What is the matter?
Where is the antimatter?
Professor Michael G Green
Royal Holloway University of
London
Where the hell …?
What is the matter? . . . . Where is the antimatter?
What is matter?
What is the matter? . . . . Where is the antimatter?
Where is the antimatter?
What is the matter? . . . . Where is the antimatter?
The concept of elements
In Aristotle’s philosophy
there were four elements
Dalton (1808) listed, with weights,
many elements we recognize today
What is the matter? . . . . Where is the antimatter?
The periodic table
Mendeleev (1869) introduced
the periodic table
What is the matter? . . . . Where is the antimatter?
The plum pudding model
J J Thomson believed the electrons
were embedded in a positively
charged matrix - plum pudding
What is the matter? . . . . Where is the antimatter?
The structure of atoms
Rutherford (1912)
showed that atoms
contain a central
nucleus
-10
10 m
Electrons orbit nucleus
with well-defined
energy and ill-defined
positions
What is the matter? . . . . Where is the antimatter?
The structure of nuclei
-14
10
m
What is the matter? . . . . Where is the antimatter?
Nucleus contains
protons with charge
+e and uncharged
neutrons
The structure of nucleons
-15
10 m
Neutrons and
protons contain
quarks
What is the matter? . . . . Where is the antimatter?
The structure of quarks?
?
<10
-18
m
There is no evidence
for further structure
What is the matter? . . . . Where is the antimatter?
Evidence for substructure
Atom absorbs energy
Electron energy
increases
Only certain energy levels
(orbits) allowed
Later ‘de-excites’
What is the matter? . . . . Where is the antimatter?
Evidence for substructure
Measure size of struck objects
(Rutherford 1912)
1970 - substructure of protons and neutrons
discovered using electrons as projectiles
What is the matter? . . . . Where is the antimatter?
The constituents of matter
charge
quarks
2
+ e
3
1
- e
3
electron
u
d
-e
e
Protons contain uud - charge = +e
Neutrons contain udd - charge = 0
What is the matter? . . . . Where is the antimatter?
Prediction of antimatter
Paul Dirac predicted existence of
the positron in 1928
Dirac’s equation implies:
positron mass = electron mass
positron charge = +e
The only equation in
Westminster Abbey?
What is the matter? . . . . Where is the antimatter?
Discovery of antimatter
Anderson (1932) discovered the positron
predicted by Dirac
What is the matter? . . . . Where is the antimatter?
What is antimatter?
e+
e-
E=
Electrons and positrons
annihilate to produce
g-rays (energy)
2
mc
What is the matter? . . . . Where is the antimatter?
Production of e
e
+
pairs
Inverse process also occurs,
with g-rays becoming an
electron-positron pair
What is the matter? . . . . Where is the antimatter?
How to produce antimatter
e+
g
e-
e+ e-
e-
thin metal
region of magnetic field
e-
Eg > few MeV since
m e c2 = 0.5 MeV
What is the matter? . . . . Where is the antimatter?
The neutrino
‘Invented’ by Pauli (1928), named by Fermi (1933)
Discovered by Reines & Cowan (1956)
quarks
2e
+
3
1
- e
3
leptons
u
0
n
d
-e
e
What is the matter? . . . . Where is the antimatter?
The muon
Discovered in cosmic rays by
Neddermeyer and Anderson (1936)
Appears identical to electron but
200 times as heavy
Decays within 2.2 msec
‘Who ordered that?’ - I I Rabi
What is the matter? . . . . Where is the antimatter?
Strange particles
In 1947 Rochester and Butler discovered yet more new
objects, now known to contain a third quark
By the early 1960s beautiful patterns of particles
containing three quarks or a quark and an antiquark were
seen which were predictive (recall Mendeleev)
What is the matter? . . . . Where is the antimatter?
The fundamental particles (1963)
n
u
d
s
quarks
m
e
leptons
What is the matter? . . . . Where is the antimatter?
The zoo grows larger
u
d
ne
nm
1995
1956
1963
s
b
e
m
t
1947
1978
1895
1936
1973
c
t
1976
six quarks
What is the matter? . . . . Where is the antimatter?
six leptons
nt
A particle accelerator
Energy of electrons is about 20kV
What is the matter? . . . . Where is the antimatter?
The LEP accelerator
Energy of electrons and positrons is about 100GeV
What is the matter? . . . . Where is the antimatter?
CERN
Europe’s research laboratory for
particle physics in Geneva.
What is the matter? . . . . Where is the antimatter?
LEP
What is the matter? . . . . Where is the antimatter?
Inside the LEP tunnel
LEP is 27km in circumference
Four bunches of electrons and
positrons circulate inside the
vacuum pipe
100ms for a complete circuit
About one electron-positron
collision per second
What is the matter? . . . . Where is the antimatter?
Electron-positron collisions
Annihilation produces energy mini Big Bang
g
g
eElectron
(matter)
e+
g
g
Positron
(antimatter)
Particles and antiparticles
are produced
E = mc2
What is the matter? . . . . Where is the antimatter?
The ALEPH detector
End view
What is the matter? . . . . Where is the antimatter?
Collisions in ALEPH
What is the matter? . . . . Where is the antimatter?
ALEPH - a LEP particle detector
What is the matter? . . . . Where is the antimatter?
Three neutrinos ...
Number of different neutrinos
= 2.984 ± 0.008
u
c
t
ne
nm
nt
d
s
b
e
m
t
s measures rate at which
e+e- collisions occur
What is the matter? . . . . Where is the antimatter?
… and no further substructure
Excited states produced
if substructure exists
g
e*
e+
ee-
e
mass
What is the matter? . . . . Where is the antimatter?
The story so far
u
c
t
ne
nm
nt
d
s
b
e
m
t
The everyday world contains two quarks and the electron.
Additional quarks and leptons have been observed with six of
each in total; most decay very rapidly.
All particles have an antiparticle.
When energy turns to mass equal numbers of particles and
antiparticles are produced.
What is the matter? . . . . Where is the antimatter?
Matter-antimatter asymmetry in the
Universe
When energy turns to mass equal numbers of particles
and antiparticles are produced.
This observation creates problems for our understanding
of the present day Universe, which appears to contain
only matter and essentially no antimatter
What is the matter? . . . . Where is the antimatter?
Evolution of
the Universe
life on earth,
molecules
form
15 billion years
1 billion years
The Universe began
with a “Big Bang”
1 million years
about 15 billion years ago
heavy
elements
stars and
formed
galaxies
in stars
microwave
exist,
300,000 years
background
atoms
radiation
form
fills universe
3 minutes
1 second
10 -10s
?
quark "soup"
matter
dominates
10 15 deg
helium
nuclei
formed
neutrons
and protons
formed
10 10 deg
10 9deg
6000 o
4000 o
-255 o
What is the matter? . . . . Where is the antimatter?
-270 o
The Big Bang
What happened
at times less
than 10-9s
is uncertain
What is the matter? . . . . Where is the antimatter?
Evolution with matter-antimatter
symmetry
Eventually such a universe contains only photons
(almost true for our Universe - cosmic microwave background)
What is the matter? . . . . Where is the antimatter?
The Sakharov conditions
Antimatter can turn into matter if:
(a) proton decay occurs
(b) there is a matter-antimatter
asymmetry (CP violation)
(c) there is thermal nonequilibrium
Sakharov (1964)
What is the matter? . . . . Where is the antimatter?
Proton decay
Life on earth implies protons exist, on average, for >1023 seconds
Searches for proton decay have set limits >1032 seconds
e+
d
u
u
proton
X
up0
u
Proton decay converts
quarks into leptons important only in early
stages of the Big Bang
but a small effect will
remain
However antiprotons will decay similarly
What is the matter? . . . . Where is the antimatter?
Parity violation
Macroscopic systems obey the
same physical laws in a mirror
system, e.g. planetary motion
“parity conservation”.
θ
θ
v
I (q ) = 1 - cos q
c
b-decay (weak interaction)
does not conserve parity.
Discovered in 1956 in
polarized 60Co decay.
What is the matter? . . . . Where is the antimatter?
P violation - CP conservation
P
Parity violation leads to an
asymmetry for neutrinos -only
left-handed ones exist.
nL
C
nR
CP
nL
Changing particle to antiparticle (C) then
applying the parity operation (P) produces the
right-handed antineutrino, which exists
“CP conservation”
What is the matter? . . . . Where is the antimatter?
nR
Matter-antimatter asymmetry
In 1964 it was discovered that the radioactive decay of antimatter
differs by a tiny amount from the decay of matter.
Since then progress in understanding has been very slow:
• experiments are very difficult;
• astronomy is an observational science, not experimental
(cannot repeat the Big Bang).
BUT we have learned that the matter-antimatter asymmetry can
only occur if there are three pairs of quarks.
What is the matter? . . . . Where is the antimatter?
CP violation in
--u,c,t
sK0
W
d
W
u,c,t
d
-0
K
s
0
K
d
-0
K
decays
--u,c,t
W
s
sW
u,c,t
Phases of the amplitudes for the two processes are not equal
‘CP violation’ (1964)
Occurs only because there are three families of quarks
What is the matter? . . . . Where is the antimatter?
K0
d
CP violation
Leads to beautiful interference
effects and non-exponential decay
distributions
COUNTS / 0.5 x 10-10s
10 6
10 5
10 4
5
10
15 t(10 -10s)
10 3
k0
p+ p- DISTRIBUTION
10 2
10 1
5
10
15
20
25
30
t (10 -10 s)
What is the matter? . . . . Where is the antimatter?
A universe with CP violation
Perhaps one in every 109 antiquarks turned into a quark very
early in the life of the Universe
After the matter-antimatter annihilation a small amount of
matter will be left (about one proton for 109 photons)
What is the matter? . . . . Where is the antimatter?
Current problems
1. We have never observed proton decay
2. Precise measurements of CP violation in K0 decay are
difficult and there are uncertain theoretical corrections
3. Cosmological models do not easily explain the ratio of 109
photons for each proton in the Universe
What is the matter? . . . . Where is the antimatter?
CP violation in
0
B
decays
Similar effect expected in B0
--u,c,t
b
B0
W
d
W
u,c,t
d
-0
B
b
d
-0
B
--u,c,t
W
b
b
W
u,c,t
First measurements starting 1999, Stanford, California
What is the matter? . . . . Where is the antimatter?
B0
d
Weak decay eigenstates
In the Standard Model the weak decay eigenstates are
u
  and
 d' 
c
 
 s' 
where d’ and s’ are a mixture
of d and s of the form
d’= d cosθc + s sinθc
s’= -d sinθc + s cosθc
We write this as
 d'   cosc
  = 
 s'   - sin c
sin c  d 
 
cosc  s 
What is the matter? . . . . Where is the antimatter?
CP violation parameters
Further, there are relations among the elements of V such as
Vud V*ub + Vcd V*cb + Vtd V*tb = 0
that can be represented by a triangle.
It is a condition for CP violation to occur that b is non-zero.
What is the matter? . . . . Where is the antimatter?
CP violation parameters
Prior to 1999 the shape of this triangle had been approximately
determined from measurement of several parameters of V.
However the angle had not been measured directly.
What is the matter? . . . . Where is the antimatter?
BaBar experiment at
SLAC
What is the matter? . . . . Where is the antimatter?
The process e+e-  B0B0
9.0 GeV e-
3.1 GeV e+
e
Centre of mass energy = 10.58 GeV - a resonance
energy for the production of B0 B0
B0 and B0 are moving in the laboratory system
B0
B0
What is the matter? . . . . Where is the antimatter?
B decays
The two particles decay
We identify them
and measure the
separation of the
decay points
The separation
(typically 1mm)
is translated to a
time difference
(typically 1ps)
What is the matter? . . . . Where is the antimatter?
Predicted distributions
We measure many examples of the process to produce a
distribution in Dt
Without CP violation
the distribution of Dt
is exponential
(a) CP violation makes the
distribution asymmetric
(b) experimental resolution
modifies it
What is the matter? . . . . Where is the antimatter?
Evidence for CP violation
The data show clear
evidence of CP violation
The size of the effect is
consistent with the
prediction of the
Standard Model of
particle physics
What is the matter? . . . . Where is the antimatter?
Values of sin 2b
Recently direct measurements have been made
(summer 2002 values)
OPAL
CDF
ALEPH
3.2+1.8-2.0±0.5
0.79+0.41-0.44
0.84+0.84-1.05
Belle
BaBar
0.72 ± 0.07 ± 0.04
0.75 ± 0.09 ± 0.04
What is the matter? . . . . Where is the antimatter?
Summary
The everyday world is made from up and
down quarks and the electron.
Experiments tell us that six quarks and six
leptons exist.
The “extra” ones seem to be needed to
explain why there is an asymmetry between
matter and antimatter and hence why we
exist.
However it is likely to be a long time
before we have a good understanding of
what happened in the first fraction of a
second of the Universe’s existence
What is the matter? . . . . Where is the antimatter?
What is the matter?
Where is the antimatter?
THE END
There follows 6 slides showing a series of summary sheets
which have been sent out to schools throughout the UK.
What is the matter? . . . . Where is the antimatter?
Particle Physics Summary Sheets
- the story so far
GO
Particle Physics Summary Sheets
- the story so far
Available as double sided A4 sheets for
individual student use, or as a set of 6
single sided A3 wall posters, they are
entitled 'Particle Physics - the story so far'.
For details of how to get the summary sheets and posters
call 01784 443448 or
e-mail: physics@rhul.ac.uk