[Research Press Release] Particle physics: Quantifying a quirky quark quartet (Nature)
4 December 2025
Measurements of the properties of exotic particles called all-charm tetraquarks, reported by the CMS Collaboration at the Large Hadron Collider (LHC), are published in Nature this week. The results can be used to test theories about the forces that hold protons and neutrons together.
Quarks are fundamental particles that form the building blocks of most known matter, combining to form protons, neutrons, and other composite particles, all known as hadrons. Protons and neutrons are made up of quark triplets, and two-quark composites (mesons) also exist, but evidence at particle accelerators has been found for the existence of more ‘exotic’ hadrons made up of four or five quarks. However, the exact nature of these composites has been uncertain: could these exotic states be combinations of two-quark or three-quark hadrons, or could they be a single compact object? Measurements of four-quark hadrons (tetraquarks) reported by the CMS Collaboration favour the latter explanation, though they are not conclusive.
A series of experiments at the LHC resulted in the observation of all-charm tetraquarks, made up of two quarks and two antiquarks (the antimatter counterpart to a quark). The CMS Collaboration studied these tetraquarks and now report measurements of the charge reversal symmetry, parity and spin of these exotic particles. The results are expressed as quantum numbers: parity and charge reversal symmetries are positive each and spin is 2, all of which suggest that the all-charm tetraquark is more likely a compact object rather than a pair of mesons. The findings advance our understanding of exotic hadrons, which challenge theories of hadronic matter, the authors conclude.
- Article
- Open access
- Published: 03 December 2025
The CMS Collaboration. Determination of the spin and parity of all-charm tetraquarks. Nature 648, 58–63 (2025). https://doi.org/10.1038/s41586-025-09711-7
News & Views: Quarks in ‘exotic’ quartets prefer to stick together
https://www.nature.com/articles/d41586-025-03591-7
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