An exotic version of an atomic nucleus is doing double duty. A study of the hypertriton simultaneously confirms a basic symmetry of nature and potentially reveals new insights into what lurks inside ultradense neutron stars. 

The hypertriton is a twin of the antihypertriton — the antimatter version of the nucleus. Both hypernuclei have the same mass, researchers with the STAR Collaboration report March 9 in Nature Physics. 

A hypernucleus is an atomic nucleus in which a proton or neutron has been swapped out with a particle called a hyperon. Like protons and neutrons, hyperons are each made of three smaller particles called quarks. Whereas protons and neutrons contain common varieties known as up quarks and down quarks, hyperons are more unusual. They contain at least one quark of a type called a strange quark. 

The matching masses of hypertritons and antihypertritons reaffirms the solid footing of a pillar of physics known as charge-parity-time, or CPT, symmetry. To visualize such symmetry, imagine taking the universe and swapping out all the particles with their antimatter opposites, flipping it in a mirror and running time backward. If you could do that, the universe would behave identically to its nonflipped version, physicists believe. If CPT symmetry were discovered not to hold, physicists would need to reconsider their theories of the universe.

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