Antihelium

We've had muonic helium; now it's the turn of antihelium. This is two antiprotons, with an antineutron or two to keep the strongforce strong.

I don't know if anyone has attempted making antitritium to let it decay. Antihelium doesn't decay... until it crashes into normal matter. Also: an antihelium ion should be negatively charged. Anyone try sticking that onto an ion like, er, helium?

The finding here is survival rate. They used the Large Hadron Collider at CERN, hadron-subset of Ions which name can be massaged into "aLICE". By pounding Hydrogen-1 - that is, ionic protons - at Lead-208, sparks fly of energy, and some sparks become antihelium. Then Alice sees how long those new ions last before they hit something else and go blooey. This allows for direct measurement of antihelium transparency: a unit of measure for the number and energies of antinuclei.

Ultimately the hope is to constrain antihelium from cosmic rays, always nature's highest-energy particle-accelerator; against antihelium from darkmatter (if it even exists). The latter should be slower-energy.