Lead-208, with its unique composition of 82 protons and 126 neutrons, has captivated the attention of scientists. Its notable feature is the presence of a neutron “skin” that envelops its nucleus. This article explores the significance of understanding the neutron skin, its implications in quantum chromodynamics, and its relevance to the study of neutron stars.
Scientists recognize the importance of studying the neutron skin to gain insights into quantum chromodynamics. By examining the behavior of quarks and gluons within the nucleus, researchers deepen their understanding of this fundamental force. Furthermore, this research helps elucidate the structure of neutron stars, which form after a supernova explosion when a star between 10 and 25 times the mass of the sun collapses.
The scientists entrusted with unraveling the mysteries of the neutron skin have relied on data from heavy-ion runs conducted by the Large Hadron Collider (LHC) at CERN. Leveraging 670 data points obtained from Runs 1 and 2, primarily from the ALICE experiment, alongside some contributions from ATLAS and CMS, theoretical physicists have successfully determined the thickness of the lead-208 neutron skin to be approximately 0.217±0.058 femtometers.
Measuring the neutron skin presents a formidable challenge due to the absence of charge in neutrons, rendering them unresponsive to electron scattering like protons. Nonetheless, the strong nuclear force, responsible for binding quarks and gluons in atomic nuclei, significantly influences neutrons. To overcome this hurdle, scientists observe the outcome of lead-208 nuclei collisions in the LHC’s heavy-ion runs.
In the moment of collision, the energy and pressure within the colliding lead-208 nuclei rupture the gluons, liberating the quarks and unleashing a quark-gluon plasma. This substance, reminiscent of the early moments after the Big Bang, represents a crucial element in understanding both the structure of lead-208 and the composition of neutron stars. As the temperature and pressure recede, the quark-gluon plasma transforms into detectable particles, enabling physicists to analyze the properties of this unique state of matter.
The distribution of protons and neutrons within lead-208 determines the size and shape of the quark-gluon plasma. By meticulously observing and analyzing the behavior of this plasma, scientists gain valuable insights into the structure of the lead-208 nucleus. These insights, subsequently, allow for the precise calculation of the neutron skin’s thickness, marking the first successful measurement of this parameter via the strong force.
In 2021, the Lead Radius Experiment (PREX) collaboration at the Thomas Jefferson National Accelerator Facility also determined the thickness of the lead-208 neutron skin, obtaining a result of approximately 0.283±0.071 femtometers. However, the PREX experiment employed techniques involving the electroweak force rather than the strong force. Although both approaches offer valuable insights, the recent determination utilizing existing LHC data provides a competitive level of uncertainty when compared to other experimental findings.
The successful determination of the neutron skin’s thickness using existing data marks a significant milestone. Nevertheless, scientists continue to explore more dedicated measurements to enhance the precision of lead-208 neutron skin extraction from LHC data. This pursuit promises to further our comprehension of neutron star formation, quantum chromodynamics, and the intricate nature of atomic nuclei.
The study of lead-208 and its neutron skin not only contributes to quantum chromodynamics research but also sheds light on the enigmatic structure of neutron stars. The collaboration between scientists investigating seemingly disparate areas of physics underscores the interdisciplinary nature of scientific exploration. As our understanding deepens, we unravel the complexities of the universe, one femtometer at a time.