Distorted quarkonia and spin alignment

TL;DR

This paper explores how measurements of spin alignment in quarkonia can reveal their shape changes under electromagnetic fields, providing a new probe of their internal structure in high-energy collisions.

Contribution

It demonstrates that spin alignment measurements can distinguish between spin state mixing and shape change contributions in quarkonia.

Findings

Spin alignment is affected by both spin state mixing and shape change.

Proper axis choice can isolate the shape change contribution.

Spin alignment measurements serve as a probe of quarkonium structure.

Abstract

It is well-known that atoms can change their shape when subject to external electromagnetic fields. Analogous phenomenon is expected for particles, which are much smaller in size with the corresponding shape change much harder to observe. We point out that 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 contribution from spin states mixing and orbital contribution from shape change. By a proper choice of quantization axis, it is possible to switch off the spin contribution, leaving only the orbital contribution. This makes spin alignment measurement a valuable probe of quarkonium structure.