📚 Families of Particles | 粒子家族
At first glance, the range of particles studied in A-Level Physics can seem like a chaotic ‘particle zoo’. Once you group them into families according to spin, composition and interactions, the structure becomes much easier to understand.
乍一看，A-Level 物理中研究的各种粒子似乎像一个杂乱的粒子动物园。一旦你根据自旋、组成和相互作用将它们分组归类，整个结构就会变得清晰易懂。
This article covers the main families you need for CIE A-Level Physics: fermions and bosons, hadrons and leptons, baryons and mesons, quarks and leptons, plus the conservation laws that link them together.
本文涵盖 CIE A-Level 物理需要掌握的主要家族：费米子与玻色子、强子与轻子、重子与介子、夸克与轻子，以及将它们联系在一起的守恒律。
1. The Particle Zoo and Classification | 粒子动物园与分类
Particles are classified using three key ideas: their spin, whether they feel the strong nuclear force, and whether they are elementary or composite. Spin divides particles into fermions and bosons. The strong force divides them into hadrons and leptons. Composition divides hadrons into baryons and mesons, with quarks as their building blocks.
粒子可以通过三个关键概念来分类：自旋、是否感受强核力，以及它们是基本粒子还是复合粒子。自旋将粒子分为费米子和玻色子。强核力将粒子分为强子和轻子。组成则将强子分为重子和介子，夸克是它们的构成单元。
In CIE questions, you are often asked to place an unfamiliar particle into a family. Always check its quark content or its lepton number first, then decide whether it is a hadron, lepton, baryon or meson.
在 CIE 考题中，你经常需要将陌生粒子归入某个家族。一定要先检查它的夸克组成或轻子数，然后再判断它是强子、轻子、重子还是介子。
2. Fermions and Bosons | 费米子与玻色子
Fermions have half-integer spin, such as 1/2, and obey the Pauli exclusion principle. This means no two identical fermions can occupy the same quantum state. All quarks and leptons are fermions, so they are the matter particles.
费米子具有半整数自旋，例如 1/2，并且服从泡利不相容原理。这意味着两个全同费米子不能占据相同的量子态。所有夸克和轻子都是费米子，因此它们是物质粒子。
Bosons have integer spin, such as 0 or 1, and do not obey the Pauli exclusion principle. Force carriers – the photon, W and Z bosons, and gluons – are bosons. Mesons are also bosons because a quark-antiquark pair can combine to give integer spin.
玻色子具有整数自旋，例如 0 或 1，并且不服从泡利不相容原理。力的传递粒子——光子、W 和 Z 玻色子以及胶子——都是玻色子。介子也是玻色子，因为夸克-反夸克对可以组合得到整数自旋。
3. Hadrons and Leptons | 强子与轻子
Hadrons are composite particles made of quarks. They experience the strong nuclear force, which binds quarks together. Examples include protons, neutrons and pions. Hadrons are not fundamental particles.
强子是由夸克组成的复合粒子。它们感受强核力，强核力将夸克束缚在一起。例子包括质子、中子和 π 介子。强子不是基本粒子。
Leptons are considered elementary particles as far as current experiments can tell. They do not feel the strong force. Charged leptons feel the electromagnetic force and the weak force; neutral leptons, the neutrinos, feel only the weak force.
目前实验显示，轻子被认为是基本粒子。它们不感受强核力。带电轻子感受电磁力和弱力；中性轻子，即中微子，只感受弱力。
A useful exam rule is: if a particle feels the strong force, it is a hadron. If it does not, it is a lepton. This simple test works for all particles on the CIE syllabus.
一个有用的考试规则是：如果粒子感受强核力，它就是强子；如果它不感受强核力，它就是轻子。这个简单判断适用于 CIE 考纲中的所有粒子。
4. Quarks: The Building Blocks of Hadrons | 夸克：强子的构成单元
There are six flavours of quark: up, down, charm, strange, top and bottom. Their symbols, charges, baryon numbers and strangeness values are shown below.
夸克有六种味：上夸克、下夸克、粲夸克、奇夸克、顶夸克和底夸克。它们的符号、电荷、重子数和奇异数如下表所示。
| Flavour 味 | Symbol 符号 | Charge 电荷 | Baryon number 重子数 | Strangeness 奇异数 |
|---|---|---|---|---|
| up 上 | u | +2/3 e | +1/3 | 0 |
| down 下 | d | -1/3 e | +1/3 | 0 |
| charm 粲 | c | +2/3 e | +1/3 | 0 |
| strange 奇 | s | -1/3 e | +1/3 | -1 |
| top 顶 | t | +2/3 e | +1/3 | 0 |
| bottom 底 | b | -1/3 e | +1/3 | 0 |
Each quark has a corresponding antiquark with the opposite charge, opposite baryon number and opposite strangeness. The strange quark has strangeness -1, so the anti-strange quark s̄ has strangeness +1.
每种夸克都有对应的反夸克，电荷相反、重子数相反、奇异数也相反。奇夸克的奇异数为 -1，因此反奇夸克 s̄ 的奇异数为 +1。
5. Baryons and Mesons | 重子与介子
Hadrons are divided into baryons and mesons. A baryon is made of three quarks, so its baryon number is +1. An antibaryon is made of three antiquarks and has baryon number -1. Baryons have half-integer spin.
强子分为重子和介子。重子由三个夸克组成，因此它的重子数为 +1。反重子由三个反夸克组成，重子数为 -1。重子具有半整数自旋。
The proton has quark composition uud and the neutron has udd. Using the table, the proton charge is +2/3 +2/3 -1/3 = +1 e, and the neutron charge is +2/3 -1/3 -1/3 = 0.
质子的夸克组成为 uud，中子的夸克组成为 udd。用上表计算，质子电荷为 +2/3 +2/3 -1/3 = +1 e，中子电荷为 +2/3 -1/3 -1/3 = 0。
A meson is made of one quark and one antiquark. Its baryon number is 0 and its spin is integer. Pions and kaons are common mesons. For example, π⁺ has composition u d̄, π⁻ has ū d, and K⁺ has u s̄.
介子由一个夸克和一个反夸克组成。它的重子数为 0，自旋为整数。π 介子和 K 介子是常见的介子。例如，π⁺ 的组成为 u d̄，π⁻ 为 ū d，K⁺ 为 u s̄。
6. Leptons and Lepton Number | 轻子与轻子数
Leptons come in three generations. The first generation contains the electron e⁻ and the electron neutrino νₑ. The second contains the muon μ⁻ and the muon neutrino ν_μ. The third contains the tau τ⁻ and the tau neutrino ν_τ.
轻子分为三代。第一代包含电子 e⁻ 和电子中微子 νₑ。第二代包含 μ 子 μ⁻ 和 μ 子中微子 ν_μ。第三代包含 τ 子 τ⁻ 和 τ 子中微子 ν_τ。
Each lepton has lepton number +1, while each antilepton has lepton number -1. In CIE problems, it is often useful to check electron lepton number and muon lepton number separately, because the weak interaction normally conserves each family number.
每个轻子的轻子数为 +1，每个反轻子的轻子数为 -1。在 CIE 问题中，分别检查电子轻子数和 μ 子轻子数通常很有用，因为弱相互作用通常分别守恒每一代轻子数。
For example, in beta-minus decay, a neutron changes into a proton, an electron and an anti-electron neutrino. The electron lepton number before is 0, and after is +1 for the electron and -1 for the anti-neutrino, giving a total of 0.
例如，在 β⁻ 衰变中，一个中子变成一个质子、一个电子和一个反电子中微子。衰变前电子轻子数为 0；衰变后电子的电子轻子数为 +1，反中微子的电子轻子数为 -1，总和仍为 0。
7. Antiparticles and Annihilation | 反粒子与湮灭
Every particle has an antiparticle. The antiparticle has the same mass as the particle but opposite charge, opposite baryon number, opposite lepton number and opposite strangeness, where these quantum numbers apply.
每种粒子都有反粒子。反粒子与粒子的质量相同，但电荷、重子数、轻子数和奇异数等量子数都相反。
When a particle meets its antiparticle, they can annihilate. Their total mass is converted into energy in the form of photons. Conversely, pair production occurs when a photon with enough energy creates a particle-antiparticle pair.
当粒子遇到它的反粒子时，它们会发生湮灭。它们的总质量转化为光子形式的能量。反之，当光子具有足够能量时，可以产生粒子-反粒子对，这称为粒子对产生。
The minimum photon energy for pair production is given by the rest energy of the pair. For an electron-positron pair this is 2mₑc², where mₑ is the electron mass and c is the speed of light.
粒子对产生所需的最小光子能量等于粒子对的静止能量。对于电子-正电子对，这个最小能量是 2mₑc²，其中 mₑ 是电子质量，c 是光速。
8. Conservation Laws | 守恒律
Baryon number is conserved in all particle interactions. Lepton number is also conserved in all interactions. Charge, energy and momentum are always conserved. These laws allow you to decide whether a proposed decay or interaction is possible.
重子数在所有粒子相互作用中守恒。轻子数也在所有相互作用中守恒。电荷、能量和动量总是守恒。这些守恒律可以帮助你判断一个假想的衰变或相互作用是否可能发生。
Strangeness is conserved in strong and electromagnetic interactions, but it is not conserved in weak interactions. In a weak decay, strangeness can change by 0 or ±1. This is very useful for identifying weak decay processes.
奇异数在强相互作用和电磁相互作用中守恒，但在弱相互作用中不守恒。在弱衰变中，奇异数可以改变 0 或 ±1。这对于识别弱衰变过程非常有用。
For beta-minus decay, we can write the equation as n → p + e⁻ + ν̄ₑ. Check baryon number: 1 → 1 + 0 + 0. Check charge: 0 → +1 -1 + 0. Check lepton number: 0 → 0 + 1 -1. All are conserved.
对于 β⁻ 衰变，我们可以写出方程 n → p + e⁻ + ν̄ₑ。检查重子数：1 → 1 + 0 + 0。检查电荷：0 → +1 -1 + 0。检查轻子数：0 → 0 + 1 -1。全部守恒。
9. Particle Interactions and Decay | 粒子相互作用与衰变
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