Fundamental particles
Section: Particle Physics | Syllabus: Cambridge AS Level Physics (9702)
What are Fundamental Particles?
Fundamental Particle A particle that has no internal structure and cannot be broken down into smaller constituents. It is not made of other particles.The Standard Model of particle physics classifies matter into two main categories of fundamental particles:
- Quarks - combine to form protons, neutrons, and other hadrons
- Leptons - include electrons and neutrinos
Protons and neutrons are NOT fundamental particles - they are made of quarks. Electrons ARE fundamental particles.
Quarks
Quark A fundamental particle that combines to form composite particles called hadrons. Quarks have fractional electric charges.The Six Quark Flavours
There are six types (flavours) of quarks, arranged in three generations:
| Generation | Quark | Symbol | Charge |
|---|---|---|---|
| 1st (lightest) | Up | u | +(2/3)e |
| Down | d | -(1/3)e | |
| 2nd | Charm | c | +(2/3)e |
| Strange | s | -(1/3)e | |
| 3rd (heaviest) | Top | t | +(2/3)e |
| Bottom | b | -(1/3)e |
Antiquarks
Each quark has a corresponding antiquark with the same mass but opposite charge:
| Antiquark | Symbol | Charge |
|---|---|---|
| Anti-up | u | -(2/3)e |
| Anti-down | d | +(1/3)e |
Remember the charges: up-type quarks (u, c, t) have charge +(2/3)e, and down-type quarks (d, s, b) have charge -(1/3)e.
Hadrons
Hadron A composite particle made of quarks, held together by the strong nuclear force. Hadrons are classified as baryons or mesons. Word OriginsThe word hadron comes from the Greek word for "heavy" and lepton comes from the Greek word for "light".
Hadrons are divided into two categories:
Baryons
Baryon A hadron composed of three quarks (qqq) or three antiquarks (qqq). Examples include protons and neutrons.Mesons
Meson A hadron composed of a quark-antiquark pair (qq). Examples include pions and kaons.| Property | Baryon | Meson |
|---|---|---|
| Quark composition | 3 quarks (qqq) | Quark + antiquark (qq) |
| Examples | Proton, neutron | Pion (π), kaon (K) |
| Meson | Quark Composition | Charge |
|---|---|---|
| Pion (π^+) | ud | +(2/3)e + (1/3)e = +e |
| Kaon (K^+) | us | +(2/3)e + (1/3)e = +e |
Quark Composition of Nucleons
The proton and neutron are the most important baryons to know:
FIG 11.13: Quark Composition of Proton and NeutronTwo circles side by side: (1) Proton containing three quarks u, u, d with charges +⅔, +⅔, −⅓ giving total charge +1; (2) Neutron containing u, d, d with charges +⅔, −⅓, −⅓ giving total charge 0. Different colours distinguish up quarks from down quarks.
Proton
Quark composition: uud (two up quarks and one down quark)
Charge = (+(2/3)) + (+(2/3)) + (-(1/3)) = +(4/3) - (1/3) = +1 = +e ✓Neutron
Quark composition: udd (one up quark and two down quarks)
Charge = (+(2/3)) + (-(1/3)) + (-(1/3)) = +(2/3) - (2/3) = 0 ✓| Nucleon | Quark Composition | Total Charge |
|---|---|---|
| Proton | uud | +e |
| Neutron | udd | 0 |
Proton = Positive, so needs more up quarks (uud)
Neutron = Neutral, so needs more down quarks (udd)
Leptons
Lepton A fundamental particle that does not experience the strong nuclear force. Leptons include electrons, muons, taus, and their associated neutrinos.At AS Level, the key leptons to know are:
| Lepton | Symbol | Charge | Fundamental? |
|---|---|---|---|
| Electron | e^- | -e | Yes |
| Electron neutrino | _e | 0 | Yes |
| Positron (antilepton) | e^+ | +e | Yes |
| Electron antineutrino | _e | 0 | Yes |
Electrons and neutrinos are fundamental leptons - they are not made of quarks. This is why electrons and neutrinos are not affected by the strong nuclear force.
Quark Changes in Beta Decay
Beta decay involves the transformation of one quark into another, which changes a neutron into a proton or vice versa.
Beta-minus (β⁻) Decay at the Quark Level
In β⁻ decay, a down quark changes into an up quark:
d arrow u + e^- + _eThis transforms a neutron (udd) into a proton (uud):
n arrow p + e^- + _e FIG 11.14: Beta-minus Decay Showing Quark ChangeNeutron (u, d, d) on the left transforms to proton (u, u, d) on the right. One d quark changes to u quark (highlighted), emitting an electron (e⁻) and electron antineutrino (ν̄ₑ). Shows the quark-level process d → u + e⁻ + ν̄ₑ.
Verification of Charge Conservation in β⁻ DecayInitial: d quark has charge -(1/3)e
Final: u quark (+(2/3)e) + electron (-e) + antineutrino (0)
Total final charge: +(2/3)e - e + 0 = -(1/3)e ✓
Beta-plus (β⁺) Decay at the Quark Level
In β⁺ decay, an up quark changes into a down quark:
u arrow d + e^+ + _eThis transforms a proton (uud) into a neutron (udd):
p arrow n + e^+ + _e FIG 11.15: Beta-plus Decay Showing Quark ChangeProton (u, u, d) on the left transforms to neutron (u, d, d) on the right. One u quark changes to d quark (highlighted), emitting a positron (e⁺) and electron neutrino (νₑ). Shows the quark-level process u → d + e⁺ + νₑ.
Verification of Charge Conservation in β⁺ DecayInitial: u quark has charge +(2/3)e
Final: d quark (-(1/3)e) + positron (+e) + neutrino (0)
Total final charge: -(1/3)e + e + 0 = +(2/3)e ✓
| Decay Type | Quark Change | Nucleon Change | Emitted Particles |
|---|---|---|---|
| β⁻ decay | d arrow u | n arrow p | e^- and _e |
| β⁺ decay | u arrow d | p arrow n | e^+ and _e |
When asked about quark changes in beta decay:
• β⁻: d → u (down becomes up, releasing negative charge)
• β⁺: u → d (up becomes down, releasing positive charge)
Summary Tables
Quark Charges Summary
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