Friday, 2 April 2010

The Antihypertriton

So, part of the reason to write this blog is to talk about some of the interesting isotopes out there - some of the uses they get put to, some of the reasons why they're of interest to physicists, or astronomers, or geologists, or oncologists, or radiologists, or engineers, or ... well, you get the idea. So let's kick off with a specific nucleus.

Each isotope is defined by the number of protons and neutrons it contains. The number of protons tells you what element the nucleus is (1=Hydrogen, 2=Helium and so on. This number is also called the atomic number) and the number of neutrons specifies which isotope of that element you are dealing with. I thought I would write about nuclei with atomic numbers going up as high as around 120, and down as low as zero (and I thought that zero would be a neat trick, talking about neutron clusters with no protons present), but I was a little shortsighted in how low you can go in atomic number.

In an article just published in the journal Science, the STAR collaboration - a team of scientists from around the world working at the Relativistic Heavy Ion Collider (RHIC - think LHC but smashing heavy gold nuclei together rather than single protons) at Brookhaven Lab in New York State, USA - have reported observation of a really exotic nucleus with atomic number -1, so it has an antiproton instead of a proton, and with a particle that doesn't usual feature in nuclei called an antihyperon. This kind of "antinucleus" doesn't make up normal matter. Nature has provided us with a whole set of particles (protons, neutrons and so on) along with a complete mirror image set, known as antiparticles. Their behaviour seems to be pretty much the same as regular matter, yet most of what we see is made of regular matter, and it's not clear why. It's thought that the big bang created basically the same amount of matter and antimatter, but that some process led to matter being more dominant now. We can't go back and observe the big bang, but by colliding two heavy nuclei together, we can get a small scale version of the high density and energy that occurred just after the big bang, and see what happens. This occurrence of the antihypertriton seems to concur with the expectation that matter and antimatter behave exactly the same - but its nice to have experimental evidence that the big bang should have behaved that way, and it's pretty cool to be able to write about a nucleus so exotic that no-one has seen one on the Earth before.

The LHC may get all the press attention, but they haven't seen an antihypertriton yet :-)