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PARTICLE PHYSICS
A brief intro to the quantum world of particles
WHAT IS PARTICLE PHYSICS?

Ordinary matter in our world is constructed of
just 3 types of particles:
Proton
 Neutron
 Electron

There are HUNDREDS of other types of
particles, most of which are unstable, that exist
in our universe
 Particle Physics is the study of all of these
particles—stable and unstable, ordinary and notso-ordinary.
 (It’s definitely a journey into a LOT of new and
interesting names of things!)

BRIEF HISTORY…
Protons, Neutrons, Electrons—historically
believed to be smallest particles of matter
 1950’s and 60’s, during nuclear reaction tests and
experiments, hundreds of other particles
discovered!

Pions (p+, p-, p0)
 Kaons (K+, K-, K0)
 Etas (h, h’)
 Hyperons (S+, S-, S0)


Each was determined to have a very short
lifetime (ranging from 10-10 s to 10-24 s)
ELEMENTARY PARTICLES


Any particle which is not made of any smaller
component particles
Three Classes of Elementary Particles:
Leptons
Responsible for ordinary matter
 Quarks
 Exchange Particles
Involved with Fundamental Interactions

LEPTONS



Lepton
Name
Symbol
Electric
Charge/e
Rest mass/ Spin/(h/2p)
MeV·c-2
Electron
e-
-1
0.511
½
Electron
neutrino
ne
0
-
½
Muon
m-
-1
106
½
Muon
Neutrino
nm
0
-
½
Tau
t-
-1
1780
½
Tau
Neutrino
nt
0
-
½
Electron and electron neutrino are seen in beta decay
Neutrinos once believed to be massless, now known to
have a very, very small mass
Existence of all 6 has been supported with solid
experimental evidence
QUARKS
Quark
Flavor



Symbol
Electric
Charge /e
Rest mass/
Mev·c-2
Spin/
(h/2p)
Up
u
+ 2/3
330
½
Down
d
- 1/3
333
½
Strange
s
- 1/3
486
½
Charm
c
+ 2/3
1500
½
Bottom
b
- 1/3
4700
½
Top
t
+ 2/3
175500
½
Existence of all 6 has been supported with solid
experimental evidence
Quarks can never exist independently
Quarks combine to form larger particles (i.e. protons
and neutrons)
QUANTUM NUMBERS

Numbers (usually) used to characterize the
properties of particles

Electric Charge: units = e, where e = 1.6 x 10-19 C







For example, the quantum number for electric charge of an
electron is -1 and for a proton, +1
Flavor:--not specified by a number, and only really
used for quarks
Color
Strangeness
Baryon number
Generation lepton number
Some quantum numbers are conserved in particle
reactions, others aren’t…
SPIN
The quantum number spin is a property that is
analagous to rotation and angular momentum,
but not the same
 It’s based on principles related to Einstein’s
theory of relativity
 For elementary and composite particles: the unit
of spin = h/2p
 All known particles have some quantity of spin
that is a multiple of that unit

Bosons have an integral spin
 Fermions  have a half-integral spin

MORE ON SPIN

Depict a particle with spin using circle with
arrow through it:
B
Spin up

Spin down
All particles with spin will align parallel (or
antiparallel) to the magnetic field’s direction
BOSONS AND FERMIONS

Bosons
All bosons have an integral spin (i.e. 1 or 2)
 Examples: Photon, W+ boson,…


Fermions
All Fermions have a ½-integral spin (i.e. ½ )
 Examples: quarks, leptons, protons, and neutrons

PAULI EXCLUSION PRINCIPLE


It is impossible for two identical fermions to
occupy the same quantum state if they have the
same quantum numbers
Example: electron distribution in energy levels…
FUNDAMENTAL INTERACTION
Range
Relative Strength
(to 2 protons
touching)
Exchange
particle(s)
Any particle with
mass
infinite
10-38
Graviton
Weak interaction
Any particle
10-18 m
10-5
W bosons, Z
boson
Electromagnetic
Any charged
particle
Infinite
1
Photon
Strong
interaction
Only quarks
10-15 m
100
gluons
Fundamental
Interaction
Felt By…
Gravitation
EXCHANGE PARTICLES
Exchange
Particle
Symbol
electric
Charge /e
Spin/ (h/2p)
Photon
g
0
1
0
Electromagnetic
W bosons
W+
W-
+1
-1
1
1
80.4
80.4
weak
weak
Z boson
Z0
0
1
91.2
weak
Gluons
Gij
0
1
0
Strong
Graviton
g
0
2
0
Gravitational

Rest
Mass/
MeV·c-2
Associated
Interaction
There is solid experimental evidence for the
existence of all of these, EXCEPT for the graviton
VIRTUAL PARTICLES
An interaction between particles will often take
place while violating the law of conservation of
energy (sorry, but it’s true!)
 This can happen IF the time in which the energy
violation is happening is small enough to be
undetectable, according to Heisenberg’s
uncertainty principle
 The exchange particle involved in the interaction,
therefore, is a virtual particle
 The exchanged particle is NOT observed in any
way.

RANGE OF AN INTERACTION

We can estimate the range through which an
interaction can occur by estimating:
the mass  (m)
 the time of the interaction, assuming maximum
velocity of c (Range/c)


We also use the Uncertainty
Principle and Einstein’s
mass-energy equivalence to
derive a mathematical
expression:
h
E  t 
4p
h
2 R
mc    
 c  4p
h
R
4p  mc
 
HADRONS


Baryons

Mesons

Made by combining 3
quarks

Made by combining 1
quark and 1 anti-quark

Anti-baryons are made
by combining 3 antiquarks

Anti-mesons are created
by combining the antiparticles of each
Examples:
Proton = uud
 Neutron = udd
 Anti-proton = uu d


Examples:

Pion (p+) =

Anti-pion =
ud
ud  p -
BARYON NUMBER
protons and neutrons have a baryon number = +1
 All anti-protons and anti-neutrons have a baryon
number = -1
 Quarks have baryon number = +1/3
 Anti-quarks have baryon number = -1/3


Baryon number is always conserved
STRANGENESS
Particles were discovered during cosmic ray
experiments in the 50’s and 60’s that had
unexpected or unusual properties
 Strange particles
 Example of an unusual property:


Decayed much, much more slowly than expected,
based on observations of similar particles
- = 10-10 s
 Decay of S

Decay of
S 0 = 10-20 s
These properties were assigned a new quantum
number, called “strangeness”
 Now known to be associated with the presence of
a strange quark or anti-strange quark

STRANGENESS
Presence of a strange quark = -1 unit of
strangeness
 Presence of an anti-strange quark = +1 unit of
strangeness

(yes, I know it seems backwards…think of it as a
“strange” assigning of quantum numbers…)
COLOR

What’s wrong with having a baryon that has a
total spin of 3/2?
Color is a new quantum number that was created
to explain the existence of particles that otherwise
would violate the Pauli Exclusion Principle. This
has nothing to do with visible light!
 Quarks carry one of 3 possible colors: Red, Blue,
or Green
 Antiquarks carry anti-colors

COLOR

Color combinations that result in a white, or
colorless, particle:
Red + Blue + Green
 Red + anti-red
 Blue + Anti-blue
 Green + Anti-green


Example:

Energy is supplied to a meson, and two new mesons
are created. Refer to the diagram on the board.
What are the colors of the quarks indicated by X, Y,
and Z?