Teasing Strange Matter from Ordinary

Data on Lambda particle formation came from experiments in Experimental Hall B, shown here, part of the Continuous Electron Beam Accelerator Facility.

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The Science

Like the more familiar protons and neutrons, Lambda particles are made up of three quarks bound together by gluons. Unlike protons and neutrons, which contain a mixture of up and down quarks, Lambdas contain an up quark, a down quark, and a strange quark. Nuclear physicists recently made the first observations of how Lambda particles are produced by a specific process called semi-inclusive deep inelastic scattering. The experiment involved colliding electrons into nuclei. Physicists found that these collisions sometimes produce Lambdas when an electron exchanges a packet of energy called a virtual photon with the target nucleus. This virtual photon then interacts with a pair of quarks, known as a diquark. Next, these diquarks find a strange quark and form Lambdas.

The Impact

Physicists are seeking a better understanding of how different particles form as part of their effort to decipher the strong interaction. This is the fundamental force that holds quark-built particles together. The theory that describes the strong interaction is quantum chromodynamics (QCD), and comparing experimental measurements to models of QCD predictions allows physicists to test this theory. The finding that Lambdas form when the virtual photon interacts with a diquark differs from the QCD model’s current predictions. The result indicates that the model may be missing information or is inaccurate. It also indicates that diquarks are present inside the nucleus.

Summary

The new result comes from an experiment conducted in the Continuous Electron Beam Accelerator Facility (CEBAF), a Department of Energy user facility. In the experiment, nuclear physicists tracked what happened when electrons from CEBAF probed the quarks inside protons and neutrons. They directed CEBAF’s electron beam at different targets, including carbon, iron, and lead. When a high-energy electron from CEBAF strikes the target, it breaks apart a proton or neutron inside one of the target’s nuclei. The experiment studied SIDIS production, in which new particles form after the electron probe interacts with a single quark or diquark. The struck object then moves through the nucleus and joins with other quarks to form a new particle.

The original experiment was conducted in 2004. Nearly a decade later, researchers revisited the dataset for this careful analysis. They attempted to extract information about how Lambda particles form in the two fragmentation regions. This analysis showed, for the first time, that Lambdas may form when the electron probe interacts with a diquark. This observation is not predicted by QCD, the theory that describes this process. The researchers hope that future experiments in CEBAF and at the upcoming Electron-Ion Collider may help shed light on this process with higher precision.

Contact

Lamiaa El Fassi
Mississippi State University
le334@msstate.edu

Funding

This work was supported in part by the Department of Energy Office of Science, Office of Nuclear Physics, the Physics and Astronomy Department and Office of Research and Economic Development at Mississippi State University, the Chilean Agencia Nacional de Investigacion y Desarrollo, the Italian Istituto Nazionale di Fisica Nucleare, the French Centre National de la Recherche Scientifique, the French Commissariat á l’Energie Atomique, the United Kingdom Science and Technology Facilities Council, the Scottish Universities Physics Alliance, the National Research Foundation of Korea, and the U.S. National Science Foundation.

Publications

Chetry, T., et al. (CLAS Collaboration), First Measurement of Λ Electroproduction off Nuclei in the Current and Target Fragmentation Regions. Physical Review Letters 130, 142301 (2023). [DOI: 10.1103/PhysRevLett.130.142301]