First Observation of Antihelium

One the big questions that trouble cosmologists and particle physicists is the distribution of matter and antimatter in the Universe. It certainly looks is if matter dominates the cosmos but looks can be deceiving. We may just live in a corner of the universe that happens to be dominated by matter.

Today, we find there’s a little extra antimatter in our corner thanks to the work of the STAR collaboration at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in the US.

These guys banged together 10^9 gold nuclei at energies of 200 GeV and spotted 18 antinuclei of helium-4 in the ensuing wreckage. That’s an impressive achievement by an standards–at the very least we now know antihelium-4 can exist.

These kinds of impacts create a hot blob of more or less equal numbers of quarks and antiquarks, a so-called quark gluon plasma. This cools down forming various particles and their antiparticles.

Of course, the bigger the antiparticle, the less likely we are to see it. In fact, each extra baryon in an antinucleus makes it 1000 times harder to make. So although positrons first cropped up in 1932, antiprotons and neutrons didn’t appear until 1955 and we had to wait until 1970 for a Russian team to announce the first observation of antihelium-3.

Now, 40 years later, we have antihelium-4. (It seems unlikely that we’ll see the next in line, antilithium-6, any time soon and, in fact, the STAR team admit it cannot be produced with current collider technology.)

What’s important about this observation is that antihelium-4 seems to occur at exactly the rate predicted by thermodynamics. So unless there’s some other mechanism for making it in vastly greater quantities, we’re unlikely to see a naturally occurring version, no matter how hard we look.

So “any observation of antihelium or even heavier antinuclei in space would indicate the existence of a large amount of antimatter elsewhere in the Universe,” say the STAR collaboration.

And, as it turns out, we are intending to look. The Space Shuttle Endeavour, currently scheduled for launch next month, is carrying the Alpha Magnetic Spectrometer to the International Space Station for precisely this reason.

Alpha is specially designed to look for particles of antimatter in cosmic rays. If antihelium is made only by known mechanisms, it will be too rare to trouble Alpha. But if the experiment gets even a sniff of antihelium or anything heavier, expect an explosion of interest from cosmologists and particle physicists.

This is one of the few genuinely useful pieces of science that is planned for the space station. Let’s hope it goes smoothly.

Ref: arxiv.org/abs/1103.3312: Observation Of The Antimatter Helium-4 Nucleus

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