Spectrum of anti-hydrogen?

What does the spectrum of anti-hydrogen look like?

This question came up at our Maths/ Physics Seminar Series on Wednesday, during a presentation I gave on the forthcoming experiments at the LHC (slides here).  It’s a good question, I never thought to ask it before. Before I look it up, here is my guess at an answer – any comments welcome.

First a definition: as you know, antimatter is the name given to matter consisting of elementary particles in which the electric charge (or other quantum property) of each particle is the reverse of that in ordinary matter (see blog title). Just as a Hydrogen atom consists of an electron orbiting a proton, an anti-Hydrogen atom consists of a positron orbiting an anti-proton. However, although antiparticles are often found in cosmic rays or produced in accelerators,  anti-atoms are very rare: only a few atoms of anti-Hydrogen are made at accelerator facilities around the world.

My guess is that the sprectrum of anti-H looks exactly like that of Hydrogen. After all, the emission spectrum of Hydrogen is due to an excited electron jumping from the excited energy level down to a lower level(s): presumably the positron in anti-H has the same separation of energy levels, so I can’t see how there would be any difference in the light emitted.

Pictorial representation of H and anti-H

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However, there is a problem with this answer: how do we detect anti-atoms if their spectrum is the same as normal atoms? By deflection in a magnetic field, you say – this is how the positron was first discovered. But anti-atoms are neutral and in any case antimatter is not always matter of opposite charge, sometimes it is another quantum property that is swapped (consider the anti-neutrino). Indeed, how do we distinguish anti-neutrinos from neutrinos? I’m not sure, but I know we can.

Also, I think I  read somewhere that we have detected clusters of antimatter in some places in the universe. Again, how do we know it’s antimatter? These sort of unexpected questions are what makes giving a seminar worthwhile..