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Chapter Questions
Explain Dirac's reasoning for the existence of antielectrons.
How do scientists know that antiparticles exist in nature?
Explain how positron emission tomography (PET) works.
A proton and an antiproton, both with negligible kinetic energy, annihilate each other to produce two photons.
An isolated slow-moving electron and positron annihilate each other. Why are two photons produced instead of just one?
Use Newtonian circular motion concepts to show that the radius $r$ of the circle in which a charged particle spirals while moving perpendicular to a magnetic field is proportional to the particle's speed $y$ .
A cosmic ray photon enters a cloud chamber from above. While inside the chamber the photon converts into an electron-positron pair. Draw a sketch of this situation. Label the photon, electron, and positron tracks and indicate the direction of the magnetic field within the cloud chamber.
A particle enters a cloud chamber from above traveling at 0.50$c$ and leaves a curving track. The magnetic field within the cloud chamber has magnitude 0.120 $\mathrm{T}$ and points outof the page. The track curves to the right and has a radius of 0.020 $\mathrm{m}$ . Determine everything you can about this particle.
Compare and contrast the fundamental interactions.
Even though the electromagnetic interaction is so much stronger than the gravitational interaction, the gravitational interaction clearly is much more relevant in everyday life. Do you agree with this statement? Explain either way.
Explain the mechanism behind elementary particle interactions.
Explain how the energy to produce interaction mediators is provided.
What is the difference between a real particle and a virtual particle?
Make an analogy between the interactions of elementary particles and some real-life interactions. Indicate which object in one system corresponds to each object in the other. Describe the limitations of your analogy.
Why are neutrinos difficult to detect?
Describe the evidence for neutrinos having nonzero mass.
What criteria do scientists use to classify clementary particles?
Compare and contrast leptons and hadrons.
How do we know that baryons are made of quarks? What is the experimental evidence that supports this idea?
Compare and contrast mesons and baryons.
How do quarks interact with each other inside a baryon?
In what way is the interaction of electrically neutral at-oms similar to the interactions of color-neutral protons and neutrons?
Describe the phenomenon of confinement.
What were four important steps in the building of the Standard Model?
What is the significance of the Higgs particle and the Higgs mechanism?
What major piece of physics is not part of the Standard Model?
Describe several open questions of the Standard Model.
What is inflation, and what eventually happened as a resultof the density fluctuations in the universe?
What is the origin of the elementary particles?
How would the universe be different if the number of particles in it had equaled the number of antiparticles?
What process produced the cosmic microwave background?
Our bodies contain significant amounts of carbon, oxygen, nitrogen, and other "heavy" elements. If only "light" elements such as hydrogen and helium were produced during Big Bang nucleosynthesis, how were the heavier elements produced?
What is the evidence that a large proportion of the mass of the universe is in the form of dark matter? Explain carefully.
Describe as many hypotheses as you can about the nature of dark matter.
What is the experimental evidence for dark energy? Explain carefully.
Describe as many hypotheses as you can that explain the accelerated expansion of the universe.
What is supersymmetry, and why is it a uscful idea in physics?
What is the cosmological constant problem? Describe a possible resolution for it.
What are the potential benefits of continued research in physics?
An electron and a positron are traveling directly toward each other at a speed of 0.90$c(90 \% \text { of light speed) with re- }$ spect to the lab in which the experiment is being performed. The electron and positron collide, annihilate, and produce a pair of photons. Determine the wavelength of each photon. Ifyou think they will have the same wavelength, explain why.
In order to discover the $W^{+}$ and $W$ particles, a large particle accelerator at CERN collided protons and antiprotons together at extremely high speeds. What is the minimum speed the proton and antiproton would need to produce a $W^{ \pm}$ pair?
Why don't you notice the solar neutrinos passing through your body? (a) The Sun is very far away so the neutrinos have very little energy by the time they get to Earth.(b) Nearly all the neutrinos oscillate into other types that can pass easily through your body.. (c) Solar neutrinos only participate in nuclear fusion reactions and since nuclear fusion doesn't happen in your body you don't notice the neutrinos.(d) Neutrinos only interact very weakly with atoms.(e) Only a very small number of them pass through your body cach second.
In 1987 a supernova was detected in the Large Magellanic Cloud. Neutrinos coming from the supernova were detected, but there were fewer than expected. What might be the reason for that? (a) The supernova actually occurred from Earth than astronomers thought.(b) Some of the neutrinos were converted into other types of neutrinos as they traveled to Earth.(c) The model used to predict the number of neutrinos produced in the supernova makes some unreasonable assumptions.(d) The detector used was not as efficient as the designers thought.(e) All of these are possible explanations.
Originally a different way to resolve the solar neutrino problem was suggested-the nuclear reaction rate wasn't as high as astrophysicists thought. What would have to be true for this suggestion to be reasonable? (a) The mass of the Sun would have to be significantly different than thought.(b) The brightness of the Sun would have to be significantlydifferent than thought.(c) The size of the Sun would have to be significantly different than thought.(d) The orbits of the planets would have to be significantly different than thought.(e) Both (b) and (c) are correct.
