140-143 - Standard Model, Antiparticles and Conservation Laws

Classifying Particles

Imagine a detector has recorded several short-lived particles. A list of names is not very useful by itself, so physicists sort particles into families. The standard quark-lepton model gives Edexcel four categories to know here.

Baryons and mesons are both hadrons because they are made from quarks. The difference is their quark structure: three quarks for a baryon, quark plus antiquark for a meson. Leptons are treated as fundamental in this course, so an electron is not made from quarks.

The word "model" matters. A good model is not just a filing cabinet for particles already found; it makes predictions. In the standard quark-lepton model, the pattern of quarks came in a symmetry that predicted the top quark before it was observed experimentally. This is the kind of evidence that makes a classification scheme more powerful than a vocabulary list.

Misconception contrast: a photon is not a tiny charged particle moving between matter particles. In this lesson it is listed separately from baryons, mesons and leptons. It has no baryon number or lepton number, and it is neutral.

For Edexcel know or state marks, be precise about the structure. "A baryon is heavy" or "a meson is smaller" is not the model. The credit-bearing idea is the quark content.

The six quarks form three paired generations: up/down, charm/strange, top/bottom. Finding bottom without its partner left a gap in this pattern, so the symmetry predicted top. This illustrates a testable model prediction, rather than a new conservation law.

When quark charges are supplied, add them to check a composition. With $u=+\tfrac23e$ and $d=-\tfrac13e$, $uud$ has charge $+e$ and $udd$ has zero charge. Both are baryons because each contains three quarks. An antiquark reverses the quark's charge: $u\bar d$ has charge $+e$, but is a meson because it contains a quark and an antiquark. Equal charge alone does not imply the same particle family.

Antiparticles

Antiparticles have the same rest mass as their partners and reversed additive quantum numbers.

Every particle has a corresponding antiparticle. The easiest way to deduce antiparticle properties is to ask which additive properties must reverse sign.

An electron has charge $-1$ in units of the elementary charge $e$, so its antiparticle, the positron, has charge $+1$. The electron has lepton number $+1$, so the positron has lepton number $-1$.

A proton has charge $+1$ and baryon number $+1$, so an antiproton has charge $-1$ and baryon number $-1$. A neutron has charge $0$, but the antineutron is still a different particle: its baryon number is $-1$, not $+1$.

This is a useful place to be careful. "Opposite particle" does not mean every property changes sign. Mass does not become negative. A neutral particle can still have a distinct antiparticle if another property, such as baryon number, changes sign. Some neutral particles, such as photons, are their own antiparticles, but exam questions on this topic usually give the relevant symbols and properties.

For an Edexcel deduce response, write the property you are changing. A bare answer such as "opposite" is too vague if the particle is neutral.

The Conservation Ledger

Particle equations are judged like a ledger. Add the relevant quantity before the arrow, add it after the arrow, and compare the totals. Here the three checks are:

[DIAGRAM: asset_name: Four particle categories and neutron beta-minus charge baryon lepton totals with correct antineutrino notation; asset_slug: 040_standard_model_antiparticles_conservation_laws__diagram_01; recommended_method: matplotlib; description: Four particle categories and neutron beta-minus charge baryon lepton totals with correct antineutrino notation]

Worked route:

Consider $\beta$-minus decay written as