

At the subatomic scale, joules are an inconveniently large unit. A single electron has a rest energy of roughly $8.19 \times 10^{-14}$ J — a number that is awkward to work with. Particle physicists therefore use the electronvolt (eV) and its multiples. One electronvolt is the kinetic energy gained by an electron accelerated through a potential difference of 1 volt: $1 \text{ eV} = 1.6 \times 10^{-19}$ J. One megaelectronvolt (MeV) is $10^6$ eV, or $1.6 \times 10^{-13}$ J.

When the specification says you should know rest energies in MeV, it means you should be able to state them from memory for the key particles and use them in annihilation and pair production calculations. The electron rest energy of 0.511 MeV is the most commonly tested value.

Worked example: Converting Rest Energy Between Joules and MeV

The rest mass of an electron is $9.11 \times 10^{-31}$ kg. Show that its rest energy is approximately 0.511 MeV.

1. $E = mc^2$ — Einstein's mass-energy equivalence. $m$ is rest mass in kg, $c = 3.0 \times 10^8$ m/s.
2. $E = 9.11 \times 10^{-31} \times (3.0 \times 10^8)^2$ — Squaring the speed of light: $3.0 \times 10^8 \times 3.0 \times 10^8 = 9.0 \times 10^{16}$ m$^2$ s$^{-2}$.
3. $E = 9.11 \times 10^{-31} \times 9.0 \times 10^{16} = 8.20 \times 10^{-14}$ J — $9.11 \times 9.0 = 81.99 \approx 82.0$, and $10^{-31} \times 10^{16} = 10^{-15}$, so $E \approx 82.0 \times 10^{-15}$ J $= 8.20 \times 10^{-14}$ J. [M1]
4. $E = \frac{8.20 \times 10^{-14}}{1.6 \times 10^{-19}} = 5.125 \times 10^{5}$ eV — Dividing: $8.20 \div 1.6 = 5.125$ and $10^{-14} \div 10^{-19} = 10^{5}$. [M1]
5. $E = \frac{5.125 \times 10^{5}}{10^6} = 0.5125$ MeV $\approx 0.511$ MeV — Dividing by $10^6$ converts eV to MeV. The small discrepancy from 0.511 MeV is due to rounding $c$ to $3.0 \times 10^8$ m/s. [A1]

Answer: Rest energy of electron $\approx 0.511$ MeV

Because the positron has exactly the same rest mass as the electron, its rest energy is also exactly 0.511 MeV. This symmetry is not a coincidence — it is a direct consequence of the rule that particle and antiparticle masses are always equal.

4. The Neutron and Antineutron — A Special Case

The neutron-antineutron pair deserves a closer look because it challenges a common assumption. Both particles are electrically neutral, so charge alone cannot distinguish them. This raises a genuine question: if charge is the quantity that is 'opposite' between a particle and its antiparticle, and both have zero charge, are the neutron and antineutron actually different particles?

The answer is yes — they are genuinely distinct. The neutron is made of quarks (two down quarks and one up quark), while the antineutron is made of the corresponding antiquarks (two anti-down quarks and one anti-up quark). Their quark compositions are opposite, even though the net charge of both is zero. At A Level, the key point to state is that the antineutron is the antiparticle of the neutron, they have the same rest mass and rest energy, and both carry zero charge.

5. The Neutrino and Antineutrino

The neutrino is one of the most elusive particles in physics — it has negligible mass, zero charge, and interacts so weakly with matter that billions pass through your body every second without effect. Its antiparticle, the antineutrino, shares all of these properties: negligible mass, zero charge, and the same near-zero rest energy.

Like the neutron-antineutron pair, the neutrino and antineutrino are distinguished by a quantum number rather than charge. That quantum number is lepton number, which you will study in detail when you reach conservation laws. For now, the essential point is that the antineutrino is produced in beta-minus decay alongside an electron, while the neutrino is produced in beta-plus decay alongside a positron — a distinction that becomes critical when you apply conservation laws.

6. Exam Technique: Scoring Full Marks on Antiparticle Questions

AQA questions on this topic tend to fall into three categories: recall of antiparticle names and symbols, comparison of particle and antiparticle properties, and application of these properties in the context of annihilation or pair production. Each type has its own mark-winning approach.

For comparison questions (command word: compare), you must address both similarities and differences. A common error is to state only the difference in charge and forget to state that mass and rest energy are identical. Examiners award separate marks for each property, so a complete answer must cover all three: mass, charge, and rest energy.

For extended response questions that ask you to explain or evaluate antiparticle properties, structure your answer around the three key quantities — mass, charge, rest energy — and then add the quantum number distinction where relevant. This mirrors the mark scheme structure and ensures you do not miss available marks.

Mastering these three properties — same mass, same rest energy, opposite charge — and knowing the four named antiparticle pairs gives you the complete foundation for the particles and radiation topic. Every subsequent concept, from annihilation to conservation laws, builds directly on this framework.