To investigate this, the research group launched experiment to try to bind a kaon to a nucleus. To do the experiment, the researchers decided to use a helium-3 target—a nucleus made up of two protons and a single neutron. By knocking out a neutron from the helium-3 target they were able to greatly reduce the energy of the kaon by using the recoil from the ejection and replacing the neutron with a kaon, forming a tightly bound nucleus with two protons and a single kaon.

“What is important about this research,” says Masahiko Iwasaki, the leader of the team, “is that we have shown that mesons can exist in nuclear matter as a real particle—like sugar that is not dissolved in water. This opens up a whole new way to look at and understand nuclei. Understanding such exotic nuclei will give us insights into the origin of the mass of nuclei, as well as to how matter forms in the core of neutron stars. We intend to continue experiments with heavier nuclei to further our understanding of the binding behavior of kaons.”

Far from being empty, the vacuum of space could be brimming with mysterious virtual particles. We now have a machine powerful enough to tear it apart and see.

By Jon Cartwright

IMAGINE a place far from here, deep in the emptiness of space. This point is light years from Earth, vastly distant from any nebula, star or lonely atom. We have many words for what you would find in such a place: a void, a vacuum, a lacuna. In fact, this nothingness is a sea of activity.

Today the collaboration for the LHCb experiment at CERN’s Large Hadron Collider announced the discovery of two new particles in the baryon family. The particles, known as the Xi_b’^- and Xi_b*^-, were predicted to exist by the quark model but had never been seen before. A related particle, the Xi_b*, was found by the CMS experiment at CERN in 2012. The LHCb collaboration submitted a paper reporting the finding to Physical Review Letters.

Like the well-known protons that the LHC accelerates, the new particles are baryons made from three quarks bound together by the strong force. The types of quarks are different, though: the new X_ib particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton. But the particles are more than just the sum of their parts: their mass also depends on how they are configured. Each of the quarks has an attribute called “spin”. In the Xi_b’^- state, the spins of the two lighter quarks point in the opposite direction to the b quark, whereas in the Xi_b*^- state they are aligned.

“Nature was kind and gave us two particles for the price of one,” said Matthew Charles of the CNRS’s LPNHE laboratory at Paris VI University. “The Xi_b’^- is very close in mass to the sum of its decay products: if it had been just a little lighter, we wouldn’t have seen it at all using the decay signature that we were looking for.”