If an atomic nucleus is a family, then in an ordinary helium house live two protons and two neutrons, holding hands tightly. Now imagine that one family member is an oddball who loves to wander far from home. That's exactly what a hypertriton looks like: at the center, an ordinary pair (a proton and a neutron), and around them, at a large distance, roams a hyperon — a particle with a strange quark inside.

Scientists had long been unable to catch this "stretched" atom: hyperons live for infinitesimal fractions of a second and decay immediately. But at the Large Hadron Collider at CERN, for the first time, they managed to see the birth of a hypertriton in proton collisions. It turned out that the hyperon sits about 9.5 femtometers from the nucleus center. A femtometer is a billionth of a millimeter, but for an atomic nucleus, that's a huge distance, like a person holding onto a house with a rope several blocks long.

Such a loose structure is not just a curiosity. Inside neutron stars, where matter is compressed to the limit, hyperons may appear in huge numbers. Knowing how they bind to ordinary particles helps physicists better understand what the densest matter in the universe is made of. And it's also a rare case where particle collisions at a collider help study not the particles themselves, but how they assemble into tiny but real atomic nuclei.