&Bullet; physics 14, p33

The detection of a new particle that contains both charm and strange quarks could offer new insights into the formation of hadrons.

By creating particles from novel combinations of quarks in high-energy particle collisions, physicists can develop theories of quantum chromodynamics that describe how quarks and gluons interact. Now the BESIII collaboration at the Beijing Electron Positron Collider in China has discovered another example of such a combination – a “Tetraquark” called

${Z}_{c\mathrm{so}}$

[1] . The result provides information about how quarks are distributed within a hadron.

The

${Z}_{c\mathrm{so}}$

Tetraquark is a charged particle that consists of four quarks: a charm quark, a charm antiquark, a strange quark and an up antiquark. Theoretical models predicting the existence of this particle also suggest that its Quark components could produce more combinations of quantum states than those in other known particles. Because compared to, for example, three-quark baryons and two-quark mesons

${Z}_{c\mathrm{so}}$

has more possible internal configurations: the quarks could be evenly distributed as a diquark-antidiquark pair, or they could be in a loosely bound “molecular” state.

The researchers discovered the new tetraquark by measuring the momentum of primary particles and decay products that result from electron-positron collisions. Using Monte Carlo simulations to reconstruct the decay paths from these measurements, they were able to select a signal that was the one predicted

${Z}_{c\mathrm{so}}$

$3rd.98th\phantom{\rule{2.22198pt}{0ex}}\text{GeV}/{c}^{2}$

, or 4 times the proton. This signal was detected at the 5.3 sigma level, which is beyond the safety threshold that constitutes a detection.

To understand better

${Z}_{c\mathrm{so}}$

Tetraquark, the team plans to collect further data in an area around the particle energy. These measurements allow them to explore the production and decay mechanisms of the particle and to search for its excited states and its neutral counterpart.

Correction (March 23, 2021): An earlier version gave an incorrect number for the energy targeted by the planned experiments.

–Sophia Chen

Sophia Chen is a freelance science writer based in Columbus, Ohio.

## References

1. M. Ablikim et al. (BESIII Collaboration), “Observation of a structure close to the threshold in
${K}^{+}$

Recoil mass spectra in

${e}^{+}{e}^{–}\to {K}^{+}\left({\mathrm{D.}}_{\mathrm{so}}^{–}{\mathrm{D.}}^{*0}+{\mathrm{D.}}_{\mathrm{so}}^{*–}{\mathrm{D.}}^{0}\right)$

Phys. Rev. Lett.126, 102001 (2021).

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