What are the
Elementary
Constituents of
Matter?
What are the forces that
control their behaviour at the
most basic level?
History of Constituents of
Matter
AD
•In Nuclear Reactions momentum and massenergy is conserved – for a closed system the
total momentum and energy of the particles
present after the reaction is equal to the total
momentum and energy of the particles before the
reaction
•In the case where an alpha particle is released
from an unstable nucleus the momentum of the
alpha particle and the new nucleus is the same as
the momentum of the original unstable nucleus
Wolfgang Pauli
1
1
0 __ 0
n0  p1e 1  0
•Large variations in the emission velocities of the 
particle seemed to indicate that both energy and
momentum were not conserved.
•This led to the proposal by Wolfgang Pauli of another
particle, the neutrino, being emitted in  decay to carry
away the missing mass and momentum.
•The neutrino (little neutral one) was discovered in 1956.
1
1
0 __ 0
n0  p1e 1  0
1.008665 u
1.007825 u
0.0005486 u
1u=
1.660 10 27 kg
1J=
1.6 10 19
eV
Mass difference
 1.008665  (1.007825  0.0005486 )
 0.0002914 u
 0.0002914 1.660 10 27
 4.83724 10
31
kg
kg
E  mc
2
 (4.837241031)(3.0 108 ) 2J
 4.353516 10 14 J

4.353516  10 14
1.602  10
19
 271755eV
 0.272 MeV
It has been found by experiment that the emitted beta particle
has less energy than 0.272 MeV
Neutrino accounts for the ‘missing’ energy
+
+
Cockroft and Walton
•First artificial splitting of
nucleus
•First transmutation using
artificially accelerated
particles
•First experimental verification
2
of E = mc
Irish Nobel Prize
E.T.S. Walton 1951
John Cockcroft
Ernest Walton
Experimental verification of
1
7
4
1H  3 Li  2 He
1 MeV
Proton + Lithium

4
2 He
E =
2
mc
 Energy
17.3 MeV
Two alpha particles + Energy
• Ancient Greeks:
Earth, Air, Fire, Water
• By 1900, nearly 100
elements
• By 1936, back to three
particles: proton, neutron,
electron
CERN LEP APPLET
http://www.hep.ucl.ac.uk/masterclass/Acc_sim2/simulator.html
The Four Fundamental Forces
20
Forces
Electromagnetic
Weak
Strong
Gravity
atoms
molecules
optics
electronics
telecom.
beta
decay
solar
fusion
particles
inverse
square law
short
range
short
range
inverse
square law
photon
W , Z0
±
gluon
graviton
Institute of Physics
Peter Kalmus
nuclei
falling
objects
planet
orbits
stars
galaxies
Particles and the Universe
m
E
c
2
Particle
zoo
11
Feel weak force
“predicted”  later discovered
Neutrinos
100,000,000,000,000 per second pass
through each person from the Sun
Antiparticles
Equal and opposite properties
“predicted”  later discovered
Annihilate with normal particles
Now used in PET scans
1950s, 1960s
Many new particles created
in high energy collisions
Convert energy to mass. Einstein E = mc2
> 200 new “elementary” (?) particles
Institute of Physics
Peter Kalmus
Particles and the Universe
Thomson (1897): Discovers electron
1x10 10 m
1x10 15 m
0.7 x10 15 m
 0.7 x10 18 m
_
60
60
0
0
27 Co28 Ni  1e 0 
Q = -1e almost all trapped in atoms
Q= 0 all freely moving through universe
Just as the equation x2=4 can have two
possible solutions (x=2 OR x=-2), so
Dirac's
equation
could
have
two
solutions, one for an electron with
positive energy, and one for an electron
with negative energy.
Dirac interpreted this to mean that for
every particle that exists there is a
corresponding
antiparticle,
exactly
matching the particle but with opposite
charge. For the electron, for instance,
there should be an "antielectron" called
the positron identical in every way but
with a positive electric charge.

 e e

1928 Dirac predicted existence of antimatter
1932 antielectrons (positrons) found in
conversion of energy into matter
1995 antihydrogen consisting of antiprotons and
positrons produced at CERN
In principle an antiworld can be built from
antimatter
Produced only in accelerators and
in cosmic rays
 rays  e   e 


e  e  2hf
Q
2
3
Q
1
3
Q  1
Q0
James Joyce
Murray Gell-Mann
1
3
1

3
1

3

2

3
2

3
2

3
12
Today’s building blocks

Leptons
Quarks
(do not feel strong force)
(feel strong force)
electron
e-neutrino
4 particles
ee
-1 up
0 down
very simple
multiply by 3 (generations)
multiply by 2 (antiparticles)
u
d

2
2
1


 1
3
3
3
proton = u u d
2
3
+2/3
1
-1/3

3
2 +2/3
1 1
= +1
+2/3
  -1/3
0
3
3 3
neutron = u d d
+2/3 -1/3 -1/3 = 0
First generation
http://lectureonline.cl.msu.edu/~mmp/applist/q/q.htm