Particle physics and the quark model

A proton and a neutron are both baryons, but neither is fundamental in the quark model. Their valence compositions are uud and udd respectively. Adding the signed quark charges gives +e and zero. A neutron’s zero net charge therefore does not mean that every constituent has zero charge. Mesons have a quark and an antiquark; for example, an up quark combined with an anti-down quark gives charge +e because reversing the down quark’s charge gives +e/3.

Electrons and neutrinos belong to the lepton family. They are not made from quarks in the Standard Model. Photons are a different kind of particle: they carry electromagnetic interactions and are neither baryons nor leptons. Use a small classification map to separate these categories before learning individual symbols. The map is a description of composition and properties, not a picture of tiny hard balls inside an atom. In particular, a neutron is not built from a proton with an electron attached.

The six quark flavours form three generations: up and down, charm and strange, top and bottom. The pattern of the model predicted a top quark before its observation. This illustrates how a scientific model can predict missing members rather than merely catalogue known particles. For charge bookkeeping, up, charm and top each have +2e/3, while down, strange and bottom each have −e/3; corresponding antiquarks reverse those charges.

Make separate totals on the two sides of an interaction for charge, baryon number and lepton number. Assign baryons +1 and antibaryons −1; mesons and leptons have baryon number zero. At this level, leptons have lepton number +1 and antileptons −1, while hadrons and photons have zero. An antiparticle has the same rest mass as its partner; changing signs in conserved quantities does not mean changing the sign of mass or energy.

For neutron beta-minus decay, n → p + e⁻ + electron antineutrino, the final charge is +1 − 1 + 0 = 0 in units of e. Baryon number remains one. The electron contributes lepton number +1 and the antineutrino −1, leaving the initial total of zero unchanged. Energy and momentum must also balance. Passing these bookkeeping checks means a proposed process survives those tests; it does not establish that the process is energetically possible or has a significant probability.

In annihilation, particle and antiparticle rest energy can become other forms of energy. An electron and positron with negligible initial kinetic energy have combined rest energy about 1.022 MeV. In a two-photon final state with the pair initially at rest, opposite photon momenta balance and each photon carries about 0.511 MeV. One photon alone could not conserve both energy and momentum for that isolated initially stationary system. If initial kinetic energy matters, include it in the total energy balance.

An electric field can do work on a charged particle, changing its kinetic energy. A magnetic field bends its path without doing work: for perpendicular motion in a uniform field, p = B|Q|r. Compare tracks at the same field and charge magnitude before concluding that a larger radius means greater momentum. Charged particles can leave ionisation tracks; neutral particles do not produce the same direct track but may be inferred from decay products and missing momentum. Lack of a visible line is not evidence that nothing travelled there.

Small-scale structure is investigated using particles with short de Broglie wavelengths. Increasing momentum reduces λ = h/p and can therefore probe shorter length scales. High collision energy can also supply the rest energy needed to create heavier particles. These are related reasons for high-energy machines, but they answer different questions: one concerns resolving structure and the other concerns which final states are energetically accessible. Neither means that an accelerating particle’s electric charge becomes larger.

In a linear accelerator, electric fields across gaps increase particle energy; the field timing must accelerate rather than slow successive particles. A cyclotron uses magnetic bending and repeated acceleration across a gap. A heated source can release electrons by thermionic emission before an electric field accelerates them. At very high speeds, relativistic effects matter. For example, unstable fast particles can travel farther in the laboratory than a rest-frame lifetime estimate would suggest, so avoid blindly combining a rest lifetime with a laboratory flight distance.