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Lecture 24: Diagrams, Decays, and Lifetimes
April 29, 2025
Reading Assignment
- Read: Supplementary Reading Ch 11
Objectives
- (Continuing objective) Describe quantum and elementary particle theories in your own words, and discuss applications of the material.
- Construct correct interaction diagrams for given strong and weak interactions among particles, showing the underlying quark structure.
- Given a particle's mass and quark constituents, list possible decay schemes and estimate decay times, using particle tables and the conservation laws.
- Describe in words and diagrams a possible proton decay and explain why such an occurrence must be very rare.
Homework
- Wednesday's Assigned Problems:
X14 (below); Supp CH 11: 1, 3, 4, 5, 7, 8, 9, 11
Problem X14 Draw a complete reaction diagram for the following interaction: \[e^+ + \Lambda \to \Sigma^+ +\overline\nu_e\] In the diagram, be sure to label all messengers, and label all particles with color/anti-color, where relevant.
- Monday's Hand-In Problems from Lecture 24:
Supp CH 11: 2, 6, 10, 12
Note: this is only the first half of the hand-in set.
Lecture Materials
- Click here for the Lecture overheads. Answers: CT1 - 1, 6; CT2 - 6; CT3 - 2; CT4 - 3; CT5 - 4
- Here are some videos from Fermilab about particle physics. This is NOT required material. If you're interested in hearing a bit more about these
topics, you might find them interesting. One warning: when they show
Feynman diagrams, time is flowing left to right, instead of from bottom to
top.
- Quantum Electrodynamics (the electromagnetic force)
- Quantum Chromodynamics (the strong force)
- The Weak Force (Note: when he talks about the range of possible masses for the particles, he's talking about virtual particles. Real particles have a precise mass.)
Videos of example problems
To see the problem statement, click on the link below. To play the video example, click on the underlined words "Video Demonstration" near the top of the page with the problem statement.- Example with interaction diagram involving a W particle.
- Video example: interaction Diagram involving a gluon
- Video example: particle Lifetimes
Pre-Class Entertainment
- My Babe - Little Walter
- Unsquare Dance - Dave Brubeck
- I'm Gonna Be (500 Miles) - The Proclaimers
- Message of Love - The Pretenders
- I Love Rock'n Roll - Joan Jett
- Ramble On - Led Zeppelin
Assigned Problems Guide
- X14: medium-long. You'll get quicker at making interaction diagrams once you've made a few. Find the quark content of the two baryons and then try to line things up.
- Supp 11-1: medium-long. More interaction diagram practice. (a) is very similar to the example in lecture, (b) is pretty similar to X14.
- Supp 11-3: medium-long. This one is very different - make sure you can do the other problems first. The idea is that we can measure the decay rate of the proton even if it's extremely long lived by watching a large number of protons. So you use the density of water to find the mass. Then use the mass to find the number of moles, and then the number of molecules of water. For each water molecule there are two protons that could decay (from the two H). Pretty different!
- Supp 11-4: medium-quick. You have to conserve baryon number, so you need a lighter baryon plus a meson. There should only be one possibility that conserves charge.
- Supp 11-5: quick. Focus on the messenger particle type, when you are given it.
- Supp 11-7: medium-quick. Baryons have to decay into lighter baryons plus other stuff. This implies something about strangeness conservation for the $\Omega^{-}$.
- Supp 11-8: medium. This is very much like the lecture comparison of the $\Sigma^{*+}$ and the $\Sigma^+$.
- Supp 11-9: medium. Note: meson number is NOT conserved, so mesons can decay into leptons, which goes by the weak interaction. Can a meson decay into photons? If so, when? Think of the Gumby rule.
- Supp 11-11: quick. Redraw the diagram, then add the colors!