Locked Up Forever: How Physicists Measured the Force That Holds Quarks express
The proton is not a solid ball but a tiny prison. Inside it, three quarks are locked up, and the gluon field keeps them from escaping — something like invisible threads that only grow stronger if the quarks try to move apart. For a long time, this property, known as confinement, remained almost mystical: it was observed in experiments, but the force acting on each quark could not be directly measured.
Recently, a group led by Xiangdong Ji proposed a way to estimate it, and obtained the first convincing numbers. In the new work, the authors improved the calculation. They incorporated results from so-called light-cone sum rules of quantum chromodynamics, like using readings from an additional instrument where ordinary accelerator data and lattice calculations are too imprecise. They also reformulated the problem: instead of fitting the answer to a pre-selected formula, they reconstructed the force as a blurred image — using a regularized inverse problem (a method that removes noise). This reduces dependence on assumptions.
This is not just a test of an idea. Confidence that confinement indeed works through a linear potential brings us closer to understanding the strongest bond in nature. And with future, more precise collider data, the uncertainty of the calculation could be reduced severalfold — making the 'photograph' of the proton's interior even sharper.
If a quark could be pulled out of a proton, the energy expended would immediately create a new quark-antiquark pair, and the lone quark would again find itself confined, but in another particle.