Quantum Physics

Problems 100330.12 Quarks &30.13 Colored QuarksRecent theories postulate that all hadrons are composedof smaller units known as quarks which have fractionalelectric charges and baryon numbers of 1/3 and comein six “flavors”: up, down, strange, charmed, top, andbottom. Each baryon contains three quarks, and eachmeson contains one quark and one antiquark.According to the theory of quantumchromodynamics, quarks have a property called color,and the strong force between quarks is referred to as thecolor force. The color force increases as the distancebetween particles increases, so quarks are confinedand are never observed in isolation. When two boundquarks are widely separated, a new quark–antiquarkpair forms between them, and the single particle breaksinto two new particles, each composed of a quark–antiquark pair.30.15 The Cosmic ConnectionObservation of background microwave radiation by Penziasand Wilson strongly confirmed that the Universestarted with a Big Bang about 15 billion years ago andhas been expanding ever since. The background radiationis equivalent to that of a blackbody at a temperatureof about 3 K.The cosmic microwave background has very small irregularities,corresponding to temperature variations of0.000 3 K. Without these irregularities acting as nucleationsites, particles would never have clumped together to formgalaxies and stars.CONCEPTUAL QUESTIONS1. If high-energy electrons with de Broglie wavelengthssmaller than the size of the nucleus are scatteredfrom nuclei, the behavior of the electrons is consistentwith scattering from very massive structuresmuch smaller in size than the nucleus, namely,quarks. How is this similar to a classic experimentthat detected small structures in an atom?2. What factors make a fusion reaction difficult toachieve?3. Doubly charged baryons are known to exist. Whyare there no doubly charged mesons?4. Why would a fusion reactor produce less radioactivewaste than a fission reactor?5. Atoms didn’t exist until hundreds of thousands ofyears after the Big Bang. Why?6. Particles known as resonances have very short halflives,on the order of 10 23 s. Would you guess theyare hadrons or leptons?7. Describe the quark model of hadrons, including theproperties of quarks.8. In the theory of quantum chromodynamics,quarks come in three colors. How would you justifythe statement “All baryons and mesons arecolorless?”9. Describe the properties of baryons and mesons andthe important differences between them.10. Identify the particle decays in Table 30.2 that occurby the electromagnetic interaction. Justify youranswer.11. Kaons all decay into final states that contain no protonsor neutrons. What is the baryon number ofkaons?12. When an electron and a positron meet at low speedsin free space, why are two 0.511-MeV gamma raysproduced, rather than one gamma ray with an energyof 1.02 MeV ?13. Two protons in a nucleus interact via the strong interaction.Are they also subject to a weak interaction?14. Why is a neutron stable inside the nucleus? (In freespace, the neutron decays in 900 s.)15. An antibaryon interacts with a meson. Can a baryonbe produced in such an interaction? Explain.16. Why is water a better shield against neutrons thanlead or steel is?17. How many quarks are there in (a) a baryon, (b) an antibaryon,(c) a meson, and (d) an antimeson? How doyou account for the fact that baryons have half-integralspins and mesons have spins of 0 or 1? [Hint :1quarks have spin .]2