Investigate the properties of the vacuum in the core
(Nanowerk News) By creating special atoms, RIKEN physicists have measured how the extremely high density of atomic nuclei affects the properties of empty space (Natural Physics, “Restored chiral symmetry at high density of matter observed in pionic atoms”). This could help explain where matter gets most of its mass.
As its name suggests, the strong force is the strongest of the four fundamental forces, about 100 times stronger than the electromagnetic force, the second strongest force. It is so strong that it causes pairs of quarks—the building blocks of protons and neutrons—and antiquarks (quark antiparticles) to spontaneously form in a vacuum.
While these quark-antiquark pairs cannot be observed directly, they change the nature of the vacuum and can be detected through their effects on other processes. In particular, they break the symmetry of the vacuum.
This breaking of the symmetry of the vacuum has a surprising effect. Protons and neutrons are composed of three quarks, but the mass of the quarks is only about 1% of the total mass of the proton or neutron. A significant proportion of their remaining mass is thought to come from the symmetry breaking of the vacuum.
An increase in the temperature or density of the existing matter is theoretically expected to partially restore the symmetry of the vacuum. While several experiments have verified this effect at high temperatures, none have been performed at high material densities.
Now, Kenta Itahashi of the RIKEN Nishina Center and co-workers have measured with high precision vacuum symmetry partial restorations at high densities, finding excellent agreement with theory.
They did this by creating special atoms from metallic lead using the RIKEN Radioactive Isotope Beam Factory, a facility that creates a beam of heavy ions by gradually accelerating them. Instead of having electrons orbiting the nucleus, these atoms have pions—very short-lived particles made up of quarks and antiquarks. Unlike electrons which experience only the electromagnetic force, pions interact with the nucleus through the strong and electromagnetic forces. Because the nucleus is about 100 trillion times denser than normal matter, this allows researchers to investigate the effect of high density on the symmetry of the vacuum.
The experiment itself only took ten days, but the subsequent analysis of the results took nearly ten years to complete. The initial results only slightly overlapped with those predicted by theory, but after correcting for various effects and updating the parameters as more accuracy became available, the results were in good agreement with theory.
“We were surprised by the consistency between the results achieved and the theoretical basis,” Itahashi said. “Thanks to the continuous efforts of our collaborators, we were able to achieve unprecedented accuracy.”
