![Chapter 15.9: Quarks
Original Model
● 1963 – Gell-Mann and George Zweig
independently propose that hadrons have two
(for mesons) or three (for baryons) constituents
● For Zweig, these would be called “aces”; the
lasting name comes from Gell-Mann's term of
“quarks”, borrowed from Finnegan's Wake
● Given the names up (u), down (d), and strange
(s) [although once proposed as “sideways”]](https://image.slidesharecdn.com/quarkspresentation-130429234418-phpapp02/85/Quarkspresentation-3-320.jpg)
Quarkspresentation
- 1. Chapter 15.9: Quarks Overview ● Chapter highlights a short history of predictions and experimental findings of quarks and quark properties ● Useful supplementary materials: Table 15.3 (p. 574), 15.4 (p. 575), 15.5 (p. 577), Figure 15.14 (p. 575)
- 2. Chapter 15.9: Quarks Introduction ● Leptons do not break down into smaller constituents, thus they are elementary particles ● Theoretical evidence through Gell-Mann's eightfold-way as well as experimental evidence via decay into other particles, strongly suggest that there is a more elementary build-up to hadrons
- 3. Chapter 15.9: Quarks Original Model ● 1963 – Gell-Mann and George Zweig independently propose that hadrons have two (for mesons) or three (for baryons) constituents ● For Zweig, these would be called “aces”; the lasting name comes from Gell-Mann's term of “quarks”, borrowed from Finnegan's Wake ● Given the names up (u), down (d), and strange (s) [although once proposed as “sideways”]
- 4. Chapter 15.9: Quarks Original Model ● The u, d, and s quarks each have a fractional e charge in order to add up to the total charge of -1, 0, or 1 e (this and other values evident on Table 15.3) ● Each quark has a respective antiquark with opposite charge, baryon number, and strangeness
- 5. Chapter 15.9: Quarks Original Model ● 1) Mesons are combinations of a quark and antiquark (quarkonium) with a baryon number of 0, and bound together by gluons transmitting the strong force ● 2) Baryons have three quarks, form a quark “molecule” through gluons ● 3) Antibaryons have three antiquarks ● See Table 15.4
- 6. Chapter 15.9: Quarks Original Model Murray Gell-Mann (left) and George Zweig (top)
- 7. Chapter 15.9: Quarks Developments ● 1967 – Discrepancies between this model and experiments caused the proposal of charmed quark (c) with the charm property. ● 1974 – J/Ψ particle discovered by Richtor's team at SLAC and Ting's at Brookhaven National Laboratory. Contains properties of cc,̄ was very massive for a meson (~3100 MeV/c²), as well as a relatively long lifetime.
- 8. Chapter 15.9: Quarks Developments ● 1975 – Stanford U gives strong evidence for tau-lepton; physicists propose new properties of topness (t) and bottomness (b) ● 1977 – Lederman at Fermi National Laboratory finds massive Y-meson with composition bb¯ ● 1995 – Fermi Lab announces discovery of top quark (mass 173 GeV/c²) through decay of tt¯ pairs into W bosons and b quarks into further decay
- 9. Chapter 15.9: Quarks Conclusion ● Currently 12 proposed fermions (Table 15.5) ● As of 2005 (last edition of textbook) an isolated quark had not yet been found due to proposed “color” force keeping the quarks inside the hadron that intensifies with distance. This would require infinite energy to give two truly free quarks
- 10. Chapter 15.9: Quarks Conclusion ● In 2009 Fermilab reported the discovery of a single top quark (1:20 billion proton-antiproton collisions) by using weak interactions instead of strong force (which produces pairs of top quarks)