Accelerators sometimes produce particles that don't fit the usual two- or three-quark scheme. Two such particles, Zb(10610) and Zb(10650), have been puzzling physicists for over a decade. They look like heavy cousins of pions, but with a clear sign of a four-quark composition. The question is how those four quarks are packed.
There are two main possibilities. First: all four quarks sit in a single dense clump — a tetraquark. Second: two quarks form one meson, the other two form a second meson, and these two mesons are bound together at a distance, like atoms in a molecule. For Zb(10610) and Zb(10650), mixed scenarios are also possible — the particle spends part of its time as a clump and part as a meson pair.
The authors of the new study propose a way to distinguish these scenarios without directly 'dissecting' the particle. They calculated what happens when a beam of high-energy photons (61–90 GeV) is aimed at a carbon or tungsten nucleus. A photon can collide with a proton or neutron in the nucleus and produce a Zb particle. The particle then travels through the nuclear matter, and here its internal structure shows itself. A compact tetraquark passes through the nucleus almost unimpeded, like a bullet through fog. A loose two-meson molecule, being larger, interacts more often with nucleons and gets absorbed, like a sponge that's harder to push through a fine mesh.
The calculations show that the yield of Zb particles and their momentum distributions differ noticeably between scenarios. For example, at a photon energy of 75 GeV and for particles emitted at small angles (0–5 degrees), the ratio of yields on tungsten versus carbon would be one value for a compact tetraquark and another for a molecular state. These differences are large enough for future experiments to detect reliably.
Planned electron-ion colliders in the US and China will provide the high luminosity and precision needed for such measurements. If the data match one of the predictions, we'll finally learn how these mysterious particles are built. And that, in turn, will help us understand how quarks assemble into more complex combinations than the usual pairs and triples.
So physics is gradually learning to 'see through' exotic hadrons without destroying them. And perhaps within the next few years, the puzzle of the Zb particles will be solved.
The names Zb(10610) and Zb(10650) reflect their masses in megaelectronvolts: 10610 and 10650 MeV/c². That's about 11 times heavier than a proton.
The idea of looking inside a particle without breaking it echoes stories of 'soft' scanning of objects, like in science fiction about non-invasive study of alien artifacts.