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Distorted quarkonia and spin alignment

Guowei Yan1 and Shu Lin1,2

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Phys. Rev. D 113, 054045 – Published 25 March, 2026

DOI: https://doi.org/10.1103/jg98-9vx9

Abstract

It is well known that atoms can change their shape when subject to external electromagnetic fields. Analogous phenomena are expected for particles that are much smaller in size, with the corresponding shape change much harder to observe. We point out that the shape change of particles can be accessed by measurements of their spin alignment. Motivated by recent measurements of spin alignment in relativistic heavy ion collisions, we consider quarkonia in electromagnetic fields as an example. We show that the quarkonia spin alignment receives both spin contributions from spin state mixing and orbital contributions from shape changes. By a proper choice of quantization axis, it is possible to switch off the spin contribution, leaving only the orbital contribution. This makes the spin alignment measurement a valuable probe of quarkonium structure.

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Phys. Rev. D 113, 054045– Published 25 March, 2026
  • Received 2 August 2025
  • Accepted 3 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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