Particles that mediate force Book pg 299-300 Exchange particles
Review Baryon number B Total # of baryons must remain constant All baryons have the same number B = 1 (p, n, Λ, Σ, Ξ) All non baryons (leptons and mesons) have B = 0 (π, k, e, μ, τ) An anti particle has the opposite baryon number B = -1 from its particle Quarks: B = 1 3, antiquark: B = - 1 3
Review - Leptons e, e + v, v μ, μ + v μ, v μ τ, τ + v τ, v τ All have a spin of 1 2 All neutrinos have a charge of zero
Review: Hadrons Mesons, Baryons, Quarks Mesons Quark antiquark u, d, s, c, b, t spin: 1, S = 0, except for s 2 quark π +, π, π 0 - spin = 0, S = 0 k +, k, k 0 - spin = 0, S = 1 k : s = - 1 J/PSI or J/Ψ spin = 1, S = 0 η 0 - spin = 0, S = 0 Baryons Proton uud S = 0 Neutron udd S = 0 Λ 0 S = -1 Σ 0, Σ +1, Σ 1 S = -1 Ξ 0, Ξ S = - 2 Ω S = 0 if of Ω bbb otherwise S = -3 All have a spin of 1 2 except Ω : spin = 3 2
Review Lepton number L Total number in each generation (L e, L μ, L τ ) must always remain the same Electron and electron neutrino v e : L = 1 μ and v μ : L = 1 τ and v τ : L = 1 All other particles have L = 0 An antiparticle has the opposite lepton number L = - 1 from its particle
Review rules for neutrino Lepton & neutrino on same side e, μ, τ - antineutrino v e +, μ +, τ + - neutrino v Lepton & neutrino on opposite side e, μ, τ - neutrino v e +, μ +, τ + - antineutrino v
Review Rules for Strangeness S S = 0 for leptons S = 0 for protons, neutrons, pions S + +1 for k 0 and k + mesons S = - 1 for k, Λ and Σ baryons S = - 2 for Ξ S = -3 for Ω, but S = 0 if Ω bbb All antiparticles have opposite strangeness
Four fundamental interactions Gravitational Weak Electromagnetic Strong 1970: electro weak interaction
The force acts EM force Cause for basic collisions b/w charged particles such as p + p p + p Weak force Only in leptons (e, v, τ and antiparticles) Reactions that produce neutrinos Neutrinos are neutral and not effected by EM force, such as v e + μ e + v μ Strong force Reactions b/w hadrons (mesons, baryons, quarks) such as p + p p + p + π 0
Exchange particles gauge bosons Elementary particles that transmit the forces of nature Force Exchange particle Rest mass m p Q spin strong G - gluons 0 0 1 weak W + W Z 0 Higgs boson 89 89 99 > 86 EM photons 0 0 1 gravity gravitons 0 0 2 +1-1 0 0 1 1 1 1 Observation: Only weak force exchange particles have mass Current neutral weak interaction mediated by Z 0 Charged current weak interaction by W +, W
Graviton Inverse square force Infinite range Affects all particles Acts on all mass / energy Rest mass = 0 Has not been found
Electro weak force EM force: photons E = hf = hc λ Transmitted spontaneously since photons travel at c Weak nuclear reaction W +, W, Z 0
Strong nuclear force Pions (mesons) pi meson Composite nucleons constantly exchange mesons with nearby nucleons w/o being altered themselves Their range is comparable to the separation between adjacent nucleons Gluons Only occurs with hadron Force that holds nucleus together Caused by leakage from gluon exchange Gluons are responsible for quark colour Linking b/w quarks and antiquarks by gluon clumps called glueballs
Summary Force Exchange particle Acts on Roles played by strong G - gluons Quarks, gluons, hadrons weak W + W Z 0 Higgs boson Observation: Only weak force exchange particles have mass EM photons Electrically charged particles Binding atomic nuclei, fusion process in stars Quarks and leptons Transmutation of elements, collision of particles Breaking up of stars (super nova) Binding atoms, creation of magnetic fields gravity gravitons All particles Binding planets, stars, galaxies, clusters
Exam Questions Applying conservation laws in particle reactions L must be conserved by family. Thus L II and L I are not conserved. A pion is a meson and has B = 0. p and n each have B = 1. Baryon number not conserved. Baryon number not conserved. Charge not conserved. cgrahamphysics.com
Exam question continued Applying conservation laws in particle reactions Gluons. cgrahamphysics.com
Exam question 2 Applying conservation laws in particle reactions Conservation of charge. Conservation of baryon number. Conservation of lepton number (by family). Also strangeness, parity, isotopic spin, angular momentum. cgrahamphysics.com
Exam question continued Applying conservation laws in particle reactions Family I lepton number is not conserved. Equation needs Family I lepton with no charge and L = -1. e fits the bill. n p + e - + e. cgrahamphysics.com