14.1Early Discoveries14.2The Fundamental Interactions14.8Accelerators14.3Classification of Elementary Particles14.4Conservation Laws and Symmetries14.5Quarks14.6The Families of Matter14.7Beyond the Standard ModelCHAPTER 14 Elementary ParticlesIf I could remember the names of all these particles, Id be a botanist.Enrico FermiSteven 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)
Elementary ParticlesFinding answers to some of the basic questions about nature is a foremost goal of science:
What are the basic building blocks of matter? What is inside the nucleus? What are the forces that hold matter together? How did the universe begin? Will the universe end, and if so, how and when?Elementary Particles
14.1: Early DiscoveriesIn 1930 the known elementary particles were the proton, the electron, and the photon.Thomson identified the electron in 1897, and Einstein defined the photon in 1905. The proton is the nucleus of the hydrogen atom. Despite the rapid progress of physics in the first couple of decades of the twentieth century, no more elementary particles were discovered until 1932, when Chadwick proved the existence of the neutron.That would have seemed sufficientJames Chadwick (1891-1974)
But particle physics measurements were happeningEnergetic particles collide with stationary particles in a bubble chamber, vaporizing the nearby matter and leaving a visible track.A magnetic field (pointing into the screen) causes charged particles to take curved paths.
The PositronIn 1928 Dirac introduced the relativistic theory of the electron when he combined quantum mechanics with special relativity.He found that his wave equation had negative, as well as positive, energy solutions.His theory can be interpreted as a vacuum being filled with an infinite sea of electrons with negative energies.If enough energy is transferred to the sea, an electron can be ejected with positive energy leaving behind a hole that is the positron, denoted by e+.Paul Dirac (1902-1984)E0Vacuum Electron & positron
Anti-particlesDiracs theory yields anti-particles, which:Have the same mass and lifetime as their associated particles.Have the same magnitude but are opposite in sign for such physical quantities as electric charge and various quantum numbers.All particles, even neutral ones (with some exceptions like the neutral pion), have antiparticles.Magnetic field into screen
The PositronCarl Anderson identified the positron in cosmic rays. It was easy: it had positive charge and was light.Carl Anderson (1905-1991)Andersons cloud chamber photo of positron trackCosmic rays are highly energetic particles, mostly protons, that cross interstellar space and enter the Earths atmosphere, where their interaction with particles creates cosmic showers of many distinct particles.
Positron-Electron InteractionThe ultimate fate of positrons (anti-electrons) is annihilation with electrons.After a positron slows down by passing through matter, it is attracted by the Coulomb force to an electron, where it annihilates through the reaction:
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 Treks dilithium crystals supposedly contain anti-matter, which powers the Enterprise.
Feynman DiagramsFeynman presented a particularly simple graphical technique to describe interactions.It predicts that, when two electrons approach each other, according to the quantum theory of fields, they exchange a series of photons called virtual photons, because they cannot be directly observed.The action of the electromagnetic field (for example, the Coulomb force) can be interpreted as the exchange of photons. In this case we say that the photons are the carriers or mediators of the electromagnetic force.Example of a Feynman space-time diagram. Electrons interact through mediation of a photon. The axes are normally omitted.
Yukawas MesonThe Japanese physicist Hideki Yukawa had the idea of developing a quantum field theory that would describe the force between nucleonsanalogously to the electromagnetic 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)
Yukawas MesonYukawas meson, called a pion (or pi-meson or -meson), was identified in 1947 by C. F. Powell (19031969) and G. P. Occhialini (19071993)Charged pions have masses of 140 MeV/c2, and a neutral pion 0 was later discovered that has a mass of 135 MeV/c2, a neutron and a proton. Feynman diagram indicating the exchange of a pion (Yukawas meson) between a neutron and a proton.
Other Mesons, Quarks, and GluonsYukawas pion is responsible for the nuclear force.
Later well see that the nucleons and mesons are part of a general group of particles formed from even more fundamental particles: quarks. The particle that mediates the strong interaction between quarks is called a gluon (for the glue that holds the quarks together); its massless and has spin 1, just like the photon.Computed image of quarks and gluons in a nucleon
The Weak InteractionIn the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam predicted that particles that they called W (for weak) and Z should exist that are responsible for the weak interaction.
They have been observed. Sheldon Glashow (1932- )Abdus Salam (1926-1996)
The GravitonIt has been suggested that the particle responsible for the gravitational interaction be called a graviton.
The graviton is the mediator of gravity in quantum field theory and has been postulated because of the success of the photon in quantum electrodynamics theory.
It must be massless, travel at the speed of light, have spin 2, and interact with all particles that have mass-energy.
The graviton has never been observed because of its extremely weak interaction with objects.
The Fundamental InteractionsOne of the main goals of particle physics is to unify these forces (to show that theyre all just different aspects of the same force), just as Maxwell did for the electric and magnetic forces many years earlier.
The Fundamental InteractionsA finite range effectively confines the particle, which, by the uncertainty principle, gives it a minimal momentum and hence a minimum kinetic energy and mass. Photons and gravitons are massless. W and Z bosons are heavy.
14.8 AcceleratorsParticle accelerators generate high enough energies to create particles 1 GeV/c2 or greater.
AcceleratorsThere are three main types of accelerators used presently in particle physics experiments: synchrotrons, linear accelerators, and colliders.
Synchrotron RadiationOne difficulty with cyclic accelerators 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 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 AcceleratorsLinear 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, and thus the distance between gaps becomes increasingly larger as the particles accelerate.
Linacs are sometimes used as pre-acceleration device for large circular accelerators.
CollidersBecause of the limited energy available for reactions like that found for the Tevatron, physicists decided they had to resort to colliding beam experiments, 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 ColliderCounter-propagating protons will 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.
14.3: Classification of Elementary ParticlesParticles with half-integral spin are called fermions and those with integral spin are called bosons. This is a particularly useful way to classify elementary particles because all stable matter in the universe appears to be composed, at some level, of constituent fermions. Fermions obey the Pauli Exclusion Principle. Bosons dont.
Photons, gluons, W, and the Z are called gauge bosons and are responsible for the strong and electroweak interactions. Gravitons are also bosons, having spin 2. Fermions exert attractive or repulsive forces on each other by exchanging gauge bosons, which are the force carriers.
The Higgs Boson One other boson that has been predicted, but not yet detected, is necessary in quantum field theory to explain why the W and Z have such large masses, yet the photon has no mass. This missing boson is called the Higgs particle (or Higgs boson) after Peter Higgs, who first proposed it. The Standard Model proposes that there is a field called the Higgs field th