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)