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Department of Physics and
Astronomy
Option 212: UNIT 2
Elementary Particles
SCHEDULE
26-Jan-15 13.00pm LRB
28-Jan-15 12.00pm
LRB
(2-Feb-15 10.00am
4-Feb-15 12.00pm LRB
E
Intro lecture
Problem solving
Problem Workshop)
Follow-up
UNIT 2: OUTLINE SYLLABUS:
1st Lecture Introduction
Hadrons and Leptons
Spin & Anti-Particles
The conservation laws: Lepton Number
Baryon number
Strangeness
2nd Lecture Problem solving
Check a decay for violation of conservation laws
Quarks
Properties of a particle given quark combination
3rd Lecture Follow-up
Fundamental forces and field particles
The standard model
Recommended Books
 Chapter 41, PA Tipler
 Quarks Leptons and The
Big Bang, J Allday
 The Cosmic Onion, F Close
Web Sites
 Brief introduction to Particle Physics
http://superstringtheory.com/experm/index.html
 CERN web site
http://home.web.cern.ch/
 212 Option - Lecture notes in MS Powerpoint & PDF
http://www.star.le.ac.uk/mrb1/lectures.html
INTRODUCTION
to
Elementary Particle Physics
*
Fundamental building blocks of
which all matter is composed:
Elementary Particles
* Pre-1930s it was thought there
were just four elementary particles
electron
proton
neutron
photon
1932 positron or anti-electron discovered, followed
by many other particles (muon, pion etc)
We will discover that the electron and photon are
indeed fundamental, elementary particles, but
protons and neutrons are made of even smaller
elementary particles called quarks
Cosmic Rays
CLASSIFICATON OF PARTICLES
An elementary particle is a point particle without structure
that is not constructed from more elementary entities
With the advent of particle accelerator
in the 1950’s many new elementary
particles were discovered.
The question arose whether
perhaps there were too
many to all be elementary.
This has led to the need
for classification of
particles.
FUNDAMENTAL INTERACTIONS AND THE
CLASSIFICATION OF PARTICLES
Fundamental interactions
o gravitation
o electromagnetic
o strong nuclear force
o weak nuclear force
Participating particles
• all particles with mass
• those carrying charge
• Hadrons (and quarks)
• Leptons (and quarks)
HADRONS
Hadrons interact through strong forces.
There are two classes, mesons and
baryons.
Mesons have zero or integral spin (0
or 1) with masses that lie between the
electron and the proton.
Baryons have half integral spin (1/2 or
3/2) and have masses that are always
greater than or equal to that of the
proton.
Hadrons are not elementary particles.
As we will see later, they are made of
quarks
LEPTONS
Leptons interact through weak interactions, but not via the strong force.
All leptons have spin of 1/2. There are
six kinds of lepton: electron e-, muon
m-, and tau t -, and 3 neutrinos ne, nm, nt
Note that each distinct neutrino is
associated with one of the other
leptons
Leptons were originally
named because they were
“Light-particles”, but we
now know the Tau is twice
as heavy as a proton
Neutrinos were originally
thought to be massless, but
they probably have a small
mass
Read more in Tipler p. 1336
Beta Decay and the discovery of the neutrino (Tipler p.1314)
-×
He
+
e
2
3
1H
3
1H
-
3
-+ n
He
+
e
2
e√
3
• In Beta decay a neutron decays into a proton plus an electron
• If decay energy shared by proton and emitted electron, energy of
electron would be unique
• But observed electrons have a range of energies – must be a third
particle involved: the neutrino
• Third particle must have no charge or mass, as they are accounted for
by the He nucleus and electron.
Spin
A particle has an intrinsic spin angular momentum
Spin ½ particles:
Electrons, protons, neutrons and neutrinos all have an
intrinsic spin characterised by the quantum number s = 1/2
Particles with half-integer spin (1/2, 3/2, 5/2, …) are called
Fermions
They obey the Pauli exclusion principle
Particles with integer spin (s = 0, 1, 2, …. ),
e.g. mesons, are called Bosons
They do not need to obey the Pauli exclusion
principle, and any number can occupy the same
quantum state
Matter & Antimatter
Every particle has an antiparticle partner
Read Tipler P.1339 to find out how Dirac predicted the existence of anti-particles in 1927
Here are some examples
e- - electron
p - proton
e+ - positron
p - antiproton
n - neutron
n - antineutron
n - neutrino
n - antineutrino
Antimatter
For each particle there is
an associated
antiparticle
Anti-particles always created
in particle-anti particle pairs
s
Electron Pair Production
s
g -> e- + e+
Eg  2 x 511 keV
e-
e+
Antimatter
* Antiparticle has the same mass
and magnitude of spin as the
particle
Electron Pair Annihilation
* Antiparticle has the opposite
charge to the particle
* The positron is stable but has a
short-term existence because our
Universe has a large supply of
electrons
* The fate of a positron is
annihilation
s
s
e- + e+ ->2g
e- s Each photon gets
eg = mec2
pg = mec
moc2
s = 1/2
e+
moc2
s = 1/2
Some Fundamental Particles
Particle
Mass less
boson
photon
Symbol
g
Rest energy MeV
0
Charge
Spin
Antiparticle
0
1
g
Leptons
Neutrino
Electron
Muon
n
em-
0
0.511
105.7
0
-1
-1
1/2
1/2
1/2
n
e
m
Meson
Pion

o
140
135
+1
0
0
0
o
Baryons
Proton
neutron
p+
no
938.3
939.6
+1
0
1/2
1/2
p-
-
n
The Conservation Laws
Can a conceivable reaction or decay occur?
• Conservation of energy
The total rest mass of the decay products must be less
than the initial rest mass of the particle before decay
• Conservation of linear momentum
When an electron and a positron at rest annihilate, two
photons must be emitted
• Angular momentum must be conserved in a decay or
reaction
• Net electric charge before must equal net charge
after a decay or reaction
The Conservation Laws
Can a conceivable reaction or decay occur?
• Conservation of Baryon number
We assign Baryon Number B=+1 to all Baryons, B=-1 to
all anti-Baryons, and B=0 to all other particles
Baryon number must be conserved in a reaction
• Conservation of Lepton number
Lepton number must be conserved in a reaction
BUT…..
The Conservation Laws
Can a conceivable reaction or decay occur?
• Conservation of Lepton number contd:
…..because the neutrino associated with an electron is
different to a neutrino associated with a muon, we assign
separate Lepton numbers Le, Lm and Lt to the particles
e.g. for e and ne, Le=+1, for their anti-particles Le=-1,
and for all other leptons and other particles Le=0
• Conservation of Strangeness
There are other conservation laws which are
not universal, e.g. strange particles have a
property called strangeness which must be
conserved in a decay or reaction
Some Fundamental Particles
Category
Particle
Symbol
Photon
photon
g
Leptons
Neutrino
Electron
Muon
Tau
n
emt-
Pion

o
K+
Ko
Hadrons
Mesons
Kaon
Baryons
Rest energy MeV
0
0
0.511
105.7
1784
140
135
493.7
497.7
938.3
p+
939.6
no
1115.6
L
1189.4

1192.5

1197.3
See also Tipler Table 41-1 Page 1337
For strangeness, examine Figure 41-3 Page 1344
Proton
Neutron
Lambda
Sigma
B Le Lm Lt S
Antiparticle
0
0
g




  
  
  
  




n
e
m
t












 
 
 
 
o
K
_
Ko
+1
+1
+1
+1
+1
+1




















-
-
-
-
p_n_
L_ 
_
_
-
0 0
0