Pattern of global spin alignment of and K^*0 mesons in heavy-ion collisions

Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange(1), a full understanding of the strong interaction remains a considerable challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that, in relativistic heavy-ion collisions, in which quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely phi and K-*0, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for phi is unexpectedly large, whereas that for K-*0 is consistent with zero. The observed spin-alignment pattern and magnitude for phi cannot be explained by conventional mechanisms, whereas a model with a connection to strong force fields(2-6), that is, an effective proxy description within the standard model and quantum chromodynamics, accommodates the current data. This connection, if fully established, will open a potential new avenue for studying the behaviour of strong force fields. At the Relativistic Heavy Ion Collider, observations of two meson species produced by heavy-ion collisions, phi and K-*0, show surprising patterns of global spin alignment, being unexpectedly large and consistent with zero, respectively.