CIE AS/A Level Physics 11.2 Fundamental particles Study Notes- 2025-2027 Syllabus
CIE AS/A Level Physics 11.2 Fundamental particles Study Notes – New Syllabus
CIE AS/A Level Physics 11.2 Fundamental particles Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Physics latest syllabus with Candidates should be able to:
understand that a quark is a fundamental particle and that there are six flavours (types) of quark: up, down, strange, charm, top and bottom
recall and use the charge of each flavour of quark and understand that its respective antiquark has opposite charge (no knowledge of any other properties of quarks is required)
recall that protons and neutrons are not fundamental particles and describe protons and neutrons in terms of their quark composition
understand that a hadron may be either a baryon (consisting of three quarks) or a meson (consisting of one quark and one antiquark)
describe the changes to quark composition that take place during β⁻ and β⁺ decay
recall that electrons and neutrinos are fundamental particles called leptons
Understanding Quarks as Fundamental Particles
A quark is a fundamental particle that forms the basic building block of hadrons — particles such as protons and neutrons that participate in the strong nuclear interaction.
Key Characteristics of Quarks:
- Quarks are elementary particles — they are not made up of smaller components.
- They combine in specific ways to form composite particles (hadrons):
- Baryons (e.g., protons and neutrons) — made of 3 quarks.
- Mesons — made of 1 quark and 1 antiquark.
- Quarks experience all four fundamental forces: gravitational, electromagnetic, weak nuclear, and strong nuclear interactions.
- Each quark has distinct properties — charge, mass, and a quantum property known as “flavour.”
The Six Flavours (Types) of Quarks:
| Quark Flavour | Symbol | Relative Mass (approx.) | Charge (in units of e) |
|---|---|---|---|
| Up | u | 1 | \( \mathrm{+\dfrac{2}{3}} \) |
| Down | d | 1 | \( \mathrm{-\dfrac{1}{3}} \) |
| Strange | s | ≈ 20 | \( \mathrm{-\dfrac{1}{3}} \) |
| Charm | c | ≈ 1300 | \( \mathrm{+\dfrac{2}{3}} \) |
| Top | t | ≈ 180000 | \( \mathrm{+\dfrac{2}{3}} \) |
| Bottom | b | ≈ 4300 | \( \mathrm{-\dfrac{1}{3}} \) |
There are six known quark flavours — up, down, strange, charm, top, and bottom. These combine in specific groups to form all known hadrons in the universe.
Example
Identify the quark composition of a proton and a neutron.
▶️ Answer / Explanation
| Particle | Quark Composition | Total Charge Calculation | Net Charge |
|---|---|---|---|
| Proton | \( \mathrm{uud} \) | \( \mathrm{(+\dfrac{2}{3}) + (+\dfrac{2}{3}) + (-\dfrac{1}{3}) = +1} \) | \( \mathrm{+1e} \) |
| Neutron | \( \mathrm{udd} \) | \( \mathrm{(+\dfrac{2}{3}) + (-\dfrac{1}{3}) + (-\dfrac{1}{3}) = 0} \) | \( \mathrm{0} \) |
Hence: A proton consists of two up quarks and one down quark, while a neutron consists of one up quark and two down quarks.
Charges of Quarks and Antiquarks
Each quark has a specific electric charge that is a fraction of the elementary charge \( \mathrm{e = 1.6 \times 10^{-19} \ C.} \) The corresponding antiquark has the same mass but an opposite charge.
| Quark Flavour | Symbol | Charge (\( \mathrm{e} \)) | Antiquark Symbol | Antiquark Charge (\( \mathrm{e} \)) |
|---|---|---|---|---|
| Up | u | \( \mathrm{+\dfrac{2}{3}} \) | \( \bar{u} \) | \( \mathrm{-\dfrac{2}{3}} \) |
| Down | d | \( \mathrm{-\dfrac{1}{3}} \) | \( \bar{d} \) | \( \mathrm{+\dfrac{1}{3}} \) |
| Strange | s | \( \mathrm{-\dfrac{1}{3}} \) | \( \bar{s} \) | \( \mathrm{+\dfrac{1}{3}} \) |
| Charm | c | \( \mathrm{+\dfrac{2}{3}} \) | \( \bar{c} \) | \( \mathrm{-\dfrac{2}{3}} \) |
| Top | t | \( \mathrm{+\dfrac{2}{3}} \) | \( \bar{t} \) | \( \mathrm{-\dfrac{2}{3}} \) |
| Bottom | b | \( \mathrm{-\dfrac{1}{3}} \) | \( \bar{b} \) | \( \mathrm{+\dfrac{1}{3}} \) |
Explanation:
- Quarks with a charge of \( \mathrm{+\dfrac{2}{3}} \) are called up-type quarks (up, charm, top).
- Quarks with a charge of \( \mathrm{-\dfrac{1}{3}} \) are called down-type quarks (down, strange, bottom).
- Each quark has an antiquark with the same mass but the opposite charge and opposite quantum numbers.
Antiquarks are the antimatter counterparts of quarks. They have the same magnitude of charge but the opposite sign.
Example
Determine the quark composition and total charge of a \( \mathrm{\pi^+} \) meson (pi-plus meson).
▶️ Answer / Explanation
The \( \mathrm{\pi^+} \) meson consists of an up quark and an anti-down quark:
\( \mathrm{\pi^+ = u\bar{d}} \)
Total charge: \( \mathrm{(+\dfrac{2}{3}) + (+\dfrac{1}{3}) = +1.} \)
Hence, the \( \mathrm{\pi^+} \) meson has a charge of \( \mathrm{+1e.} \)
Example
What is the quark composition and total charge of a neutron?
▶️ Answer / Explanation
A neutron consists of one up quark and two down quarks:
\( \mathrm{n = udd} \)
Total charge: \( \mathrm{(+\dfrac{2}{3}) + (-\dfrac{1}{3}) + (-\dfrac{1}{3}) = 0.} \)
Therefore, a neutron is electrically neutral.
Protons and Neutrons — Not Fundamental Particles
Protons and neutrons, collectively known as nucleons, are not fundamental particles because they are each composed of smaller, more basic particles called quarks.
Explanation:
- Quarks are the fundamental constituents of hadrons (including protons and neutrons).
- They are held together by the strong nuclear force, which is mediated by particles called gluons.
- The combination of quark charges in each nucleon gives rise to the overall charge of the particle.
Quark Composition of Nucleons:
| Particle | Quark Composition | Charge of Each Quark | Total Charge | Classification |
|---|---|---|---|---|
| Proton | \( \mathrm{uud} \) | \( \mathrm{+\dfrac{2}{3}, +\dfrac{2}{3}, -\dfrac{1}{3}} \) | \( \mathrm{+1e} \) | Baryon (3 quarks) |
| Neutron | \( \mathrm{udd} \) | \( \mathrm{+\dfrac{2}{3}, -\dfrac{1}{3}, -\dfrac{1}{3}} \) | \( \mathrm{0} \) | Baryon (3 quarks) |
Interpretation:
- A proton consists of two up quarks and one down quark — giving a net charge of +1e.
- A neutron consists of one up quark and two down quarks — giving a net charge of 0.
- Both are baryons because they are made up of three quarks.
Protons and neutrons are composite particles made of quarks bound by the strong nuclear force — they are not fundamental.
Example
Show that the charge of a proton equals +1e using its quark composition.
▶️ Answer / Explanation
Proton = \( \mathrm{uud} \)
Charge = \( \mathrm{(+\dfrac{2}{3}) + (+\dfrac{2}{3}) + (-\dfrac{1}{3}) = +1.} \)
Hence, the proton has a total charge of +1e.
Example
Show that a neutron has no net charge using its quark composition.
▶️ Answer / Explanation
Neutron = \( \mathrm{udd} \)
Charge = \( \mathrm{(+\dfrac{2}{3}) + (-\dfrac{1}{3}) + (-\dfrac{1}{3}) = 0.} \)
Hence, the neutron is electrically neutral.
Classification of Hadrons: Baryons and Mesons
A hadron is a composite particle made up of quarks that are held together by the strong nuclear force. Hadrons are not fundamental because they contain quarks as subparticles.
Types of Hadrons:
| Type of Hadron | Constituents | Example | Charge Calculation | Key Properties |
|---|---|---|---|---|
| Baryons | 3 quarks (qqq) | Proton (\( \mathrm{uud} \)), Neutron (\( \mathrm{udd} \)) | Sum of three quark charges → integer charge | Stable or long-lived; participate in strong interaction; heavy. |
| Mesons | 1 quark + 1 antiquark (q\( \bar{q} \)) | π⁺ (\( \mathrm{u\bar{d}} \)), π⁰ (\( \mathrm{u\bar{u}} \) or \( \mathrm{d\bar{d}} \)) | Sum of one quark and one antiquark charge → integer charge | Short-lived; act as force carriers in strong interactions between baryons. |
Explanation:
- Baryons are heavy hadrons composed of three quarks. The most common examples are the proton and neutron, which form atomic nuclei.
- Mesons are lighter hadrons composed of one quark and one antiquark. They are typically unstable and exist for a short time.
- Both baryons and mesons are bound states of quarks held together by the strong force (via gluons).
Key Distinction:
All hadrons contain quarks, but baryons have three quarks while mesons have a quark–antiquark pair.
Example
Classify each of the following as a baryon or a meson: (a) Proton (\( \mathrm{uud} \)), (b) \( \mathrm{\pi^-} \) meson (\( \mathrm{d\bar{u}} \))
▶️ Answer / Explanation
- (a) Proton has 3 quarks → Baryon.
- (b) \( \mathrm{\pi^-} \) has 1 quark and 1 antiquark → Meson.
Hence: Proton = baryon, \( \mathrm{\pi^-} \) = meson.
Example
Explain why hadrons are not considered fundamental particles.
▶️ Answer / Explanation
Hadrons are made up of smaller particles — quarks — bound by the strong nuclear force. Since they can be broken down into quarks, they are not fundamental. Fundamental particles, like quarks and leptons, cannot be divided further.
Changes to Quark Composition During β⁻ and β⁺ Decay
Definition of Beta Decay:
Beta decay is a type of radioactive decay in which a nucleus changes its composition by converting one type of nucleon (proton or neutron) into another, accompanied by the emission of a beta particle and a neutrino or antineutrino.
There are two types of beta decay:
- β⁻ decay: A neutron changes into a proton, emitting an electron and an antineutrino.
- β⁺ decay: A proton changes into a neutron, emitting a positron and a neutrino.
(a) β⁻ Decay (Beta-minus decay)
Process Description: A neutron (neutral) decays into a proton (positive) by converting one of its down quarks into an up quark.
\( \mathrm{n \rightarrow p + e^- + \bar{\nu}_e} \)
Quark-level process:
\( \mathrm{d \rightarrow u + e^- + \bar{\nu}_e} \)
| Before Decay | After Decay | Quark Change | Particle Emitted |
|---|---|---|---|
| Neutron (\( \mathrm{udd} \)) | Proton (\( \mathrm{uud} \)) | One down quark → up quark | Electron (\( \mathrm{e^-} \)) and antineutrino (\( \mathrm{\bar{\nu}_e} \)) |
Explanation:
- A down quark (charge \( \mathrm{-\dfrac{1}{3}} \)) changes into an up quark (charge \( \mathrm{+\dfrac{2}{3}} \)).
- This increases the overall charge of the nucleon by +1, turning a neutron into a proton.
- An electron and an antineutrino are emitted to conserve charge, lepton number, and energy.
Key Idea:
In β⁻ decay, a down quark changes to an up quark, producing an electron and an antineutrino.
Example
Write the nuclear equation and identify the quark change when carbon-14 undergoes β⁻ decay.
▶️ Answer / Explanation
Nuclear equation: \( \mathrm{^{14}_6C \rightarrow ^{14}_7N + e^- + \bar{\nu}_e} \)
Quark change: In one neutron of carbon-14, a down quark becomes an up quark → neutron (\( \mathrm{udd} \)) changes to proton (\( \mathrm{uud} \)).
Hence, carbon-14 becomes nitrogen-14 due to β⁻ decay.
(b) β⁺ Decay (Beta-plus decay)
Process Description: A proton (positive) decays into a neutron (neutral) by converting one of its up quarks into a down quark.
\( \mathrm{p \rightarrow n + e^+ + \nu_e} \)
Quark-level process:
\( \mathrm{u \rightarrow d + e^+ + \nu_e} \)
| Before Decay | After Decay | Quark Change | Particle Emitted |
|---|---|---|---|
| Proton (\( \mathrm{uud} \)) | Neutron (\( \mathrm{udd} \)) | One up quark → down quark | Positron (\( \mathrm{e^+} \)) and neutrino (\( \mathrm{\nu_e} \)) |
Explanation:
- An up quark (charge \( \mathrm{+\dfrac{2}{3}} \)) changes into a down quark (charge \( \mathrm{-\dfrac{1}{3}} \)).
- This decreases the total charge by 1, turning a proton into a neutron.
- A positron and a neutrino are emitted to conserve charge and lepton number.
Key Idea:
In β⁺ decay, an up quark changes to a down quark, producing a positron and a neutrino.
Example
Write the nuclear equation and identify the quark change when sodium-22 undergoes β⁺ decay.
▶️ Answer / Explanation
Nuclear equation: \( \mathrm{^{22}_{11}Na \rightarrow ^{22}_{10}Ne + e^+ + \nu_e} \)
Quark change: One up quark in a proton changes into a down quark → proton (\( \mathrm{uud} \)) becomes neutron (\( \mathrm{udd} \)).
Hence, sodium-22 transforms into neon-22 through β⁺ decay.
Electrons and Neutrinos as Leptons
Leptons are a family of fundamental particles that do not experience the strong nuclear force. They are distinct from quarks and are among the basic building blocks of matter.
Key Properties of Leptons:
- They are fundamental — not composed of smaller particles.
- They do not feel the strong nuclear force but participate in weak and electromagnetic interactions.
- There are six types (three “flavours”) of leptons, each with an associated neutrino.
| Lepton | Symbol | Charge (e) | Associated Neutrino | Interaction |
|---|---|---|---|---|
| Electron | \( \mathrm{e^-} \) | \( \mathrm{-1} \) | Electron neutrino (\( \mathrm{\nu_e} \)) | Electromagnetic, weak |
| Muon | \( \mathrm{\mu^-} \) | \( \mathrm{-1} \) | Muon neutrino (\( \mathrm{\nu_\mu} \)) | Electromagnetic, weak |
| Tau | \( \mathrm{\tau^-} \) | \( \mathrm{-1} \) | Tau neutrino (\( \mathrm{\nu_\tau} \)) | Electromagnetic, weak |
Antiparticles:
- Each lepton has a corresponding antilepton (e.g., positron \( \mathrm{e^+} \), antineutrino \( \mathrm{\bar{\nu}_e} \)).
- They have the same mass but opposite charge and lepton number.
Key Idea:
Electrons and neutrinos are fundamental leptons. They are not made of quarks and play a key role in weak nuclear processes such as beta decay.
Example
Explain why neutrinos are difficult to detect compared to electrons.
▶️ Answer / Explanation
Neutrinos are electrically neutral and interact only via the weak nuclear force, which is extremely short-ranged and weak. Therefore, they rarely interact with matter, making them very difficult to detect experimentally.
Example
During a β⁻ decay process, an antineutrino (\( \mathrm{\bar{\nu}_e} \)) is emitted along with an electron. Explain why both the electron and the antineutrino are classified as leptons and describe their differences in properties.
▶️ Answer / Explanation
Reason for classification:
- Both the electron (\( \mathrm{e^-} \)) and antineutrino (\( \mathrm{\bar{\nu}_e} \)) are fundamental leptons — they are not made up of quarks and do not experience the strong nuclear force.
- They participate in the weak nuclear interaction, which governs beta decay processes.
Differences in properties:
| Property | Electron (\( \mathrm{e^-} \)) | Antineutrino (\( \mathrm{\bar{\nu}_e} \)) |
|---|---|---|
| Electric charge | \( \mathrm{-1e} \) | \( \mathrm{0} \) |
| Mass | Small but non-zero | Extremely small (≈ 0) |
| Interaction type | Weak and electromagnetic | Only weak interaction |
| Detectability | Easily detected (ionises matter) | Very difficult to detect (rarely interacts) |
Conclusion: Both the electron and the antineutrino are leptons because they are fundamental particles involved in weak nuclear processes. However, the electron is charged and easily detected, whereas the antineutrino is neutral, nearly massless, and interacts extremely weakly with matter.