Many physicists suspect that in the core of neutron stars hides strange matter—that is, matter containing strange quarks. The cores’ high densities make strange quarks energetically favorable to the usual up and down quarks. But with no way to directly measure a neutron star’s core composition, scientists’ search for stellar strange matter has been a theoretical one. Researchers constrain their models with a few parameters from Earth-based strange-matter measurements and predict a maximum neutron star mass. But those models yield maximums of around 1.5 solar masses, which fall short of recent observations of neutron stars as massive as 2.14 solar masses (see Physics Today, January 2011, page 12). Now the STAR international collaboration at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) in New York has measured a more precise mass and binding energy of a nucleus containing strange matter. The new values could narrow or even eliminate the disagreement between theoretical models and observations.

Two RHIC detectors record the trajectories of the decay products of the antihypertriton. Credit: J. Adam et al., Nat. Phys., 2020, doi:10.1038/s41567-020-0799-7

An important experimental parameter that constrains neutron star models is the Λ hyperon binding energy. The Λ hyperon is a baryon composed of an up, a down, and a strange quark, and it is one possible form of strange matter expected in neutron stars. The hyperon binding energy embodies how strongly it interacts with the nucleons that are also in a star’s core.

The RHIC researchers measured the hyperon binding energy by tracking the decay products from the hypertriton, a lightweight nucleus composed of a proton, a neutron, and a Λ hyperon. Collisions of gold ions produce hypertritons, which decay after traveling a few centimeters. The decay products enter a detector, where they travel through a 0.5 T magnetic field. The curvature of the trajectories indicates each particle’s momentum, and from that information, the researchers compute the hypertriton’s mass before decay. They then compare it with the sum of the components’—that is, Λ hyperon and deuteron—masses; the difference is the binding energy.

Credit: J. Adam et al., Nat. Phys., 2020, doi:10.1038/s41567-020-0799-7

Measurements of the Λ hyperon binding energy in the 1960s and 1970s (see the plot) often yielded values of 0 MeV or very close to it. The RHIC measurements produced a binding energy of about 0.4 MeV, about triple that of the current accepted standard (the 1973 data point) from a nuclear emulsion measurement, which had a larger unknown systematic uncertainty. The larger the binding energy is, the larger the predicted neutron star mass. The RHIC result could help reconcile theory with the recently identified massive neutron stars.

In addition to measuring the binding energy, the RHIC team tested charge, parity, and time (CPT) reversal symmetry, which is often regarded as an exact symmetry of nature. Although systems have been shown to violate individuals or pairs of those symmetries (see, for example, Physics Today, August 2019, page 14), no one has ever demonstrated violation in all three simultaneously. To test for CPT violation for the first time in strange nuclear matter, the researchers compared the mass of the hypertriton with that of the antihypertriton, which was discovered at RHIC in 2010 and is made of the antimatter versions of the same components. If CPT symmetry holds, a hypertriton and an antihypertriton would have the same mass, and that’s what the results showed. (J. Adam et al., Nat. Phys., 2020, doi:10.1038/s41567-020-0799-7.)