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CHAPTER 14 Elementary Particles 14.1 14.2 14.8 14.3 14.4 14.5 14.6 14.7 Early Discoveries The Fundamental Interactions Accelerators Classification of Elementary Particles Conservation Laws and Symmetries Quarks The Families of Matter Beyond the Standard Model Steven Weinberg (1933 - ) “I have done a terrible thing: I have postulated a particle that cannot be detected.” - Wolfgang Pauli (after postulating the existence of the neutrino) “If I could remember the names of all these particles, I’d be a botanist.” - Enrico Fermi Elementary Particles Particle physics tries to answer the most fundamental questions about nature: What’s inside the nucleus? What are the basic building blocks of matter? What are the forces that hold matter together and break it apart? What new physical laws are required to describe these forces? Discovery of the Neutron By the 1920s, most atomic nuclei were known to be heavier than Z, the atomic number. But only protons and electrons were known, so many thought that the nucleus contained extra protons and an equal number of extra electrons, too, to compensate. But electrons can’t exist within the nucleus: Nuclear size The uncertainty principle puts a lower limit on its kinetic energy that’s much larger that the kinetic energy observed for any electron emitted from nuclei. Nuclear spin Deuterons were known. If a deuteron consisted of protons and electrons, then it must contain 2 protons and 1 electron. A nucleus composed of 3 fermions must have half-integral spin. But it had been measured to be 1. Nuclear Properties The nuclear charge is +e times the number (Z) of protons. Other Hydrogen atoms: Deuterium: Heavy hydrogen. One proton and one neutron in its nucleus. Tritium: Heavier hydrogen! One proton and two neutrons. The respective nuclei are called deuterons and tritons. Atoms with the same Z, but different mass number, are called isotopes. Number of nucleons Number of protons Hydrogen The Nuclear Force Because there’s no negative charge in the nucleus, it’s clear that a new force is involved. The nuclear force is called the strong force for the obvious reason! The nuclear potential energy vs. distance The angular distribution of nucleons scattered by other nucleons tells us the nuclear potential. Nuclear potential Coulomb repulsion Summary of Early Discoveries Thomson had identified the electron in 1897, and Einstein had defined the photon in 1905. The proton is the nucleus of the hydrogen atom (let’s give Rutherford credit for its discovery). In 1932 James Chadwick identified the neutron, actually first seen by Bothe and Becker. That seemed sufficient… Protons, neutrons, electrons, and photons—what do you give the universe that has everything? The Positron In 1928 when Dirac combined quantum mechanics with special relativity, he introduced the relativistic theory of the electron. He found that, in free space, his wave equation had negative, as well as positive, energy solutions. His theory can be interpreted as the vacuum being filled with an infinite sea of electrons with negative energies. Exciting an electron from the “sea,” leaves behind a hole with negative energy, that is, the positron, denoted by e+. Paul Dirac (1902-1984) Vacuum Electron & positron Positron! E 0 Anti-Particles Dirac’s theory yields anti-particles for all particles, which: Have the same mass and lifetime as their associated particles. Have the same magnitude but opposite sign for such physical quantities as electric charge and various quantum numbers. All particles, even neutral ones, have anti-particles (with some exceptions like the neutral pion, whose anti-particle is itself). In 1932, Carl Anderson identified the positron in cosmic rays. It was easy: it had positive charge and was light. Anderson’s cloud chamber photo of the first recorded positron track Discovery of the Positron Carl Anderson (1905-1991) Electron-Positron Interaction The ultimate fate of positrons (anti-electrons) is annihilation with electrons. After a positron slows down by passing through matter, it’s attracted by the Coulomb force to an electron, where it is annihilated through the reaction: e e 2 All anti-matter eventually meets the same fate. A lot of energy is released in this process: all of the matter is converted to energy. Star Trek’s “dilithium crystals” supposedly contain anti-matter, which powers the Enterprise. Quantum Field Theory and Feynman Diagrams Richard Feynman presented a particularly simple graphical technique to describe many-particle interactions in what we now call quantum field theory. Electromagnetism can be interpreted as the exchange of photons. We say that the photons are the carriers or mediators of the electromagnetic force. Example of a Feynman spacetime diagram. Electrons interact through mediation of a photon. The axes are normally omitted. Virtual photons mediate electromagnetism. Quantum field theory predicts that, when two charged particles interact, they actually exchange a series of photons called virtual photons, which cannot be directly observed. Two charged particles and their virtual photons: The strong, weak, and gravitational interactions are assumed to operate in the same manner, but with their mediating particles. Yukawa’s Meson The Japanese physicist Hideki Yukawa developed a quantum field theory that described the force between nucleons (protons and neutrons)—the strong force. To do this, he had to determine the carrier or mediator of the nuclear strong force analogous to the photon in the electromagnetic force, which he called a meson (derived from the Greek word meso meaning “middle” due to its mass being between the electron and proton masses). Hideki Yukawa (1907-1981) Yukawa’s Meson Yukawa’s meson, called a pion (or pi-meson or p-meson), was identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini (1907–1993) in cosmic rays at sites located at high-altitude mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later at Chacaltaya in the Andes Mountains. Charged pions have masses of 140 MeV/c2, and a neutral pion p0 with a mass of 135 MeV/c2 was later discovered. Feynman diagram indicating the exchange of a pion (Yukawa’s meson) between a neutron and a proton. The Mass of a Mediator Particle and the Range of the Force The Uncertainty Principle allows energy conservation to be violated by a time: t ~ / 2 E If we set E = mc2, the mass energy of the particle (allowing for its creation), we have: 2 t ~ / 2 mc But the particle can travel up to the speed of light, so, if r is the range of the force, t ~ r/c: r / c ~ / 2 mc 2 Or: mc2 ~ c / 2r the strong and weak forces So if r = ∞, then m = 0. But if the force is short-range, m can be large. For the strong force, r ~ 4 × 10-15 m, so mc2 ~ 150 MeV for the meson. Accelerators Particle accelerators generate particles with energies >1 TeV. Accelerators There are several types of accelerators used presently in particle physics experiments: cyclotrons, linear accelerators, and colliders. They’re all based on the same idea: as the particles move, apply a voltage that accelerates them to higher a speed. Cyclotrons and Synchrotrons A charged particle in a magnetic field travels in a circle. Accelerating it with voltage yields a cyclotron. A problem with cyclotrons, however, is that, when charged particles are accelerated, they radiate electromagnetic energy called synchrotron radiation. This problem is particularly severe when electrons, moving very close to the speed of light, move in highly curved paths. If the radius of curvature is small, electrons can radiate as much energy as they gain. Physicists have learned to take advantage of these synchrotron radiation losses and now build special electron accelerators (called light sources) that produce copious amounts of photon radiation used for both basic and applied research. Linear Accelerators Linear accelerators or linacs typically have straight electric-field-free regions between gaps of RF voltage boosts. The particles gain speed with each boost, and the voltage boost is on for a fixed period of time, so the distance between gaps becomes increasingly larger as the particles accelerate. Linacs are sometimes used as pre-acceleration device for large circular accelerators. Colliders Head-on collisions are twice as energetic as those involving hitting an object at rest, so physicists began building collidingbeam accelerators, in which the particles meet head-on. If the colliding particles have equal masses and kinetic energies, the total momentum is zero and all the energy is available for the reaction and the creation of new particles. Large Hadron Collider Counterpropagating protons each have an energy of ~7 TeV, giving a total collision energy of 14 TeV. The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.