ANTIMATTER
Bollington Scibar
September 14th 2009
Roger Barlow: Manchester University
Matter – the electron
Discovered over 100 years ago
An everyday object – electronics,
electricity, atomic physics…
Still one of the fundamental
‘elementary particles’
Properties known very well:
Mass
9.109382… 10-13 kg
Charge
-1.6021765… 10-19C
Radius
Zero (?)
e
Paul Dirac (1902-1984)
Dirac Equation
ihc .  mc   E
2
Deduced from aesthetics.
Explains the magnetic behaviour of the
electron and links it to its electrical behaviour
But …..
Dirac’s “unwanted” answers
The equation always gives two solutions.
- Sensible solution. With positive energy. E.g. E=½mv2
- Crazy solution. With negative energy. E.g. E=-½mv2
Basically it gives E2. Which has positive and negative
roots.
What next?
1. Reject the equation
2. Ignore crazy solutions
3. Take them seriously
3) Take them seriously.
Think it through…
+ charged
- charged
Ordinary electron travelling left to right
Attracted by the + plate, repelled by the - plate
Picks up energy. E=½mv2 Goes faster.
“Crazy” electron travelling left to right
Picks up energy. E= -½mv2 Goes slower
Behaves as if it were repelled by the + plate attracted by
the – plate
Battery
It’s always the same
‘Negative energy’ electrons behave like ordinary electrons with
the opposite charge: plus instead of minus.
Another particle
(or just another aspect of the basic particle)
The Positron or Anti-electron
 Mass
9.109382 10-13 kg
 Charge
+1.6021765 10-19C
 Radius
Zero (?)
Properties exactly the same (or exactly opposite)
Plenty of positrons
High energy photons
(MeVγrays) produce
electrons and positrons in
pairs.
Lots in cosmic ray showers
Some nuclei βdecay by emitting a
positron
22Na  22Ne + e++ ν
Also 11C, 13N, 15O, 18F
Half lives of minutes
What about other particles?







protons, neutrons, quarks …
Yes, they all have their antiparticle equivalents:
Antiprotons, antineutrons, antiquarks…
Often denoted by a line (bar) above: p
Experiment to study B and B particles
All properties exactly the same or exactly opposite
(But only antielectrons have their own special name)
Matter meets antimatter
If a particle meets its antiparticle:
 Total charge (etc) is 0
 Combine to give lots of lower-mass particles, typically
gamma rays (‘annihilation’)
Isn’t that dangerous?
1 gm of antimatter + 1 gm matter 2mc2=1.8 1014J
(A days output for a large power station)
But to make that gram you need – many days output from a
large power station
Another question
Why is the universe full of
matter and empty of
antimatter?
If positrons and electrons are
equal, why aren’t there
equal numbers of both
around?
(Just as well!)
Equal numbers of
particles and
antiparticles
An imbalance
develops,
~1000000001
particles to
999999999
antiparticles
Most particles
annihilate with
antiparticles
leaving the
residue
What was that difference?
We don’t know! This is one of the big 3
questions of particle physics
There are small differences in the behaviour of
Kaons. A K0 will decay to e+π - or to e- π + .
The rates are slightly different (0.3%). So
e+ and e- do differ a little
 Explained by Kobayashi and Maskawa
 Their theory also predicts similar effects with
B particles
 Predictions spot on – Nobel prize for K & M
 These are not big enough to account for the
matter/antimatter domination.
Antimatter

Part of our understanding of the universe

But we still don’t understand everything
Frequently Asked Questions
Are there antiphotons?
No. Or rather: the antiphoton is exactly the same as the photon so there isn’t a
separate species.
 What about antineutrons? Antineutrinos?
Yes. Although these are neutral so their antiparticles have the same charge (0=0) they have other properties which are different.
 Do antiparticles mean antigravity?
No. Apples and antiapples both fall downwards
 Could distant galaxies be made of antimatter?
No. Though they would look just the same. But the space where galaxies meet
would produce lots of annihilation radiation
 Does antimatter have any use?
Yes. PET scanners – Positron Emission Tomography
 What about anti-antiparticles?
These are the original particles

Positrons at work: PET scanning
Positron Emission Tomography

Prepare biologically interesting chemical with
positron emitting nucleus (e.g. FDG – like glucose)

Inject patient. Molecules move to sites where
needed

Nucleus decays giving positron

Positron encounters electron and ‘annihilates’ to 2
photons, emitted back to back

Photons are detected externally. The decay
ocurred somewhere along the line joining them

Collect more data and get 3D map
From antiparticles to Antimatter

Hydrogen
P

e
Antihydrogen
P
e
Straightforward to
make:
Make antiprotons.
(1M protons at 1 GeV
give ~25 antiprotons)
Make positrons.
(easier). Combine
them.
Worth studying to see
if properties really
are the same as
Hydrogen
Very hard/impossible
to store