LHC shines light on mysterious particles

The structure of a class of exotic particles, called pentaquarks, is being revealed by experiments at the Large Hadron Collider...

Quarks are sub-atomic particles that combine together to form the components from which the atoms that make up the matter around us are themselves made. Indeed, as Tomasz Skwarnicki of Syracuse University points out, “we are made out of quarks!” Break up an atom, and you’ll find them inside.

Physicists knew from previous experiments that quarks usually assemble into groups of three to produce familiar particles like protons and neutrons - also known as baryons - from which atoms are made. They can also make more exotic particles called “mesons”, which are combinations of just two quarks.

But, in 2015, with an experiment at the Large Hadron Collider called LHCb, Skwarnicki and his collaborators found evidence for particles composed of assemblages not of two or three quarks, but five. For obvious reasons, these are dubbed "pentaquarks". But although there was evidence for their existence, the physical structure of the pentaquark wasn't known and was a subject of significant debate among theoreticians.

Many of the models were conflicting and disagreed on the sub-structure of the peculiar particles. “There were a lot of theoretical papers published, which proposed different models of pentaquarks,” explains Skwarnicki.

The new study reveals that, rather than being composed of five quarks bonded tightly, and directly, to each other in a so-called “tightly bound” or “compact pentaquark” model, pentaquarks are instead composed of two smaller structures: one contains three quarks, and its partner the other two. The two sub-particles are bonded weakly to each other in what is known as the “molecular model”.

This molecular model of the pentaquark looks remarkably like a baryon (three quarks) and a meson (two quarks) linked loosely together, quite similar to the way atoms join together to form molecules. “This is why we call [these kinds of models] molecular models,” says Skwarnicki.

The team were able to distinguish between the models for two key reasons. First, they found that pentaquarks had long lifetimes, which meant there must be some mechanism to make the particles stable. The molecular model provides this, whereas the tightly-bound model does not.

Additionally, the team discovered a new pentaquark particle, but found that its mass was lower than previously-identified pentaquarks; this also “points in the same direction of the molecular model,” they say.

This new understanding has big implications for particle physics, because it will help scientists to predict what kinds of new particles to look for in experiments at the LHC and elsewhere in the future, although Skwarnicki concedes that this work has little practical impact on our day to day lives at the moment. That said, as he points out, "you never know where some discoveries today might impact technologies of the future!"

Nevertheless, the work will also inform the world of astrophysics. Neutron stars, which are massive cosmic structures composed of trillions upon trillions (about 10⁵⁷) of neutrons, are essentially gigantic clumps of quarks. Skwarnicki believes a deeper understanding of quark structures “could change astrophysical models of these objects.”