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Transcript

Physics 5333 Spring 2015 (Chapter 1: getting started) the elementary particles the forces the model how can we understand it? Elementary particle: an entity not able to be further decomposed with a unique set of properties mass, m charge, Q spin : s =½ integer (fermion), s = 0, 1, 2…(boson) flavor spin, charge & mass(energy) Intrinsic property “constituents” do not exist We don’t know how to account for the property by classical, quantum mechanical or relativistic (field theoretic) models what is charge? Charge (Q) is a quantity we have defined in order to describe how certain particles (with this charge) interact. If the particles don’t interact in the prescribed way, they don’t have charge. The force, F, between two charges (and the classical mathematical model, Coulomb’s Law, kQ1Q2/r2), was derived experimentally. Subsequent to this we developed the ideas of electric fields, E=F/Q1 electrostatic potentials, Ф, magnetic fields, B, (from moving Q or changing E fields) and Maxwell’s equations, the most rigorous model in physics. Still, this does not tell us what charge is. The models above have been extended to a startling new model (Quantum Electrodynamics) which “explains” why two charges interact: they exchange photons (a new kind of particle with no charge, travelling with the speed of light). Still, we do not know what charge is. about charge and the electron We do know that charge is “quantized”: it comes only in multiples of the electronic charge, e = 1.6 x 10-19 Coulombs. Furthermore, the electron itself, although having both mass and charge, e , has a “size” so small that we are able only to say it is smaller than what we can detect! This is indeed a phenomenon! the elementary particles (as far as we know at this time) six quarks (u d six leptons (e ne cs m nm t b) t nt) all have spin = ½ they are fermions that’s it! size Like the electron, these elementary particles have “sizes” smaller than we can detect. Another phenomenon! mn < 0.3 eV Mass of proton = 0.938 GeV/c2 Particle Antiparticle Q -Q mm an antiparticle is like a particle ‘going backwards in time’ Building composite particles – with sizes we can detect: Quarks (q) can be bound together to form composite particles, like protons, neutrons and pions. But, we only find in the laboratory composite particles corresponding to quark-antiquark or qqq combinations. (LHC November 2015: possibly 5-quark particle ) These composite particles of quarks are held together by the strong force mediated by the exchange of gluons. Like the electric charge which produces the Coulomb force, the color “charge” is carried by the quarks.