A quartet of charm quarks

The particles that make up most of the visible universe are held together by the strong force, and the strong force is, in a sense, the most successful and the most intractable part of physics. Its rules are known, its equations are written, and its predictions for the simplest systems are confirmed to extraordinary precision; but the moment the systems grow, the calculations become punishing, and the theory’s behaviour in the messy middle ground remains one of the great unsolved problems of the field. A paper in the 4 December issue of Nature reports new measurements from the CMS Collaboration at the Large Hadron Collider of an exotic family of particles, the all-charm tetraquarks, and each measurement is a new datum for that unsolved middle ground.

The particles in question are made of quarks, the fundamental constituents that combine into protons and neutrons. The simplest particles are built from three quarks or from a quark and its antimatter counterpart, but nature is more inventive than that: quarks can assemble into larger, rarer structures, including particles made of four quarks at once, the tetraquarks. What makes the new measurements unusual is that the four quarks are all of the same kind, all charm quarks, a particle with no quarks of any other flavour to stabilise it. The all-charm tetraquark is an exotic object even by the standards of exotic objects, and measuring its properties is a direct probe of how the strong force holds quarks together.

The results, the collaboration reports, characterise the quartet’s properties, its mass, its width, its behaviour as it decays, with precision that previous observations could not match. The numbers matter because they sit where the theory is hardest to compute. For simple particles the predictions are clean; for four identical heavy quarks, the calculations are at the edge of what methods can do, and the measurements become a test of which theoretical approach comes closest to reality. Each refinement of the measured values narrows the room in which the competing models operate.

The wider significance is the one that runs through all of hadron physics: the strong force is the force that makes the world, and the exotic particles at the LHC are its most demanding examination. A theory that can predict the properties of a four-charm state, that can explain why such a particle exists and how it decays, is a theory that has understood something essential about how quarks behave when the rules are hardest. The exotic hadrons discovered and measured over the past decade have collectively redrawn the map of what the strong force can make, and the all-charm quartet is one of the strangest shapes on that map.

The paper is a step, not an end. The measurements will be refined, the states will be examined at higher precision, and the theorists will go back to their calculations with the new numbers. But the direction is the right one. The particles that were once dismissed as impossibly exotic are now measured with the precision of established physics, and every improvement in their numbers is progress on the oldest question in the field: how the force that builds the universe’s matter actually works at the scale where it is hardest to see.