Pions form a new kind of helium – Physics World

Pions form a new kind of helium – Physics World

Like helium with a dash of antimatter

Move over, plain helium. Pionic helium is here: a helium atom in which one of the two electrons has been replaced by a negative pion, a composite particle made of one quark and one antiquark. Exotic atoms can help physicists to make exquisitely precise measurements of the fundamental constants of nature, such as the size of the proton. Pionic helium could provide a direct measurement of the mass of a related fundamental particle, the neutrino. That has been estimated indirectly, says physicist Masaki Hori, but “it is always nice to have a direct laboratory determination”. Pionic helium is the latest addition to a zoo of exotic atoms, including positronium, muonium, muonic hydrogen, muonic deuterium and antihydrogen. No dilithium crystals yet, though.

Physics World | 6 min readSource: Nature paper.
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