Particle Physics: Discoveries & Theories from Ancient Greece to Quarks, Slides of Advanced Physics

Download Particle Physics: Discoveries & Theories from Ancient Greece to Quarks and more Slides Advanced Physics in PDF only on Docsity! A Brief History of Particle Physics Ancient Greece: idea of atoms proposed, no experimental evidence 1804: Dalton explains some of chemistry using idea of atoms 1869: Mendeleev develops current form of periodic table 1895: Röntgen discovers X-rays 1896: Becquerel observes radioactivity 1897: JJ Thomson discovers the β particle / electron 1905: Einstein shows Brownian motion results from atomic collisions, quantizes light (photoelectric effect) 1907: Rutherford and Royds find α's are 4He ions 1910: Rutherford, Geiger, and Marsden discover nuclei 1919: Rutherford finds protons in nuclei, proposes neutrons 1928: Dirac finds negative energy states in relativistic quantum mechanics. Meaning unclear. Thursday, May 2, 2013 docsity.com Negative Energy States and Antiparticles 1928: Dirac finds negative energy states in relativistic quantum mechanics. Meaning unclear. Negative energy states must be filled or electrons would fall into them. 1932: Anderson discovers positron. 1949: Feynman shows electron moving backward in time = positron. Antiparticles: identical to particles, except additive quantum numbers reversed. Which description is right? They all are - same physics, differently described. Thursday, May 2, 2013 docsity.com Accelerators Early experiments used low voltages, or radioactive sources. By the late 1920s, physicists started building high voltage accelerators. DC voltage machines going up to several MV, including the "Cockcroft-Walton" shown to the right, and (tandem) van der Graaf designs, below, are still in use today. Far left: simple lab vdG left: Brookhaven 15 MV tandem vdG: negative ions are "stripped" at the center, so doubly accelerated, or more, if more than 1 e- is removed Thursday, May 2, 2013 docsity.com Cyclotrons In 1932 Lawrence built the first cyclotron - the accelerating scheme is shown below. Protons, for example, are injected near the center and extracted at the outer edge. These were a major advance over the DC machines, as they could accelerate protons to ≈100 MeV kinetic energy, rather than the 10 MeV of early DC machines. Consider a particle of speed v. The radius of its orbit in a constant magnetic field is r = mv/qB, or v = qBr/m. As a result, the period of its orbit is T = 2πr/v = 2πr/(qBr/m) = 2πm/qB, which does not depend on its speed / energy! Thursday, May 2, 2013 docsity.com Relativistic Cyclotrons When the particle becomes relativistic, however, this changes. The radius becomes larger by a factor of γ while the speed becomes nearly constant, so the period increases. No longer is T = 2πm/qB independent of energy. Initially it was thought that this would prevent cyclotrons from accelerating relativistic particles, but several machines have been built with, eg, curved pole faces to overcome this problem. The Paul Scherrer Institute 500 MeV proton cyclotron in Switzerland. The blue regions are bending dipole magnets, the 4 gray regions house the accelerating gaps. The Michigan State University superconducting K1200 cylcotron. The curvature corrections for the variation in T(r). Thursday, May 2, 2013 docsity.com iClicker Which of the following does not refer to a usual subatomic- physics experiment particle acceleration technique? A) DC voltage B) cyclotron C) radio-frequency (RF) cavity D) baseball bat E) synchrotron Thursday, May 2, 2013 docsity.com CM Energy A key parameter for new machines is the center of mass energy achieved. This is because the higher the center of mass energy, the more massive new particles one can search for. For symmetric colliders - there are some where the two beams are different and have unequal energies - the center of mass system is the same as the lab system, so Ecm = 2Elab. But if we have a target M at rest in the lab and a particle of mass m accelerated to energy Em, what is the energy in the c.m.? This is easy with 4-vectors (and subatomic units with c=1): pTotal = (Elab,0,0,plab) + (M,0,0,0) = (Elab+M,0,0,plab) Ecm2 = pTotal2 = (Elab+M)2 - plab2 = 2ElabM + M2 + m2. At very high energies, you can see that the mass terms become small, and the c.m. energy is proportional to (Elab)1/2 for a fixed target, vs Elab for colliding beams. Thursday, May 2, 2013 docsity.com CM Energy Example We want to generate a 3 GeV particle (J/ψ) in pp collisions. What energy is needed for colliding beams? What energy is needed for a fixed target experiment? First note that the total mass of all particle will be 2x the proton mass plus the J/ψ mass, or 0.94 + 0.94 + 3.0 GeV = 4.88 GeV. In the center of mass frame at threshol all the particles exist, but they have no kinetic energy; they are at rest. a) Ecm = 2Elab = 4.88 GeV ➮ Elab = 2.44 GeV - each beam is 2.44 GeV. b) Ecm2 = 2ElabM + M2 + M2 = 2M(Elab + M) = 4.882 ➮ Elab = 11.73 GeV. Thursday, May 2, 2013 docsity.com Understanding why these particles and others are grouped into families like these would require going into more detail than we have time for, and a branch of mathematics known as group theory. One aspect of strangeness was that strange particles would be produced in pairs so that total strangeness was 0, but they would decay with long lifetime to non-strange particles. This leads to the idea that the strong interaction conserves strangeness, but the weak interaction is responsible for the decays, and does not. Strangeness Thursday, May 2, 2013 docsity.com The Quark Model In the early 1960s, Gell-Mann and others proposed that strongly interacting particles (hadrons) - everything found except for electrons, muons, neutrinos, and their antiparticles (leptons) - are made up of either 2 or 3 spin-1/2 "quarks". At first it was not clear whether quarks were real particles or a mathematical / organizational trick. The quarks were named up, down, and strange. The quarks (oddly) have electric charges of ±⅓e or ±⅔e, rather than being integral multiples of e, and no one detected free quarks, so they did not seem to be real. Also, scattering of electrons at high energy and momentum transfer from protons showed that the proton contained a large number of point-like particles. This could be understood in terms of Feynman's parton model, in which particles could have an infinite number of components. So how were the infinite numbers of partons in the proton related to the 3 quarks? Thursday, May 2, 2013 docsity.com Quark Structure of Mesons and Baryons Thursday, May 2, 2013 docsity.com The Theory of Quarks and Strong Interactions In general, QCD remains too complicated to solve, but one important feature is "asymptotic freedom". At high energies quarks act like free particles, but low energy quarks interact strongly with gluons and are bound into nucleons. Most of the nucleon mass comes from the color interactions - The up and down quarks in the proton have masses of only ≈5 MeV, or 1% of the proton mass. While QCD itself cannot be exactly solved at low energy, there are solvable approaches to it such as "effective field theory", "lattice" QCD, "ads/QCD", Dyson-Schwinger, ... that each allow some aspects of it to be approximated. Lattice + Dyson-Schwinger calculations of how quarks acquire mass at low momentum through "dragging" the sea along. Thursday, May 2, 2013 docsity.com MY CS Te] AY CIT emONO Cem IE OP ROIS These are a few of the many types of mesons. Symbol] Name | Quark | Electric | Mass {Spin content snes Gevie2 |" ®. 2, 2013 docsity.com iClicker The π- meson is the antiparticle of the π+ meson. What is its quark content? A) u dbar B) s ubar C) d ubar D) d cbar E) it has gluons, not quarks Thursday, May 2, 2013 docsity.com