DAW

1
Read Das and Ferbel Chap. 13.

• DAW 3 status?
• I will be away on Friday.
• Lecture will be given by Dr. Ben Speakman and Dr.
  Alex Smith Introduction to Neutrinos.
• Test 2 take-home. Out on Friday, April 18,
  back on Monday, April 21.
• 5 problems, potentially covering anything since
  Test 1.

2
Many particles! Patterns!
Nucleons p (938 MeV) n (940 MeV)
Pions ? (140 MeV) ?0 (135 MeV) ?? (140 MeV)
Kaons K (494 MeV) K0 (498 MeV) K? (494 MeV)

• Isospin
• Members of multiplets are different states of the
  same particle nucleon doublet, pion triplet,
  kaon and anti-kaon doublets.
• Butthe original premise of isospin was that only
  EM broke an otherwise perfect symmetry. If this
  is valid, explain

Perhapsobserved elementary particles are
different assemblies of a small set of basic
building blocks Fermi and Yang 1949!
3
Fermi-Yang Pre-Quark Model
SU(2)
Problems No experimental or theoretical basis
for baryons as building blocks. With more
particles, some repetition of patterns for
excited versions of known particles, but no place
for strangeness!
4
More Patterns Strangeness, Hypercharge,
Gell-Mann and others noted that electric charge
(Q) of all particles could be related to isospin
(I3), baryon number (B) and strangeness (S)
Y Hypercharge
Interesting patterns became evident when I3 was
plotted vs. Y
? Model of Sakata (1956)
Known mesons (7) and baryons (8) can be built
from three fundamental baryons (p, n, ?) and
their antiparticles
Good step! Explained known particles, got
statistics right. But why should p, n, ? be
fundamental? Lots of tinkering, incl. weakening
of connection between the building blocks and the
physical baryons. Sakata model could not
accommodate some decays and new particle
discoveries.
5
Next step SU(3) and the Eightfold Way
Gell-Mann, Neeman (1961)
Isospin SU(2) Strangeness ? SU(3)
Pseudoscalar Mesons
Vector Mesons
Spin-1/2 Baryons
Spin-3/2 Baryons
Octet of SU(3), consisting of an isotriplet, a
singlet and two isodoublets is asserted to be the
basic unit of organization
Spin-3/2 baryons pose a puzzle not an octet.
10? 27?
6

• Many competing predictions were made of particle
  masses and decays expected under the Sakata model
  and the 10 and 27 interpretations of the
  eightfold way.
• Gell-Mann predicted that the 10 is correct and
  that the ? in the spin- 3/2 decuplet would be
  the ??, a baryon with S ?3 at 1700 MeV.

• Last step! Gell-Mann and Zweig in 1964
  independently postulated that the constitutents
  of hadrons were not known particles, but
  as-yet-unseen building blocks in the fundamental
  (3) representation of SU(3). Zweig thought
  they were observable, G-M just bookkeeping
  tools!

7
The Quark Model

• Initially (to maximize confusion) these
  hypothetical particles were named p, n and ?, but
  within a decade the modern names u, d and s had
  been adopted.

Observed hadrons constructed from quarks
Mesons quark/antiquark pairs
Baryons three quarks or antiquarks
Quark and antiquark properties
8
Quark Model Successes
Quark Model Failures

• Classified known particles (early 60s), predicted
  others found later, and accom- modated many
  not-yet imagined.
• Explained absence of some particles, like S
  1 baryon.
• Explained mass splittings, isospin, magnetic
  moments, cross-section ratios (??p/?pp
  2/3).

• Free quarks? Many looked, no one found.
• Quantum statistics problems, e.g. ? is a
  fermion, but constructed wave function is totally
  symmetric.
• No satisfactory understanding of quark binding in
  hadrons
• Family problem