Fundamental Particles | A-Level物理：基本粒子

📚 Fundamental Particles | A-Level物理：基本粒子

In A-Level Physics, the study of fundamental particles takes you beyond the familiar proton, neutron and electron into the deeper structure of matter. Fundamental particles are particles that cannot be broken down into smaller constituents. The Standard Model classifies them into quarks, leptons and force-mediating particles, and it provides the rules that govern how they combine and interact.

在 A-Level 物理中，基本粒子的学习将带你超越熟悉的质子、中子和电子，进入物质更深层的结构。基本粒子是指不能再分解为更小成分的粒子。标准模型将它们分为夸克、轻子和传递力的粒子，并给出了它们如何组合和相互作用的规则。

1. What Are Fundamental Particles? | 什么是基本粒子

Fundamental particles are particles that are not made of smaller constituents. As far as current experiments show, quarks and leptons have no internal structure and are treated as point-like particles. They are the building blocks of all matter. By contrast, hadrons such as protons and neutrons are not fundamental because they are composed of quarks.

基本粒子是指不再由更小成分组成的粒子。目前的实验表明，夸克和轻子没有内部结构，可视为点粒子。它们是所有物质的构建模块。相比之下，质子和中子等强子不是基本粒子，因为它们由夸克组成。

2. The Standard Model Overview | 标准模型概览

The Standard Model classifies fundamental matter particles into two families: quarks and leptons. Quarks experience all four fundamental forces, including the strong interaction. Leptons do not experience the strong interaction. The model also includes force-mediating particles called gauge bosons, such as the photon for electromagnetic force. CIE A-Level questions focus mainly on quarks, leptons and the hadrons formed from quarks.

标准模型将基本物质粒子分为两类：夸克和轻子。夸克受到包括强相互作用在内的四种基本力。轻子不参与强相互作用。该模型还包括传递力的规范玻色子，例如传递电磁力的光子。CIE A-Level 主要考查夸克、轻子以及由夸克组成的强子。

• Matter particles: quarks and leptons | 物质粒子：夸克和轻子
• Antimatter partners: antiquarks and antileptons | 反物质伴侣：反夸克和反轻子
• Force carriers: photon, W and Z bosons, gluons | 力的传播子：光子、W 和 Z 玻色子、胶子

3. Quark Properties | 夸克的性质

There are six flavours of quark: up (u), down (d), strange (s), charm (c), bottom (b) and top (t). CIE focuses mainly on u, d and s. Each quark has baryon number +1/3 and a fractional electric charge. The up quark has charge +2/3e, while the down and strange quarks have charge −1/3e. The strange quark carries strangeness S = −1.

夸克有六种“味”：上（u）、下（d）、奇（s）、粲（c）、底（b）和顶（t）。CIE 重点考查 u、d、s 三种。每种夸克的重子数为 +1/3，并且带分数电荷。上夸克电荷为 +2/3e，下夸克和奇夸克电荷为 −1/3e。奇夸克的奇异数 S = −1。

4. Leptons | 轻子

Leptons are fundamental particles that do not feel the strong force. The A-Level syllabus includes the electron (e⁻), the muon (μ⁻), their associated neutrinos (νₑ and νμ), and their antiparticles. Electrons and muons have integer charge −1e, while neutrinos are neutral. Leptons are assigned lepton numbers: electron lepton number Lₑ for e⁻ and νₑ, and muon lepton number Lμ for μ⁻ and νμ.

轻子是不参与强相互作用的基本粒子。A-Level 大纲包括电子（e⁻）、μ 子（μ⁻）、相应的中微子（νₑ 和 νμ）以及它们的反粒子。电子和 μ 子带整数电荷 −1e，中微子不带电。轻子具有轻子数：e⁻ 和 νₑ 的电子轻子数为 Lₑ，μ⁻ 和 νμ 的 μ 子轻子数为 Lμ。

5. Antiparticles | 反粒子

Every particle has a corresponding antiparticle with the same mass but opposite charge and opposite quantum numbers such as baryon number, lepton number and strangeness. The positron (e⁺) is the antiparticle of the electron. An anti-up quark (u̅) has charge −2/3e and baryon number −1/3. Antiparticles are represented by a bar over the symbol.

每种粒子都有对应的反粒子，质量相同但电荷、重子数、轻子数和奇异数等量子数相反。正电子（e⁺）是电子的反粒子。反上夸克（u̅）电荷为 −2/3e，重子数为 −1/3。反粒子通常在符号上方加一横杠表示。

• e⁻ ↔ e⁺ | 电子 ↔ 正电子
• u ↔ u̅ | 上夸克 ↔ 反上夸克
• d ↔ d̅ | 下夸克 ↔ 反下夸克
• n ↔ n̅ | 中子 ↔ 反中子

6. Hadrons: Baryons and Mesons | 强子：重子与介子

Hadrons are composite particles made of quarks. They are divided into baryons, made of three quarks or three antiquarks, and mesons, made of one quark and one antiquark. Baryons have baryon number B = +1, antibaryons have B = −1, and mesons have B = 0. The proton (uud) and neutron (udd) are the most familiar baryons.

强子是由夸克组成的复合粒子，分为重子和介子。重子由三个夸克或三个反夸克组成，介子由一个夸克和一个反夸克组成。重子的重子数 B = +1，反重子 B = −1，介子 B = 0。质子（uud）和中子（udd）是最常见的重子。

7. Common Baryon and Meson Compositions | 常见重子与介子的夸克组成

It is essential to know the quark compositions of common hadrons. The proton is uud and the neutron is udd. Pions are the lightest mesons: π⁺ = ud̅, π⁻ = u̅d, and π⁰ is a mixture of uu̅ and dd̅ states. Kaons contain strange quarks: K⁺ = us̅ and K⁻ = u̅s. You can check charges by adding the quark charges: for a neutron, +2/3 − 1/3 − 1/3 = 0; for a proton, +2/3 + 2/3 − 1/3 = +1; for π⁺, +2/3 + 1/3 = +1.

必须掌握常见强子的夸克组成。质子为 uud，中子为 udd。π 介子是最轻的介子：π⁺ = ud̅，π⁻ = u̅d，π⁰ 是 uu̅ 和 dd̅ 态的混合。K 介子含奇夸克：K⁺ = us̅，K⁻ = u̅s。你可以通过夸克电荷相加来检验：中子 +2/3 − 1/3 − 1/3 = 0；质子 +2/3 + 2/3 − 1/3 = +1；π⁺ 为 +2/3 + 1/3 = +1。

8. Baryon Number Conservation | 重子数守恒

Baryon number is conserved in all particle interactions. Since quarks have B = +1/3 and antiquarks have B = −1/3, baryons (qqq) have B = +1 and mesons (qq̅) have B = 0. If a reaction produces a proton, it must also produce an antibaryon or consume a baryon to keep total baryon number unchanged. For example, p + p → p + p + p + p̅ conserves baryon number.

重子数在所有粒子相互作用中守恒。因为夸克 B = +1/3、反夸克 B = −1/3，所以重子(qqq) B = +1，介子(qq̅) B = 0。如果反应产生一个质子，必须同时产生一个反重子或消耗一个重子，以保持总重子数不变。例如 p + p → p + p + p + p̅ 重子数守恒。

9. Lepton Number Conservation and Strangeness | 轻子数守恒与奇异数

Lepton number is conserved separately for electron type and muon type in A-Level treatments. For neutron decay n → p + e⁻ + ν̅ₑ, the initial electron lepton number is 0; the final electron has Lₑ = +1 and the anti-neutrino has Lₑ = −1, so the total remains 0. Strangeness is conserved in strong and electromagnetic interactions but is not conserved in weak interactions, so strange particles can decay weakly into non-strange particles.

在 A-Level 处理中，轻子数按电子型和 μ 子型分别守恒。对于中子衰变 n → p + e⁻ + ν̅ₑ，初态电子轻子数为 0；末态电子 Lₑ = +1，反中微子 Lₑ = −1，因此总量仍为 0。奇异数在强相互作用和电磁相互作用

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