Synopsis

# Doubly Strange Nucleus Observed

Physics 14, s15
Particle physicists have detected a short-lived nucleus containing two strange quarks, whose properties could provide new insights into the behavior of other nuclear particles.

Baryons are three-quark particles, such as protons, neutrons, and the lesser-known hyperons. The latter’s claim to fame is that they contain at least one strange quark. Researchers study hyperons to understand how baryons interact. One experiment, known as the J-PARC E07 experiment, has now uncovered a very rare interaction in which a hyperon with two strange quarks binds to a normal nucleus [1]. By measuring precisely how this “hypernucleus” decayed, the team behind the experiment was able to determine the particles’ binding energy.

The collaboration directed a beam of K mesons from J-PARC (the Japan Proton Accelerator Research Complex) into a diamond target. Collisions in the target produced hyperons—specifically $\Xi$ hyperons that are made up of two strange quarks and one down quark. These hyperons then traversed a stack of photographic sheets that recorded the particle tracks. Using an automated track-identification system, the team extracted an event where a $\Xi$ hyperon bonded with a nitrogen nucleus, briefly forming a doubly strange hypernucleus.

Only a handful of doubly strange hypernuclei have been seen before—in fact the events are so rare that they are often given their own names. The J-PARC E07 team has christened their event IBUKI, which is the name of a mountain on the Japanese main island. By identifying the decay products in this event, the team determined that $\Xi$ hyperons bind to nitrogen nuclei with an energy of 1.27 MeV, which agrees with theoretical predictions. As more data are processed, the researchers expect to find roughly ten more $\Xi$-hyperon events, which could provide insights for nuclear physics as well as for theories of neutron star models.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

## References

1. S. Hayakawa et al., “Observation of Coulomb-assisted nuclear bound state of ${\Xi }^{-}$${}^{14}\text{N}$ system,” Phys. Rev. Lett. 126, 062501 (2021).

Nuclear Physics

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