Quarks, Leptons and the Conservation Laws
Protons and neutrons are built from up and down quarks, while electrons and neutrinos are leptons with nothing smaller inside them. Whether a proposed reaction can happen is settled by checking charge, baryon number and lepton number.
What a learner can do afterwards
- Gives the quark content of a proton and of a neutron and checks that the charges add up
- Writes beta-minus decay at quark level, as a down quark turning into an up quark
- Tests a proposed reaction against charge, baryon number and lepton number
1 · Read
Protons and neutrons are built from up and down quarks. An up quark carries plus two thirds of a unit, a down quark minus one third. A proton is up, up, down: two thirds plus two thirds minus one third equals plus one. A neutron is up, down, down: two thirds minus one third minus one third equals zero.
Watch beta-minus decay at quark level. One down quark inside a neutron flips into an up quark. The neutron becomes a proton, and the spare charge and energy leave as an electron plus an antineutrino. Quark flavour changed, yet every tally still balances.
Quarks feel the strong force and group into hadrons such as protons and neutrons. Leptons, the family holding electrons and neutrinos, feel no strong force and hide nothing smaller inside. Whether any proposed reaction can happen is settled by three tallies: charge, baryon number, and lepton number, matched on both sides.
Run the tallies line by line and mind the antiparticles, which flip every sign. The classic slip is writing neutron to proton plus electron and stopping there: lepton number then reads zero against one. Add the antineutrino and all three tallies close.
Two quark flavours build the nucleons, one quark flip drives beta decay, and three tallies judge every reaction.
More to read
33.5 Quarks: Is That All There Is?
Atoms are not the bottom floor. Protons and neutrons are each built from three quarks, and no one has ever found a quark alone. Quarks come in six flavors with the playful names up, down, charm, strange, top, and bottom, and each flavor carries a fractional charge. The everyday pair is up with charge plus 2/3 and down with charge minus 1/3. A proton is two ups plus a down, adding to exactly plus 1, while a neutron is one up plus two downs, adding to exactly zero. Check the sums yourself: the fractions always land on whole charges for the particles we see. Alongside the quarks sit the leptons, the electron family, which feel no strong force and never hide inside hadrons.
23.2 Quarks
Beta decay looks far neater through quark glasses. A free neutron becomes a proton, and at quark level that means one down quark flips into an up quark. The flip changes the charge tally by plus 1, so an electron must leave to carry the difference away, joined by an antineutrino that balances the lepton count. Nothing about the story breaks the conservation books. Charge before equals charge after, baryon number stays at 1, and lepton number stays at 0 with the electron and antineutrino cancelling out. This is the same test physicists run on any proposed reaction. Write down charge, baryon number, and lepton number on each side, and if any tally mismatches, the reaction cannot happen.
11.3 Quarks
The full particle zoo fits a tidy pattern once quarks enter the picture. Mesons pair a quark with an antiquark, baryons pack three quarks, and antibaryons pack three antiquarks. Each quark also carries one of three color charges, and only color neutral packs ever show up alone, which is why quarks stay locked inside hadrons. Heavier flavors like charm and top appear in cosmic rays and collider debris, then decay in tiny fractions of a second down to up and down quarks. The pattern even predicted missing particles before they were found, the way blank spots in a periodic table point to unknown elements. Quarks plus leptons plus the force carriers make the Standard Model, the best catalogue of matter ever tested.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.