Chapter 46 Particle Physics and Cosmology
Atoms as Elementary Particles Atoms From the Greek for indivisible Were once thought to be the elementary particles Atom constituents Proton, neutron, and electron After 1932 these were viewed as elementary All matter was made up of these particles
Discovery of New Particles New particles Beginning in the 1940s, many new particles were discovered in experiments involving highenergy collisions Characteristically unstable with short lifetimes Over 300 have been catalogued A pattern was needed to understand all these new particles
Elementary Particles Quarks Physicists recognize that most particles are made up of quarks Exceptions include photons, electrons and a few others The quark model has reduced the array of particles to a manageable few The quark model has successfully predicted new quark combinations
Original Quark Model Three types or flavors u up d down s strange Quarks have fractional electrical charges -1e/3 and +2e/3 Associated with each quark is an antiquark The antiquark has opposite properties (charge, baryon number and strangeness)
Fundamental Forces All particles in nature are subject to four fundamental forces
Strong Force Attractive force between nucleons Strongest of all the fundamental forces Very short-ranged Less than 10-15 m (1 fm)
Electromagnetic Force About 10-2 times the strength of the nuclear force A long-range force that decreases in strength as the inverse square of the separation between interacting particles
Weak Force Is responsible for instability in certain nuclei Decay processes Its strength is about 10-5 times that of the strong force
Gravitational Force Its effect on elementary particles is negligible A long-range force It is about 10-39 times the strength of the strong force Weakest of the four fundamental forces
Explanation of Forces Forces between particles are often described in terms of the actions of field particles or exchange particles Field particles are also called gauge bosons The interacting particles continually emit and absorb field particles The emission of a field particle by one particle and its absorption by another manifests itself as a force between the two interacting particles The force is mediated, or carried, by the field particles
Forces and Mediating Particles
Paul Adrien Maurice Dirac 1902 1984 British physicist Understanding of antimatter Unification of quantum mechanics and relativity Contributions of quantum physics and cosmology Nobel Prize in 1933
Dirac s Description of the Electron Dirac developed a relativistic quantum mechanical description of the electron It successfully explained the origin of the electron s spin and its magnetic moment The solutions to the wave equation required negative energy states Dirac postulated that all negative energy states were filled These electrons are collectively called the Dirac sea Electrons in the Dirac sea are not directly observable because the exclusion principle does not let them react to external forces
Dirac s Explanation An interaction may cause the electron to be excited to a positive energy state This would leave behind a hole in the Dirac sea The hole can react to external forces and is observable
Dirac s Explanation, cont. The hole reacts in a way similar to the electron, except that it has a positive charge The hole is the antiparticle of the electron The electron s antiparticle is now called a positron
Antiparticles For every particle, there is an antiparticle From Dirac s version of quantum mechanics that incorporated special relativity This has been verified for all particles known today Some particles are their own antiparticles Photon and p o An antiparticle of a charged particle has the same mass as the particle, but the opposite charge The positron (electron s antiparticle) was discovered by Anderson in 1932 Since then, it has been observed in numerous experiments
Pair Production A common source of positrons is pair production A gamma-ray photon with sufficient energy interacts with a nucleus and an electronpositron pair is created from the photon The photon must have a minimum energy equal to 2m e c 2 to create the pair
Pair Production, cont. A photograph of pair production produced by 300 MeV gamma rays striking a lead sheet The minimum energy to create the pair is 1.02 MeV The excess energy appears as kinetic energy of the two particles
Quick Quiz Given the identification of the particles in the figure on the right, what is the direction of the external magnetic field in the bubble chamber shown in the figure below? (a) into the page (b) out of the page (c) impossible to determine
Annihilation The reverse of pair production can also occur Under the proper conditions, an electron and a positron can annihilate each other to produce two gamma ray photons e - + e +
Hideki Yukawa 1907 1981 Japanese physicist Nobel Prize in 1949 for predicting the existence of mesons Developed the first theory to explain the nature of the nuclear force
Mesons Developed from a theory to explain the nuclear force Yukawa used the idea of forces being mediated by particles to explain the nuclear force Exchange of mesons between nucleons causes the nuclear force
Mesons, cont. The proposed particle would have a mass about 200 times that of the electron Efforts to establish the existence of the particle were done by studying cosmic rays in the 1930s Actually discovered multiple particles pi meson (pion) muon Found first, but determined to not be a meson
Richard Feynman 1918 1988 American physicist Developed quantum electrodynamics Shared the Nobel Prize in 1965 Worked on Challenger investigation and demonstrated the effects of cold temperatures on the rubber O-rings used
Feynman Diagram Two Electrons The photon is the field particle that mediates the electromagnetic force between the electrons The photon transfers energy and momentum from one electron to the other The photon is called a virtual photon It can never be detected directly because it is absorbed by the second electron very shortly after being emitted by the first electron
Feynman Diagram Proton and Neutron (Yukawa s Model) The exchange is via the nuclear force Analysis predicts the rest energy of the pion to be ~130 MeV / c 2
Nucleon Interaction More About Yukawa s Model The time interval required for the pion to transfer from one nucleon to the other is The distance the pion could travel is c t Using these pieces of information, the rest energy of the pion is about 100 MeV
Nucleon Interaction, final This concept says that a system of two nucleons can change into two nucleons plus a pion as long as it returns to its original state in a very short time interval It is often said that the nucleon undergoes fluctuations as it emits and absorbs field particles These fluctuations are a consequence of quantum mechanics and special relativity
Feynman Diagram Weak Interaction An electron and a neutrino are interacting via the weak force The Z 0 is the mediating particle The weak force can also be mediated by the W The W and Z 0 were discovered in 1983 at CERN
Nuclear Force and Strong Force Historically, the nuclear force was called the strong force Now the strong force is reserved for the force between quarks Or between particles made from quarks The nuclear force is the force between nucleons It is a secondary result of the strong force Sometimes called residual strong force
Classification of Particles Two broad categories Classified by interactions Hadrons interact through strong force Leptons interact through weak force
Hadrons Interact through the strong force Two subclasses distinguished by masses and spins Mesons Integer spins (0 or 1) Decay finally into electrons, positrons, neutrinos and photons Baryons Masses equal to or greater than a proton Half integer spin values (1/2 or 3/2) Decay into end products that include a proton (except for the proton) Not elementary, but composed of quarks
Quark Composition of Particles Examples Mesons are quarkantiquark pairs Baryons are quark triplets
Leptons Do not interact through strong force All have spin of 1/2 Leptons appear truly elementary No substructure Point-like particles
Conservation Laws A number of conservation laws are important in the study of elementary particles Already have seen conservation of Energy Linear momentum Angular momentum Electric charge Two additional laws are Conservation of Baryon Number Conservation of Lepton Number
Conservation of Baryon Number Whenever a baryon is created in a reaction or a decay, an antibaryon is also created B is the baryon number B = +1 for baryons B = -1 for antibaryons B = 0 for all other particles The sum of the baryon numbers before a reaction or a decay must equal the sum of baryon numbers after the process
Conservation of Baryon Number and Proton Stability There is a debate over whether the proton decays or not If baryon number is absolutely conserved, the proton cannot decay Some recent theories predict the proton is unstable and so baryon number would not be absolutely conserved For now, we can say that the proton has a half-life of at least 10 33 years
Conservation of Baryon Number, Example Is baryon number conserved in the following reaction? Baryon numbers: Before: 1 + 1 = 2 After: 1 + 1 + 1 + (-1) = 2 Baryon number is conserved The reaction can occur as long as energy is conserved
Conservation of Lepton Number There are three conservation laws, one for each variety of lepton The law of conservation of electron lepton number states that the sum of electron lepton numbers before the process must equal the sum of the electron lepton number after the process The process can be a reaction or a decay
Conservation of Lepton Number, cont. Assigning electron lepton numbers L e = 1 for the electron and the electron neutrino L e = -1 for the positron and the electron antineutrino L e = 0 for all other particles Similarly, when a process involves muons, muon lepton number must be conserved and when a process involves tau particles, tau lepton numbers must be conserved Muon and tau lepton numbers are assigned similarly to electron lepton numbers
Conservation of Lepton Number, Example Is lepton number conserved in the following reaction? Check electron lepton numbers: Before: L e = 0 After: L e = 1 + (-1) + 0 = 0 Electron lepton number is conserved Check muon lepton numbers: Before: L µ = 1 After: L µ = 0 + 0 + 1 = 1 Muon lepton number is conserved
Strange Particles Some particles discovered in the 1950s were found to exhibit unusual properties in their production and decay and were given the name strange particles Peculiar features include: Always produced in pairs Although produced by the strong interaction, they do not decay into particles that interact via the strong interaction, but instead into particles that interact via weak interactions They decay much more slowly than particles decaying via strong interactions
Strangeness To explain these unusual properties, a new quantum number S, called strangeness, was introduced A new law, the law of conservation of strangeness was also needed It states that the sum of strangeness numbers before a reaction or a decay must equal the sum of the strangeness numbers after the process Strong and electromagnetic interactions obey the law of conservation of strangeness, but the weak interaction does not
Bubble Chamber Example of Strange Particles The dashed lines represent neutral particles At the bottom, π - + p K 0 + Λ 0 Then Λ 0 π - + p and K o π - +µ + νμ
Murray Gell-Mann 1929 American physicist Studies dealing with subatomic particles Named quarks after Three Quarks for Muster Mark from Finnigan s Wake by James Joyce Developed pattern known as eightfold way Nobel Prize in 1969
The Eightfold Way Many classification schemes have been proposed to group particles into families These schemes are based on spin, baryon number, strangeness, etc. The eightfold way is a symmetric pattern proposed by Gell-Mann and Ne eman There are many symmetrical patterns that can be developed The patterns of the eightfold way have much in common with the periodic table Including predicting missing particles
An Eightfold Way for Baryons A hexagonal pattern for the eight spin 1/2 baryons Strangeness vs. charge is plotted on a sloping coordinate system Six of the baryons form a hexagon with the other two particles at its center
An Eightfold Way for Mesons The mesons with spins of 0 can be plotted Strangeness vs. charge on a sloping coordinate system is plotted A hexagonal pattern emerges The particles and their antiparticles are on opposite sides on the perimeter of the hexagon The remaining three mesons are at the center
Eightfold Way for Spin 3/2 Baryons The nine particles known at the time were arranged as shown An empty spot occurred Gell-Mann predicted the missing particle and its properties About three years later, the particle was found and all its predicted properties were confirmed
Quarks Hadrons are complex particles with size and structure Hadrons decay into other hadrons There are many different hadrons Quarks are proposed as the elementary particles that constitute the hadrons Originally proposed independently by Gell-Mann and Zweig
Original Quark Model Rules All the hadrons at the time of the original proposal were explained by three rules Mesons consist of one quark and one antiquark This gives them a baryon number of 0 Baryons consist of three quarks Antibaryons consist of three antiquarks
Additions to the Original Quark Model Charm Another quark was needed to account for some discrepancies between predictions of the model and experimental results A new quantum number, C, was assigned to the property of charm Charm would be conserved in strong and electromagnetic interactions, but not in weak interactions In 1974, a new meson, the J/, was discovered that was shown to be a charm quark and charm antiquark pair
More Additions Top and Bottom Discovery led to the need for a more elaborate quark model This need led to the proposal of two new quarks t top (or truth) b bottom (or beauty) Added quantum numbers of topness and bottomness Verification b quark was found in a Y meson in 1977 t quark was found in 1995 at Fermilab
Numbers of Particles At the present, physicists believe the building blocks of matter are complete Six quarks with their antiparticles Six leptons with their antiparticles
More About Quarks No isolated quark has ever been observed It is believed that at ordinary temperatures, quarks are permanently confined inside ordinary particles due to the strong force Current efforts are underway to form a quark-gluon plasma where quarks would be freed from neutrons and protons
Color It was noted that certain particles had quark compositions that violated the exclusion principle Quarks are fermions, with half-integer spins and so should obey the exclusion principle The explanation is an additional property called color charge The color has nothing to do with the visual sensation from light, it is simply a name
Colored Quarks Color charge occurs in red, blue, or green Antiquarks have colors of antired, antiblue, or antigreen These are the quantum numbers of color charge Color obeys the exclusion principle A combination of quarks of each color produces white (or colorless) Baryons and mesons are always colorless
Quantum Chromodynamics (QCD) QCD gave a new theory of how quarks interact with each other by means of color charge The strong force between quarks is often called the color force The strong force between quarks is mediated by gluons Gluons are massless particles When a quark emits or absorbs a gluon, its color may change
More About Color Charge Particles with like colors repel and those with opposite colors attract Different colors attract, but not as strongly as a color and its anticolor The color force between color-neutral hadrons is negligible at large separations The strong color force between the constituent quarks does not exactly cancel at small separations This residual strong force is the nuclear force that binds the protons and neutrons to form nuclei
Quark Structure of a Meson A green quark is attracted to an antigreen quark The quark antiquark pair forms a meson The resulting meson is colorless
Quark Structure of a Baryon Quarks of different colors attract each other The quark triplet forms a baryon Each baryon contains three quarks with three different colors The baryon is colorless
QCD Explanation of a Neutron- Proton Interaction Each quark within the proton and neutron is continually emitting and absorbing gluons The energy of the gluon can result in the creation of quark-antiquark pairs When close enough, these gluons and quarks can be exchanged, producing the strong force
Electroweak Theory The electroweak theory unifies electromagnetic and weak interactions The theory postulates that the weak and electromagnetic interactions have the same strength when the particles involved have very high energies Viewed as two different manifestations of a single unifying electroweak interaction
The Standard Model A combination of the electroweak theory and QCD for the strong interaction form the Standard Model Essential ingredients of the Standard Model The strong force, mediated by gluons, holds the quarks together to form composite particles Leptons participate only in electromagnetic and weak interactions The electromagnetic force is mediated by photons The weak force is mediated by W and Z bosons The Standard Model does not actually yet include the gravitational force
The Standard Model Chart
Mediator Masses Why does the photon have no mass while the W and Z bosons do have mass? Not answered by the Standard Model The difference in behavior between low and high energies is called symmetry breaking The Higgs boson has been proposed to account for the masses Large colliders are necessary to achieve the energy needed to find the Higgs boson In a collider, particles with equal masses and equal kinetic energies, traveling in opposite directions, collide head-on to produce the required reaction
Particle Paths After a Collision
The Big Bang This theory states that the universe had a beginning, and that it was so cataclysmic that it is impossible to look back beyond it Also, during the first few minutes after the creation of the universe, all four interactions were unified All matter was contained in a quark-gluon plasma As time increased and temperature decreased, the forces broke apart
A Brief History of the Universe
Cosmic Background Radiation (CBR) CBR represents the cosmic glow left over from the Big Bang The radiation had equal strengths in all directions The curve fits a black body at 2.7K There are small irregularities that allowed for the formation of galaxies and other objects
CBR, cont. The COBE satellite found that the background radiation had irregularities that corresponded to temperature variations of 0.000 3 K
Hubble s Law The Big Bang theory predicts that the universe is expanding Hubble claimed the whole universe is expanding Furthermore, the speeds at which galaxies are receding from the earth is directly proportional to their distance from us This is called Hubble s law
Hubble s Law, cont. Hubble s law can be written as v = HR H is called the Hubble constant H 17 x 10-3 m/s ly
Remaining Questions About the Universe Will the universe expand forever? Today, astronomers and physicists are trying to determine the rate of expansion It depends on the average mass density of the universe compared to a critical density Missing mass in the universe The amount of non-luminous (dark) matter seems to be much greater than what we can see Various particles have been proposed to make up this dark matter
Another Remaining Question About the Universe Is there mysterious energy in the universe? Observations have led to the idea that the expansion of the universe is accelerating To explain this acceleration, dark energy has been proposed The dark energy results in an effective repulsive force that causes the expansion rate to increase
Some Questions in Particle Physics Why so little antimatter in the Universe? Is it possible to unify electroweak and strong forces? Why do quarks and leptons form similar but distinct families? Are muons the same as electrons apart from their difference in mass? Why are some particles charged and others not? Why do quarks carry fractional charge? What determines the masses of fundamental particles? Can isolated quarks exist?
A New Perspective String Theory String theory is one current effort at answering some of the previous questions It is an effort to unify the four fundamental forces by modeling all particles as various vibrational modes of an incredibly small string
String Theory, cont. The typical length of a string is 10-35 m This is called the Planck length According to the string theory, each quantized mode of vibration of the string corresponds to a different elementary particle in the Standard Model
Complications of the String Theory It requires space-time to have ten dimensions Four of the ten dimensions are visible to us, the other six are compactified (curled) Another complication is that it is difficult for theorists to guide experimentalists as to what to look for in an experiment Direct experimentation on strings is impossible
String Theory Prediction SUSY One prediction of string theory is supersymmetry (SUSY) It suggests that every elementary particle has a superpartner that has not yet been observed Supersymmetry is a broken symmetry and the masses of the superpartners are above our current capabilities to detect