Unraveling the Secrets of Neutron Stars

The Continuous Electron Beam Accelerator at Thomas Jefferson National Accelerator Facility
The Continuous Electron Beam Accelerator at Thomas Jefferson National Accelerator Facility
Krishna Kumar and team members at Thomas Jefferson National Accelerator Facility
Krishna Kumar and team members at Thomas Jefferson National Accelerator Facility

Neutron stars, the collapsed cores of dying massive stars, are one of the universe’s mysteries. With the exception of black holes, neutron stars are the smallest and densest things in existence. Thus, any insight into the nature of neutron stars helps illuminate the raw mechanics of the universe, but like black holes, they are difficult to observe directly.

However, new research recently published in “Physical Review Letters,” to which Krishna Kumar, Gluckstern Professor of Physics, and his research group contributed, found some of the stars’ secrets here on earth – in a lump of lead.

The international research team, known as PREx, which consists of over 90 scientists from 30 institutions, conducted their experiments at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility in Virginia. The team noted that lead is a neutron-rich material – it has 126 neutrons to its 82 protons. These neutrons surround the protons, forming a “skin” that “bulges out beyond the protons in a heavy nucleus,” like lead’s, Kumar says. The question is why: why does the neutron skin bulge? By how much? And why does it matter?

The PREx team is the first to observe this neutron skin using electron-scattering techniques, which involved an enormous machine called the Continuous Electron Beam Accelerator Facility (CEBAF). The facility shot a beam of electrons, whose spin was alternated 240 times per second, along a mile-long accelerator into a thin sheet of cryogenically cooled lead. “On average over the entire run, we knew where the right- and left-hand beams were, relative to each other, within a width of 10 atoms,” says Kumar, achieving the “sharpness” required to differentiate between the volumes occupied by neutrons compared to protons in the lead nucleus.

What they discovered is that the neutron skin is about .28 millionths of a nanometer thick – nearly twice as thick as previously theorized. While only a fraction of a millionth of a nanometer might not seem like much at all, the implications are already sending waves through the physics world, in part because they relate to earlier astrophysical observations begun in 2017, when the global astronomy community trained dozens of telescopes on a pair of neutron stars that had collapsed into one another. This cataclysmic event, first discovered by the gravitational wave detector known as LIGO, was direct evidence that neutron star mergers are a significant source for the synthesis of heavy elements in the universe. LIGO’s data provided new information on the nuclear equation of state.

Milliseconds before the final merger, the two neutron stars were so close together that they deformed into “teardrops.” What Kumar calls the “tidal deformability” of the neutron stars is affected by dense matter characteristics similar to the neutron skin in the nucleus of lead – a linkage made possible by the nuclear equation of state. “The same nuclear equation of state that governs the neutron skin of the lead nucleus impacts the bulk properties of neutron stars,” Kumar says.

Though the PREx team only released its research results this week, the Johannes Gutenberg University in Mainz, Germany, is already planning follow-up experiments.