ANTIMATTERDidsbury ScibarMay 25th 2009

Roger Barlow

Antimatter – the hype

Matter – the electron

Discovered over 100 years agoStill one of the fundamental ‘elementary particles’

Properties known very well:Mass 9.109382 10-13 kgCharge -1.6021765 10-19CRadius Zero (?) ee

Commonplace yet mysteriousJust as ‘fundamental’ as quarks, gluons, Higgs bosons…Also an everyday object – electronics, electricity, atomic physics…

Paul Dirac (1902-1984)

Dirac Equation

€

−ihcα .∇ψ + βmc 2ψ = Eψ

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? Reject the equation Ignore crazy solutions Take them seriously

Think it through…

Battery

+ charged

- charged Ordinary electron travelling left to

rightAttracted by the + plate, repelled by the - platePicks up energy. E=½mv2 Goes faster.

Anomalous electron travelling left to rightPicks up energy. E= -½mv2 Goes slowerBehaves as if it were repelled by the + plate attracted by the – plate

It’s always the same

‘Negative energy’ electrons behave like ordinary electrons with the opposite charge.

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 positron22Na 22Ne + e++ ν

Also 11C, 13N, 15O, 18FHalf lives of minutes

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

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 Hence the experiment to study B and B

particles All properties exactly the same or exactly

opposite Only antielectrons have their own special name

From antiparticles to Antimatter Hydrogen

Antihydrogen

PP

eePP

ee

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

Matter meets antimatter

If a particle meets its antiparticle: Total charge (etc) is 0 Combine to give lots of 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

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 What about anti-antiparticles?These are the original particles

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!)

Maybe distant galaxies are made of antimatter? Looks like not.

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%).

This was explained by Kobayashi and Maskawa Their theory also predicts similar effects with B

particles Predictions are spot on – Nobel prize for K and

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