Authors

Abstract

The strong force, as one of the four fundamental forces at work in the universe, governs interactions of quarks and gluons, and binds together the atomic nucleus. Notwithstandingdecades of progress since Yukawa first developed a description of the force between nucleonsin terms of meson exchange 1, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbativenature of the strong force, which leads to the phenomenon of quark confinement at distancescales on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of producedvector (spin-1) mesons, namely and K0, emerge with a surprising pattern of global spinalignment. In particular, the global spin alignment for is unexpectedly large, while that forK0is consistent with zero. The observed spin-alignment pattern and magnitude for the cannot be explained by conventional mechanisms, while a model with strong force fields2, 3accommodates the current data. This is the first time that the strong force field is experimentally supported as a key mechanism that leads to global spin alignment. We extract aquantity proportional to the intensity of the field of the strong force. Within the frameworkof the Standard Model, where the strong force is typically described in the quark and gluonlanguage of Quantum Chromodynamics, the field being considered here is an effective proxydescription. This is a qualitatively new class of measurement, which opens a new avenue forstudying the behaviour of strong force fields via their imprint on spin alignment.