New results from the world’s largest particle accelerator illuminate the structure of the pentaquark, an exotic particle consisting of five quarks bound together.
Quarks, the subatomic particles that make up protons and neutrons, usually bind together in pairs or triplets to form classes of particles called mesons and baryons, respectively. But recent analyses of data taken at the Large Hadron Collider in Geneva, Switzerland have revealed the existence of larger aggregations, like the five-quark pentaquark. Now, scientists have been able to crunch even more data to understand how the quarks are arranged in these odd pentaquark particles. It appears, in fact, that researchers observed a baryon bound to a meson, forming a weird new kind of unearthly molecule.
The LHC machinery accelerates packets of protons to nearly the speed of light, then injects them into a pair of magnetic circles that intersect at four points. The high-energy particles collide, releasing energy and mass in the form of other particles that are otherwise inaccessible here on Earth. Detectors like the LHCb detector sit at these collision points and record the resulting spray of particles. Scientists compare the data to the laws of physics as they understand them, hoping to find unobserved but predicted particles or unexpected deviations from those laws.
Back in 2015 (and confirmed in 2016), scientists first observed a pair of “peaks” in their data analysis, a literal peak on a graph where they saw more hits in the detector than otherwise expected. The peaks indicated the presence of assemblages of five quarks, known as pentaquarks, approximately 4.5 times the mass of a proton. But questions remained unanswered as to the nature of these particles, such as their internal configuration.
After taking a whole lot more data, LHCb researchers detected yet another pentaquark, and determined that one of the pentaquarks discovered in 2015 was actually two pentaquarks close in mass. This time around, the researchers realised that the peaks were very skinny—meaning they were able to get high-resolution measurements of the pentaquarks’ mass.
The high-resolution piece is important, Tomasz Skwarnicki, physics professor at Syracuse University and physicist from the LHCb collaboration, explained to Gizmodo. By the Heisenberg’s uncertainty principle, there’s a relationship between how well you can measure the energy of a particle and how well you can measure the amount of time it takes for the particle to decay. If the particle were to decay quickly, the researchers wouldn’t have been able to get the skinny peaks they observed. The theory that a pentaquark is actually a three-quark particle and a two-quark particle bound together would explain these long lifetimes.
Under this theory, it’s almost like a pair of exotic particles are bound together in some sort of strange molecule that could only exist at the energies created in the LHC. This molecule would be held together by the strong nuclear force rather than electromagnetism, the force that binds most molecules. Don’t expect pentaquarks to find any practical use here on Earth—they decay very quickly. But maybe they exist at the centres of strange objects in space, like neutron stars, Skwarnicki said.
The two-quark-three-quark molecule theory isn’t the only way to explain the observations, though, according to the paper published in Physical Review Letters. As usual, “more experimental and theoretical scrutiny” is required to completely nail down the internal structure of these pentaquarks.
But thanks to more data, we’re getting closer to really understanding them.
Featured illustration: Daniel Dominguez/CERN