Describe the Standard Model classification of matter into quarks and leptons, use the quark composition of protons and neutrons, and read particle charges from the Standard Model chart on the Reference Tables.

What this topic is asking

The final modern-physics dot point introduces the Standard Model, the classification of the fundamental particles of matter into quarks and leptons. The Physical Setting/Physics course asks you to describe this classification, to use the quark composition of protons and neutrons, and to read particle properties (especially the fractional charges of quarks) from the Standard Model chart printed on the Reference Tables. The Regents tests this with charge-adding problems and classification questions.

The classification of matter

The key idea for the Regents is that ordinary matter is built from a small set of fundamental particles. The atoms you studied earlier are made of protons, neutrons and electrons, but only the electron is itself fundamental; the proton and neutron are built from quarks. The Standard Model organizes all these fundamental particles and is summarized in a chart on the Reference Tables.

Quarks and their fractional charges

The fractional charges are the distinctive feature of quarks and the basis of common Regents calculations. Quarks are never found alone; they combine into composite particles whose charges add up to whole numbers. Reading the $+\tfrac{2}{3}e$ and $-\tfrac{1}{3}e$ values from the Standard Model chart, and adding them for a given particle, is the main skill.

Leptons

The leptons are the other family of six fundamental particles: the electron, the muon and the tau (each carrying a charge of $-1e$), together with their three neutrinos (the electron neutrino, muon neutrino and tau neutrino, each with charge $0$). The electron is the lepton you meet in atoms and circuits. Unlike quarks, leptons exist as independent particles and are not bound inside composite particles. Neutrinos are nearly massless and electrically neutral, which makes them very hard to detect.

The quark composition of protons and neutrons

This is the most-tested calculation in the topic: given the quark content and the fractional charges, add them to confirm the proton's charge of $+1e$ or the neutron's charge of $0$. It connects the Standard Model back to the structure of the atom, showing that the proton's charge is not fundamental but arises from its quarks.

Reference Tables note

The Reference Tables include a Standard Model of Particles chart listing the six quarks (up, down, charm, strange, top, bottom) with their charges ($+\tfrac{2}{3}e$ or $-\tfrac{1}{3}e$) and the six leptons (electron, muon, tau and their neutrinos) with their charges ($-1e$ or $0$). The chart is the source for any charge values you need; you read the fractional charges from it and add them for composite particles. The elementary charge $e = 1.60 \times 10^{-19}$ C is in the constants list. This completes the modern-physics picture begun in mass-energy and nuclear physics.

Try this

Q1. State the two families of fundamental particles in the Standard Model, with one example of each. [2 points]

• Cue. Quarks (for example the up quark) and leptons (for example the electron).

Q2. A proton is two up quarks ($+\tfrac{2}{3}e$ each) and one down quark ($-\tfrac{1}{3}e$). Calculate its total charge. [2 points]

• Cue. $2(+\tfrac{2}{3}e) + (-\tfrac{1}{3}e) = +\tfrac{4}{3}e - \tfrac{1}{3}e = +1e$.