2. Welcome to Physics 7C! Lecture 7 -- Winter Quarter -- 2005 Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu

3. Announcements Course policy and regrade forms on the web: http://physics7.ucdavis.edu If you received rubric code 4 on part b) of Quiz 2, please hand in your quiz for a possible regrade. Quiz today on Block 13, DLMs 9 through 12. Block 14: The Fundamental Forces of Nature. Lecture 10 will be a review for the final. Turn off cell phones and pagers during lecture.

4. Understanding our Universe: A brief introduction…

5. What is the World Made Of? In ancient times, people sought to organize the world around them into fundamental elements Aristotle: – Earth – Air – Fire – Water

6. What Else Did They Think? “By Convention there is color, by convention sweetness, by convention bitterness, but in reality there are atoms and space.” -Democritus (400 BC) Atom = Mushy Ball (c. 1900)

7. Where We Were ~100 Years Ago…

8. Rutherford’s Scattering Expt Apparatus Hypothesis Results (data) Analysis Conclusion: A Nucleus!

9. New Types of Matter! Fermilab: Bubble Chamber Photo Then…More Mysteries!

10. The Quark Idea The Stanford Linear Accelerator Center EndStation A: Beam of Electrons onto Target

11. Quarks Are Found! ‘Three Quarks for Muster Mark!’ 1990 Nobel Prize in Physics: Quarks Revealed! Structure Inside Protons and Neutrons Quarks found: 1968!

12. The Nature of Matter What, then, is fundamental?

13. Quark Discoveries… Top! Quarks (u,d,s) were postulated in 1964 by Gell-Mann and Zweig, discovered in 1968 udud The charm quark c was discovered in 1974 by Brookhaven and SLAC cscs The bottom b quark was discovered In 1977 at Fermilab …b…b The bottom quark needed a partner… => top! The top quark was finally found in 1995 at Fermilab!

14. Physics of the Top Quark Top physics is one of the more sexy things to study at the Fermilab Tevatron…

15. Quarks and Scale It is also possible that quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles. As far as we know now, however, they are fundamental!

16. What Holds it All Together? Electromagnetic: Xrays, radio, infrared, magnets Weak Force: Nuclear radioactive decay Strong Force: Binds quarks and nuclei You will learn about these two forces in block 14…

17. The Fundamentals Forces 4 Fundamental Forces Electromagnetic: Photon  Weak Force: W/Z Bosons Strong Force: Gluons * * * Gravitational Force: Graviton(?)

18. The New Periodic Table …of Particles and Forces

19. Standard Model of Particle Physics The SM states that: The world is made up of quarks and leptons that interact by exchanging bosons (force carriers). A Higgs field interacts as well, giving particles their masses. Lepton Masses: M e <M  <M  M ~0.* Quark Masses: M u ~ M d < M s < M c < M b << M t Matter as we know it

20. Nature is Fundamentally Simple The Standard Model -- Lots of Mysteries Still! O ur current theory of how elementary particles interact does not make predictions Example: We knew that the top quark existed but the S.M. could not tell us what its mass was. What makes the pattern of particles masses? Why are there only 3 families of quarks? Do protons decay? Why is there so little antimatter in the universe? What is Dark Matter? Dark energy? Do we live in a universe that has “extra dimensions”? What about gravity? Is there a Grand Unified Theory that merges the 4 forces? (A Theory of Everything?)

21. Many Puzzles Remain…

22. Summary of the Universe The world is made up of quarks and leptons These particles interact through 4 different forces The Standard Model works - but remains limited We are still missing several pieces of the puzzle Work is underway to extend understanding-- watch the next decade for discoveries… Pretty cool stuff! On to Block 14…

23. The Funny Thing About the Nucleus As you know from chemistry, the nucleus of the atom is made up of protons and neutrons. What did we just learn about the electric force between two positive charges? So, if the nucleus is loaded with positively charged protons, why doesn’t it blow apart? The force that counteracts the electromagnetic repulsion in the nucleus is called the force, and is mediated by the gluon (it’s carrier particle is the gluon, like the photon  is for electromagnetism).

24. The Strong Force Never heard of the strong force??? The strong force is responsible for binding nucleons (protons and neutrons) together inside of the nucleus, and for binding quarks together inside of the nucleon. We will focus on the former-- protons and neutrons. p p Radius proton ~ 10 -15 We’ll use our field model of forces, and extend our energy conservation model to explain some atomic behaviors. other nucleon (n or p) exerts force nucleon strong field (n or p) creates Field Model of Forces

25. Making Helium: Nuclear Fusion Notation for atomic elements: To make helium, we need 2 protons, 2 neutrons, 2 electrons: When the nucleons combine, strong bonds are formed, which decreases the potential energy in the system. However, the electric potential energy increases since the protons are now closer. How do we know if the total energy increases or decreases? Because energy is released (our sun!) and helium is stable, we know that the decrease in PE strong wins over the increase in PE electric We can quantify this! Example: carbon-14 has 14 nucleons, 6 protons, 8 neutrons

26.  Potential Energy of Nucleus How do we know if the total energy increases or decreases? Define: E nuclear = PE electric + PE strong The total change  E nuclear = observed change in the nucleus mass:  mc 2 (  E=  mc 2 … Does this look familiar?) Initial State: Final State: 1.672673 x 10 -27 kg = M p 4.00260 amu 1.674929 x 10 -27 kg = M n (1 amu = 1.660540 x 10 -27 kg) 0.000911 x 10 -27 kg = M e Mass( 2p+2n+2e ) = 6.697026 x 10 -27 kg => 6.46477 x 10 -27 kg Therefore, the mass decreased by 0.050549 x 10 -27 kg in putting the subatomic particles together to make helium. Hence E nuclear decreased. Then the decrease in PE strong is larger than the increase in PE electric.

27. Energy Interaction Diagrams To calculate PE electric : When a positively charged proton is brought closer to another positively charged proton, r final is smaller than r initial, and  PE electric is positive. Work must be done to bring them together. For PE strong : Quantify PE strong using knowledge of PE electric and  mc 2 Evaluate this nuclear reaction:

28. Next Time: Nuclear Fission, or… What makes things go boom!