ANTIMATTER Didsbury Scibar May 25 th 2009 Roger Barlow

Antimatter – the hype

Matter – the electron Discovered over 100 years ago Still one of the fundamental ‘elementary particles’ Properties known very well: Mass kg Charge C RadiusZero (?) e e Commonplace yet mysterious Just as ‘fundamental’ as quarks, gluons, Higgs bosons… Also an everyday object – electronics, electricity, atomic physics…

Paul Dirac ( ) Dirac Equation 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=½mv 2 - Crazy solution. With negative energy. E.g. E=-½mv 2 Basically it gives E 2. 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 right Attracted by the + plate, repelled by the - plate Picks up energy. E=½mv 2 Goes faster. Anomalous electron travelling left to right Picks up energy. E= -½mv 2 Goes slower Behaves 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 kg  Charge C  RadiusZero (?) Properties exactly the same (or exactly opposite)

Plenty of positrons High energy photons (MeV γ rays) produce electrons and positrons in pairs. Lots in cosmic ray showers Some nuclei β decay by emitting a positron 22 Na  22 Ne + e + + ν Also 11 C, 13 N, 15 O, 18 F Half 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 P P e e P P e 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

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  2mc 2 = J (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, ~ particles to 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 K 0 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