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

Journal Article · · Nature (London)

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. 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 major 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 distance scales 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 produced vector (spin-1) mesons, namely φ and K∗0 , emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for φ is unexpectedly large, while that for K∗0 is consistent with zero. The observed spin-alignment pattern and magnitude for the φ cannot be explained by conventional mechanisms, while a model with strong force fields2, 3 accommodates 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 a quantity proportional to the intensity of the field of the strong force. Within the framework of the Standard Model, where the strong force is typically described in the quark and gluon language of Quantum Chromodynamics, the field being considered here is an effective proxy description. This is a qualitatively new class of measurement, which opens a new avenue for studying the behaviour of strong force fields via their imprint on spin alignment.

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
Ministry of Education of the People's Republic of China; Brookhaven National Laboratory (BNL); USDOE Office of Science - Office of Nuclear Physics; Lawrence Berkeley National Laboratory (LBNL); National Science Foundation (NSF); Russian Federation - Ministry of Education and Science; National Natural Science Foundation of China (NSFC); National Research Foundation of Korea (NRF); Czech Academy of Sciences; National Science Centre (Poland) (NSC); Rosatom State Atomic Energy Corporation; Bundesministerium für Bildung und Forschung (BMBF); Helmholtz Association of German Research Centres; Japan Society for the Promotion of Science (JSPS)
DOE Contract Number:
AC02-06CH11357
OSTI ID:
1991169
Journal Information:
Nature (London), Vol. 614
Country of Publication:
United States
Language:
English

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