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 Key Point 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 +fractione Down d -fractione 2nd Charm c +fractione Strange s -fractione 3rd (heaviest) Top t +fractione Bottom b -fractione Antiquarks Each quark has a corresponding antiquark with the same mass but opposite charge : Antiquark Symbol Charge Anti-up u -fractione Anti-down d +fractione Exam Tip Remember the charges: up-type quarks (u, c, t) have charge +fractione, and down-type quarks (d, s, b) have charge -fractione.
Hadrons Hadron A composite particle made of quarks, held together by the strong nuclear force. Hadrons are classified as baryons or mesons. Word Origins The 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) Examples of Meson Compositions Meson Quark Composition Charge Pion (π^+) ud +fractione + fractione = +e Kaon (K^+) us +fractione + fractione = +e Quark Composition of Nucleons The proton and neutron are the most important baryons to know: FIG 11.13: Quark Composition of Proton and Neutron Two 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 = (+fraction) + (+fraction) + (-fraction) = +fraction - fraction = +1 = +e ✓ Neutron Quark composition: udd (one up quark and two down quarks) Charge = (+fraction) + (-fraction) + (-fraction) = +fraction - fraction = 0 ✓ Nucleon Quark Composition Total Charge Proton uud +e Neutron udd 0 Memory Aid P roton = P ositive, so needs more u p quarks (uud) N eutron = N eutral, so needs more d own 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 ν_e 0 Yes Positron (antilepton) e^+ +e Yes Electron antineutrino ν_e 0 Yes Key Point 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 u + e^- + ν_e This transforms a neutron (udd) into a proton (uud): n p + e^- + ν_e FIG 11.14: Beta-minus Decay Showing Quark Change Neutron (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⁻ Decay Initial: d quark has charge -fractione Final: u quark (+fractione) + electron (-e) + antineutrino (0) Total final charge: +fractione - e + 0 = -fractione ✓ Beta-plus (β⁺) Decay at the Quark Level In β⁺ decay, an up quark changes into a down quark : u d + e^+ + ν_e This transforms a proton (uud) into a neutron (udd): p n + e^+ + ν_e FIG 11.15: Beta-plus Decay Showing Quark Change Proton (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⁺ Decay Initial: u quark has charge +fractione Final: d quark (-fractione) + positron (+e) + neutrino (0) Total final charge: -fractione + e + 0 = +fractione ✓ Decay Type Quark Change Nucleon Change Emitted Particles β⁻ decay d u n p e^- and ν_e β⁺ decay u d p n e^+ and ν_e Exam Tip When asked about …
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