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The hexadecapole deformation (₄) of the ^238U nucleus has not been determined because its effect is overwhelmed by those from the nucleus' large quadrupole deformation (₂) in nuclear electric transition measurements. In this Letter, we identify the nonlinear response of the hexadecapole anisotropy to ellipticity in relativistic ^238U+ ^238U collisions that is solely sensitive to ₄ and insensitive to ₂. We demonstrate this by state-of-the-art hydrodynamic calculations and discuss the prospects of discovering the ₄ of ^238U in heavy-ion data at the Relativistic Heavy Ion Collider.
Xu et al. (Mon,) studied this question.