Even a weird hypernucleus confirms a fundamental symmetry of nature

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.

So far, scientists have not found any hints of CPT symmetry violation (SN: 2/19/20), but they’ve never before tested it in nuclei that contain strange quarks, so they couldn’t be sure it held there. “It is conceivable that a violation of this symmetry would have been hiding in this little corner of the universe and it would never have been discovered up to now,” says physicist Declan Keane of Kent State University in Ohio. But the equal masses of hypertritons and antihypertritons — found in experiments at the Relativistic Heavy Ion Collider, RHIC, at Brookhaven National Laboratory in Upton, N.Y. — means that CPT symmetry was upheld.

In collisions of gold nuclei at RHIC, Keane and colleagues identified the hypernuclei by looking for the particles produced when the hypernuclei decayed inside of the 1,200–metric ton STAR detector. In addition to confirming that CPT symmetry prevailed, the researchers determined how much energy would be needed to liberate the hyperon from the hypernucleus: about 0.4 million electron volts. Previous measurements — which are now decades old — suggested that amount, called binding energy, was significantly lower, with measurements mostly scattered below 0.2 million electron volts. (For comparison, the binding energy of a nucleus consisting of a proton and neutron is about 2.2 million electron volts.)

The new number could alter scientists’ understanding of neutron stars, remnants of exploded stars that cram a mass greater than the sun’s into a ball about as wide as the length of Manhattan. Neutron stars’ hearts are so dense that it’s impossible to re-create that matter in laboratory experiments, says Morgane Fortin of the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences in Warsaw. So, “there is a big question mark what’s at the very center of neutron stars.”

Some scientists think the cores of neutron stars might contain hyperons (SN: 12/1/17). But the presence of hyperons would soften the matter inside neutron stars. Softer neutron stars would more easily collapse into black holes, so neutron stars couldn’t become as massive. That feature makes hyperons’ potential presence difficult to reconcile with the largest neutron stars seen in the cosmos — which range up to about two solar masses.

But the newly measured, larger binding energy of the hyperon helps keep alive the idea of a hyperon-filled center to neutron stars. The result suggests that hyperons’ interactions with neutrons and protons are stronger than previously thought. That enhanced interaction means neutron stars with hyperons are stiffer and could reach higher masses, Fortin says. So neutron stars may still have strange hearts.