Physics Awards: March 2024

Saturday, March 30, 2024

Nuclear Physics from Particle Physics

A new theoretical analysis connects the results of high-energy particle experiments at the Large Hadron Collider with three-proton correlations inside nuclei.

How triplets of protons and neutrons behave is an important ingredient in theories of dense nuclear matter. Directly observing that behavior is beyond the reach of terrestrial labs. However, Alejandro Kievsky of Italy’s National Institute of Nuclear Physics and his collaborators have now demonstrated that it can be inferred from particle collisions recorded by the ALICE experiment at CERN’s Large Hadron Collider in Switzerland [1].

Last year, the ALICE experiment reported the results of smashing together beams of protons at an energy of 13 TeV. The kaons, protons, and deuterons that reached ALICE’s detectors carried with them correlations that arose when the particles sprang to life from a volume a few femtometers across. To determine what happens in that tiny volume, Kievsky and his collaborators analyzed the case of three squished-together nucleons (neutrons and protons) and worked out what detectable correlations would ensue after they flew apart. The effort required determining the source function, which described the initial state of the three nucleons, and the scattering wave function, which described their ensuing spatial distribution.

The researchers modeled the three-body source function in terms of the hyperradius, a generalized coordinate depending on the three nucleon–nucleon distances. Calculating the scattering wave function entailed casting the problem in terms of a generalization of spherical harmonics known as hyperspherical harmonics. By expanding the scattering wave function in partial waves, the researchers could simultaneously handle the short-range strong nuclear force and the long-range Coulomb force. Accurately treating the latter force was crucial in describing the asymptotic behavior in the most challenging case of all three nucleons being protons.

Kievsky and his collaborators’ analytical framework can cope with triplets of protons, neutrons, and combinations of the two. It can also handle deuterons and mesons. “We have opened the door to a new way of studying three-body systems,” he says. Indeed, he and his collaborators are looking forward to analyzing the correlations among two protons and a Λ baryon already measured by ALICE.

International Research Conference on High Energy Physics and Computational Science

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Thursday, March 28, 2024

Characterizing the “Knee” of High-Energy Cosmic Rays

Using observations made with an array of thousands of particle detectors, researchers have uncovered an important clue about cosmic rays that originate from outside of our Galaxy.

The cosmic rays pummeling Earth’s atmosphere are atoms stripped of their electrons and accelerated to high energies. Where and how cosmic rays are accelerated remains uncertain. However, their energy spectrum is clear. The spectrum follows a descending power law that extends from 10⁹ eV all the way to 10²⁰ eV. One of its most important features is a kink or “knee” at around 4 × 10¹⁵ eV (4 PeV), where the slope steepens. Now researchers from the Large High Altitude Air Shower Observatory (LHAASO) in southwest China have produced the most precise characterization of the knee so far [1]. The LHAASO findings shed new light on the knee’s origin, which has perplexed astronomers and physicists for nearly 70 years.

The LHAASO observations were made using the Square Kilometer Array (KM2A), one of the facility’s three experimental setups. KM2A consists of 5216 electromagnetic particle detectors and 1188 muon detectors spread over an area of 1.36 km². Together, they capture the subatomic fragments produced when cosmic rays rip apart atoms in Earth’s atmosphere. From those captures, researchers can infer the energy and mean mass of the cosmic-ray progenitors.

Thanks to its ability to conduct calorimetric measurements, KM2A is equally sensitive to cosmic rays regardless of their atomic number. This property enabled an accurate measurement of the knee and led to the discovery that the knee coincides with a shift in the mix of cosmic rays—with increasing energy—toward elements with lower atomic number. Cosmic rays above the knee are thought to originate from outside the Galaxy. The finding that they are lighter in mass could help explain their higher energies.