Chapter 29 Lecture Particle Physics Prepared by Dedra Demaree, Georgetown University
Particle Physics What is antimatter? What are the fundamental particles and interactions in nature? What was the Big Bang, and how has the universe evolved since?
Be sure you know how to: Use the right-hand rule to determine the direction of the magnetic force exerted by a magnetic field on a moving charged particle (Section 17.4). Explain beta decay (Section 28.6). Write an expression for the rest energy of a particle (Section 25.8).
What's new in this chapter Beta decay can produce antineutrinos, a form of antimatter. Every known particle has a corresponding antiparticle. In this chapter we investigate elementary particles such as the positron and their fundamental interactions. This area of physics is called particle physics.
Antiparticles By 1930, physicists had identified four particles: the electron, the proton, the neutron, and the photon. At that time, these were the only known truly elementary particles a description used to indicate the simplest and most basic particles. This view changed with the proposal and discovery of so-called antiparticles.
Antielectrons predicted Dirac predicted that free electrons would have an infinite number of possible quantum states with negative total energy. A free electron in a positive energy state should be able to transition to one of these negative energy states. How could a free electron have negative total energy? These negative energy states are occupied by an infinite number of positrons (then called antielectrons), a new type of particle that had not yet been observed.
Antielectrons detected
Pair production Under the right conditions, a photon can produce an electron and a positron:
Pair annihilation If an electron and a positron meet, it is possible for them to annihilate and produce a photon.
Conceptual Exercise 29.1 Imagine that an electron and a positron meet and annihilate each other. Assume that they are moving directly toward each other at constant speed. A. Will one or two photons be produced? Write a reaction equation for this process. B. In which directions do these photons move relative to each other after the process?
Beta-plus decay: Transforming a proton into a neutron If a proton captures a gamma-ray photon, the energy of the excited-state proton may be great enough to produce a neutron and the other particles. The proton absorbs the photon and then decays into a neutron, a positron, and a neutrino. This process is called beta-plus decay to indicate that a positron (not an electron) is produced.
Positron emission tomography Positron emission tomography (PET) is a process for imaging the brain. Fluorine-18 isotopes undergo beta-plus decay continually, producing positrons. The positrons meet electrons and annihilate each other, producing a pair of gamma-ray photons that move in opposite directions.
Other antiparticles The positively charged proton that is part of all nuclei has a negatively charged antiproton of the same mass but opposite electric charge. Even though the neutron has zero electric charge, it also has an antimatter counterpart; other properties besides charge differentiate the neutron from the antineutron. Occasionally, a particle is its own antiparticle; the photon is an example.
Fundamental interactions Fundamental interactions are the most basic interactions known, such as the electromagnetic interaction between charged particles. Nonfundamental interactions, such as friction, can be understood in terms of fundamental interactions. Friction is a macroscopic manifestation of the electromagnetic interaction between the electrons of two surfaces that are in contact.
Fundamental interactions: Gravitational interaction All objects in the universe participate in gravitational interactions due to their mass. This interaction is important for very massive (mega-) objects. It is much less important for objects in our daily lives and extremely insignificant for microscopic objects.
Fundamental interactions: Electromagnetic interaction Electrically charged objects participate in electromagnetic interactions. The interaction is electric if the objects are at rest or in motion with respect to each other. The interaction is magnetic only if the objects are moving with respect to each other. The electromagnetic interactions between nuclei and electrons are important in understanding the structure of atoms.
Comparing the electromagnetic interaction to the gravitational interaction The electrostatic force that an electron and a proton exert on each other in an atom is about 10 39 times greater than the gravitational force that they exert on each other.
Fundamental interactions: Strong interaction The binding of protons and neutrons together into a nucleus is a residual interaction of the strong interaction. The strong interaction is a very short-range interaction, exerted by protons and neutrons only on their nearest neighbors within the nucleus.
Fundamental interactions: Weak interaction The weak interaction is responsible for beta decay. Protons, neutrons, electrons, and neutrinos all participate in it. The weak interaction is significantly weaker than the strong interaction and has a significantly shorter range.
Mechanisms of fundamental interactions This particle exchange mechanism has been successful in describing the weak and strong interactions. It has also had some success in describing the gravitational interaction. The emitted and absorbed particle is called a mediator. For the electromagnetic interaction, the mediator is the photon.
Interaction mediators Photons: electromagnetic interaction Gluons: strong interaction W and Z bosons: weak interaction Gravitons: gravitational interaction The mediators of the electromagnetic, strong, and weak interactions have all been discovered. The hypothetical mediator for the gravitational interaction, the so-called graviton, has not.
Fundamental interactions
Quantitative Exercise 29.2 Convert the masses of the W ± and Z 0 particles into electron volts.
Elementary particles and the Standard Model Particle accelerators facilitate collisions between particles with total energy significantly greater than the rest energies, allowing for additional particles to be produced. The properties of these additional particles can be determined using elaborate detectors. Most of the particles produced are not stable.
Leptons Leptons interact only through weak, electromagnetic, and gravitational interactions, but not through strong interactions. The electron and the electron neutrino are examples of leptons. These two particles form a generation (or family) of leptons.
Lepton generations (families)
Hadrons Two different types of hadrons can be distinguished: baryons and mesons. The proton and the neutron are baryons. In 1935, Hideki Yukawa suggested the existence of new particles that mediated the strong interaction the first example of a meson. In 1947, physicists discovered a meson in cosmic rays that participated in strong interactions and had the correct properties to be Yukawa's meson.
Properties of hadrons
Particle
Quarks Hadrons are made up of smaller, more fundamental particles that have fractional electric charge, known as quarks. Six different quarks have been discovered experimentally. These different quark types are known in the physics community as flavors.
Quarks
The proton and quark charge The total electric charge adds to e, and the total color is neutral.
Conceptual Exercise 29.3 Which combination of quarks will combine to have the correct properties to be a neutron?
Conceptual Exercise 29.3
Particles (matter) and their interactions
Confinement No experiment has ever produced a quark in isolation. Every quark and antiquark ever produced have always been part of a hadron. This phenomenon, called confinement, is an indication of a feature of the strong interaction. The strong interaction between quarks is weakest when they are close together and gets stronger the farther apart the quarks are.
Development of the Standard Model The Standard Model is the combined theory of the building blocks of matter and their interactions. In the late 1940s, physicists Feynman, Schwinger, and Tomonaga independently combined the ideas of special relativity and quantum mechanics into a single model that explains all electromagnetic phenomena. Their model, known as quantum electrodynamics (QED), is a cornerstone of the Standard Model.
Standard Model
Higgs particle In 1967, Glashow, Salam, and Weinberg independently put forth a model that unified the electromagnetic and weak interactions into a single interaction, which they called the electroweak model. This model predicted the existence of a particle, which became known as the Higgs particle after physicist Peter Higgs.
Predictions of the electroweak model In the very distant past when the universe was much smaller and very much hotter, all particles were massless. This situation led to the existence of the Higgs particle. As the universe cooled, the Higgs particle began interacting significantly with other elementary particles, reconfiguring them into the familiar forms they have today. This is known as the Higgs mechanism.
Quantum chromodynamics In 1973, using Yang's and Mills' mathematical framework, Fritzsch and Gell-Mann formulated quantum chromodynamics (QCD), a mathematical model of the strong interaction that plays a role in the exchange of gluons between quarks. Between 1976 and 1979, scientists discovered the tau lepton and bottom quark and found direct evidence for gluons.
Additional particle discovery timeline The 1980s brought the discoveries of the predicted weak interaction mediators W and Z. The 1990s gave us the top quark. In 2000, the tau neutrino was discovered. In July 2012, CERN announced the discovery of a particle that may be the long-sought-after Higgs particle.
Unanswered questions of the Standard Model 1. Can the strong interaction be unified with the electroweak interaction? 2. Why are there only three families of quarks/leptons? 3. Are the Standard Model particles truly fundamental? 4. Are there additional particles beyond those predicted by the Standard Model?
Summary of the Standard Model Quarks and leptons, which make up the matter of the universe The theory of strong interactions (QCD) mediated by gluons The theory of electromagnetic (QED) and weak interactions mediated by photons and the W and Z particles The Higgs particle, which explains, through the Higgs mechanism, why some of the fundamental particles have nonzero mass
Cosmology Why is our universe not filled with equal numbers of particles and antiparticles? Why is there an imbalance? These questions are answered in part by particle physics and by cosmology a branch of physics that studies the composition and evolution of the universe as a whole.
Big Bang
Standard Model
Inflation When the universe first became "cold" enough that quarks and leptons emerged as distinguishable particles, a fundamental change in the structure of the universe occurred, resulting in an extremely rapid exponential expansion known as cosmic inflation. During inflation, small fluctuations in the density of the universe decreased. Areas where the density was slightly above average would later act as the seeds of galaxy formation.
Nucleosynthesis A few minutes after the Big Bang, the temperature had dropped to about 1 billion K, and the average density of the universe was close to the density of air at sea level on Earth today. For the first time, protons and neutrons were able to combine to form the simplest nuclei: deuterium, helium, and trace amounts of lithium. This process is known as Big Bang nucleosynthesis.
Atoms, stars, and galaxies When the universe had cooled enough, gravitational interactions became the dominant driver of its further evolution. Density fluctuations led to the formation of the first galaxies and stars just 500,000 years after the Big Bang. These early stars went through their life cycles, with some ending in a violent collapse and explosion known as a supernova, which created heavier elements such as carbon, oxygen, iron, and gold.
Dark matter and dark energy When astronomers measure the mass of all the stars and gas that they can see, they find that the total mass is only about one-tenth of the mass needed to account for the speed of the solar system around the center of the galaxy. The universe is "missing" about 90% of the mass needed to account for the observed motion of stars and galaxies. How can this contradiction be resolved?
Dark matter In 1933, astrophysicist Fritz Zwicky speculated that there must be some unseen dark matter present in the Coma cluster; for about 40 years, his observation was the only evidence for its existence. In the 1970s, Vera Rubin presented further evidence. It was at this point that the dark matter explanation started to become more widely accepted.
Dark matter Dark matter does not emit photons or otherwise participate in the electromagnetic interaction (this is why it is called "dark"). Dark matter cannot be some sort of dark cloud of protons or gaseous atoms, because these could be detected by the scattering of radiation passing through them.
MACHOs: Massive compact halo objects These objects could be black holes, neutron stars, or brown dwarfs. Astronomers have detected MACHOs through their gravitational effects on the light from distant objects. The small number of detected events translates into MACHOs accounting for at most 20% of the dark matter in our galaxy. There must be another (or an additional) explanation.
WIMPs: Weakly interacting massive particles WIMPs are "weakly interacting": they can pass through ordinary matter with almost no interaction, and they neither absorb nor emit light. WIMPs are "massive": their mass is not zero. Prime candidates for WIMPs include neutrinos, axions, and neutralinos. Axions and neutralinos are not Standard Model particles and require the Standard Model to be extended to accommodate their existence.
Grand unified theories Grand unified theories combine the strong, weak, and electromagnetic interactions into a single interaction. These theories predict the existence of "sterile neutrinos," which could have even fewer interactions and be far more massive than Standard Model neutrinos. Physicists do not know how to detect such a particle. If it exists in sufficient abundance, it could account for dark matter.
Supersymmetry Supersymmetry is an extension of the Standard Model: It effectively doubles the number of elementary particles. It gives insight into the cosmological constant problem. It allows for a more precise understanding of the unification of interactions in grand unified theories. It gives a potential candidate for dark matter.
Explaining the accelerating expansion of the universe Invoke a discarded feature of Einstein's general theory of relativity (our current best model of the gravitational interaction) known as the cosmological constant. Suggest the existence of a strange kind of energy-fluid that fills space and has a repulsive gravitational effect. Propose a modified version of general relativity that includes a new kind of field that creates this cosmic acceleration.
The cosmological constant model The dominant model of the universe was the steady-state model, which asserted that the universe essentially did not change in any major way as time passed. General relativity predicted that a static universe was unstable. Einstein introduced the cosmological constant into general relativity in an attempt to allow the theory to accommodate a steady-state universe.
The dark energy model The cosmological constant seems to represent a type of dark energy that is present at every point in space with equal density. Even as the universe expands, the density does not decrease because it is a property of space itself. Dark energy has a negative pressure. In general relativity, this produces a gravitationally repulsive effect on space.
Modified general relativity Some better theory of the gravitational interaction would make even better predictions than general relativity. The challenge has been to construct the new theory in such a way that it does not make predictions that contradict experiments that have already been done. Thus far physicists have been unsuccessful in achieving this goal.
The proportion of matter, dark matter, and dark energy in the universe
Cosmological constant problem Dark energy is the sum of the zero-point energies of all quantum fields in the universe. When physicists predict values for the cosmological constant, they get a result that is 10 120 times the observed value. This is the largest disagreement between prediction and experiment in all of science. This so-called cosmological constant problem is a major unsolved problem in physics.
Tip
Is our pursuit of knowledge worthwhile? Our models describe the behavior of only 4% of the content of our universe. The nature of the remaining 96% of our universe currently remains an unsolved problem. Will our eventual knowledge of the other 96% of the universe someday make people's lives better? It is impossible to say for sure, but history suggests that it very likely will.
Summary
Summary