A large Hadron Collider experiment reveals the extraterrestrial structure of a "Pentaquark"

New results from the largest particle accelerator in the world illustrate the structure of pentaquark, an exotic particle of five quarks linked together.

Quarks, the subatomic particles that make up protons and neutrons, usually bind in pairs or triplets to form classes of particles called mesons and baryons, respectively. But recent analyzes of data collected at the Large Hadron Collider in Geneva, Switzerland, have revealed the existence of larger aggregations, such as the five-quark pentaquark. Now scientists have been able to collect even more data to understand how quarks are arranged in these strange pentaquark particles. In fact, it seems that researchers have observed a baryon bound to a meson, forming a strange new type of supernatural molecule.

The LHC machine accelerates the proton bunches almost to the speed of light, then injects them into a pair of magnetic circles that intersect at four points. High energy particles collide, releasing energy and mass in the form of other particles inaccessible elsewhere on Earth. Sensors such as the LHCb detector are at these collision points and record the resulting particle projection. Scientists compare data to the laws of physics as they understand them, hoping to find unobserved but predicted particles or unexpected deviations from these laws.

In 2015 (and confirmed in 2016), scientists first observed a pair of "spikes" in their data analysis, a literal peak on a graph where they saw more hits in the detector than expected. The peaks indicated the presence of assemblages of five quarks, called pentaquarks, about 4.5 times the mass of a proton. But questions have remained unanswered about the nature of these particles, such as their internal configuration.

After taking a lot more data, LHCb researchers detected another pentaquark and determined that one of the pentaquarks discovered in 2015 was actually two pentaquarks whose mass was close. The researchers then realized that the peaks were very lean, which allowed them to obtain high resolution measurements of the mass of pentaquarks.

The high resolution piece is important, says Tomasz Skwarnicki, professor of physics at Syracuse University and physicist of the LHCb collaboration in Gizmodo. According to Heisenberg's Uncertainty Principle, there is a relationship between the extent to which you can measure the energy of a particle and how you can measure the time it takes the particle to disintegrate. If the particle disintegrated quickly, the researchers would not have been able to obtain the lean peaks they had observed. The theory that a pentaquark is actually a three-quark particle and a two-quark particle bound together would explain these long lifetimes.

According to this theory, it almost looks like a pair of exotic particles that are linked together by some sort of strange molecule that could only exist at the energies created in the LHC. This molecule would be maintained by the powerful nuclear force rather than by electromagnetism, the force that binds most molecules. Do not expect pentaquarks to find practical use here on Earth – they break down very quickly. But perhaps they exist at the center of strange objects in space, like neutron stars, said Skwarnicki.

The theory of two quark-three-quark molecules is not the only way to explain the observations, although, according to the paper published in Physical Review Letters. As usual, "a more experimental and theoretical examination" is needed to fully understand the internal structure of these pentaquarks.

But thanks to more data, we are getting closer to really understanding them.