Introduction to Elementary Particles

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PHY 585 Topics in Elementary Particle Physics (Introduction to Particle Physics for High School Physics Teachers) Fall Semester 2002 Michael Rijssenbeek
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Lecture 1 1 Sep 18, 2002 Part I Phenomenology of Elementary Particles 1 Introduction It can be justifiably argued that Particle Physics started in the early twentieth century with a series of experiments by Rutherford and collaborators. Previously J.J.Thomson had discovered the electron in cathode rays (1896), discrete spectral emission lines had been discovered in hot gaseous elements, as well as absorption lines in the solar spectrum. Avogadro’s number, the number of 6.02 × 10 23 “indivisi- ble” atoms (or molecules) in 22.4 L of an ideal gas at STP, was well known from a variety of experi- mental measurements, and the concept of an atom was established. Maxwell and others had predicted and discovered the existence of electromagnetic radiation from the laws of electromagnetism. How- ever, the model of the atom was still uncertain: at the time the Thomson “plum pudding” model was assumed, which took the atom to be uniformly filled with a positive material (the pudding), in which the electrons were imbedded as little pits (the plums). In that case, the mass density of the pudding would be about the mass density of the bulk material. Ernest Rutherford, with assistants Geiger and Marsden, performed a series of painstaking experiments which ultimately showed that almost all the mass of the atom is carried by an exceedingly small sized nucleus, and that the electrons are electromagnetically bound as planets around a central star: the “so- lar” model of the atom was born. Rutherford’s results led Niels Bohr to the idea of his quantum postu- late: the angular momentum of electrons in the atom is quantized: L = n ħ . Rutherford-type experiments have continued to be the tool of choice in particle physics: crudely speak- ing, the way to investigate the tiniest particles is to slam them with other particles and investigate the resulting collisions; a more refined description is to say that, the wavelength of normal light being much too large to measure (sub)nuclear distances, we have to use particles whose wavelengths obey the de Broglie relationship λ = h/p , with p the particle’s momentum: the larger p , the larger the resolv- ing power. It is very instructive to begin with a discussion of the Rutherford experiment, which can be treated as a purely classical non-relativistic interaction. Moreover, aside from having historical significance, Ruth- erford-type experiments are still very much around: the very highest energy colliders still serve to slam particles together (the particle beams are both beam and target), and experimenters still study the angu- lar (and energy/momentum) distributions of what comes out of the collisions! For instance, quarks were discovered in a series of electron-Hydrogen scattering experiments at the Stanford Linear Accel- erator Center (SLAC) with a high-energy version of the Rutherford method.
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