Fundamental particles
Introduced| English | Meaning |
|---|---|
| fundamental particles/ˌfʌndəˈmentl ˈpɑːtɪklz/ | Particles that have no known smaller constituents. |
| quarks/kwɑːks/ | quarks |
| flavours/ˈfleɪvəz/ | The different types within a particle family, such as the six types of quark. |
| antiquark/ˈæntɪkwɑːk/ | The antiparticle of a quark, with the same mass and opposite electric charge. |
| hadrons/ˈhædrɒnz/ | Particles composed of quarks or antiquarks and bound by the strong interaction. |
| baryons/ˈbæriɒnz/ | Heavy particles made of three quarks, such as protons and neutrons. |
| mesons/ˈmiːzɒnz/ | Hadrons described in the basic quark model as a quark paired with an antiquark. |
| leptons/ˈleptɒnz/ | Fundamental particles that do not participate in the strong interaction, including electrons and neutrinos. |
Inside the proton
- Protons and neutrons feel solid — but they are not fundamental particles 基本粒子.
- Each is built from smaller point-like particles called quarks 夸克.
- Some particles have no smaller parts; those are the truly fundamental ones.
Fundamental particle lab
Sort particles by the family or interaction that defines them.
Quarks
- Six flavours 味. Up, charm, top have charge $+\tfrac{2}{3}e$; down, strange, bottom have $-\tfrac{1}{3}e$.
- Each has an antiquark 反夸克 with the opposite charge.
The six quark flavours: up-type carry +⅔e, down-type carry −⅓e
What is the charge on an up quark?
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Up-type quarks (up, charm, top) carry $+\tfrac{2}{3}e$; down-type (down, strange, bottom) carry $-\tfrac{1}{3}e$.
Hadrons 强子: baryons 重子 and mesons 介子
- Particles built from quarks are hadrons.
- Baryons = three quarks; mesons = a quark and an antiquark.
Match each hadron to what it is made of.
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Protons and neutrons are baryons (three quarks); a pion is a meson (quark + antiquark).
Proton and neutron
- Proton = u u d: charge $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$.
- Neutron = u d d: charge $\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$.
A proton is made of which quarks?
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Proton = u u d, giving charge $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$.
A neutron is u d d. What is its total charge, in units of $e$?
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$\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$ — the neutron is uncharged.
Worked example: the quarks in an α-particle
An α-particle is two protons and two neutrons. Show, using quarks, that each hadron in it has charge $0$ or $+e$, and find the total quark content.
- Each proton is uud: $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$, so charge $+e$. It is a baryon (three quarks).
- Each neutron is udd: $\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$. Also a baryon.
- Whole α-particle: up quarks $2 \times 2 + 2 \times 1 = 6$; down quarks $2 \times 1 + 2 \times 2 = 6$.
- Check: $6\left(+\tfrac{2}{3}\right) + 6\left(-\tfrac{1}{3}\right) = 4 - 2 = +2$, the α-particle's charge of $+2e$.
A helium-3 nucleus contains two protons and one neutron. How many down quarks does it contain?
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Each proton (uud) has one down quark and the neutron (udd) has two: $1 + 1 + 2 = 4$.
β-decay in quark terms
- β$^{-}$: a neutron becomes a proton — a down quark turns into an up quark.
- β$^{+}$: a proton becomes a neutron — an up quark turns into a down quark.
β⁻ decay up close: one down quark flips to an up quark, so a neutron becomes a proton
In β$^{-}$ decay, which quark change happens?
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A neutron (u d d) becomes a proton (u u d): one down → up, emitting a $\beta^{-}$ and an antineutrino.
Leptons 轻子 and what's fundamental
- Leptons (electrons, neutrinos) are fundamental and are not made of quarks.
- Fundamental: quarks, electrons, positrons, neutrinos. Not fundamental: protons, neutrons, mesons (all hadrons).
Particles sort into hadrons (made of quarks) and leptons (fundamental, not made of quarks)
Select all the particles that are fundamental.
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Quarks and leptons (electron, neutrino) are fundamental. Protons and mesons are hadrons — made of quarks.
An electron is a lepton and is not made of quarks.
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Leptons (electrons and neutrinos) are fundamental particles, not built from quarks.
Worked example: a neutral meson
Particle Q is a meson with zero charge. It has a mass of $0.67\ \text{u}$ and a kinetic energy of $2.1 \times 10^{-16}\ \text{J}$.
- A possible composition: a quark with its own antiquark, for example $\text{u}\bar{\text{u}}$: $+\tfrac{2}{3} - \tfrac{2}{3} = 0$. ($\text{d}\bar{\text{d}}$ would also work.)
- Why not two quarks? A meson is always a quark and an antiquark; two quarks together are not a meson.
- Speed: $E_{\text{K}} = \tfrac{1}{2}mv^{2}$ with $m = 0.67 \times 1.66 \times 10^{-27} = 1.1 \times 10^{-27}\ \text{kg}$, so $v = \sqrt{\dfrac{2 \times 2.1 \times 10^{-16}}{1.1 \times 10^{-27}}} = 6.1 \times 10^{5}\ \dfrac{\text{m}}{\text{s}}$.
- Check: well below the speed of light, so the ordinary kinetic-energy formula is fine to use.
A meson is made of a quark and an ____.
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Mesons are quark–antiquark pairs; baryons are three quarks. A neutral meson pairs a quark with an antiquark of the opposite charge, such as up with anti-up.
"Fundamental" means not made of anything smaller: quarks and leptons are; protons, neutrons and every other hadron are not. Antiquarks carry the opposite charge, so a neutral meson pairs a quark with the matching antiquark. In β$^{-}$ decay the quark change is d → u — a charge rise of $+1$ that the emitted electron's $-1$ balances — and the electron is created in the decay, not released from an orbit.
You've got it
- quarks are fundamental (up-type $+\tfrac{2}{3}e$, down-type $-\tfrac{1}{3}e$; antiquarks opposite)
- baryons = 3 quarks (proton uud, neutron udd); mesons = quark + antiquark; a nucleus's quarks are just its protons' and neutrons' added up
- in β$^{-}$ decay a down quark → up quark; leptons (electron, neutrino) are fundamental