Exploring the nuclear momentum anisotropy based on intermediate-energy heavy-ion collisions

TL;DR

This study uses simulations of uranium-uranium collisions at intermediate energies to explore how momentum anisotropy affects elliptic flow and particle production, revealing significant geometric and flow effects.

Contribution

It introduces the quadrupole deformation parameter in momentum space and analyzes its impact on elliptic flow and particle yields in heavy-ion collisions.

Findings

Oblate momentum density enhances elliptic flow $v_2$

Prolate momentum density suppresses $v_2$

Momentum anisotropy influences pion production and flow slope

Abstract

We simulate ultra-central collisions of prolate uranium-uranium nuclei at intermediate energies using the isospin-dependent Boltzmann-Uehling-Uhlenbeck model to investigate the impact of momentum anisotropy on spatial geometric effects. By defining the quadrupole deformation parameter in momentum space $\beta_\text{p}$, we establish an ellipsoidal Fermi surface, aligning its rotational symmetry axis with the one in coordinate space. It is found that oblate momentum density enhances elliptic flow $v_2$, while prolate momentum density has the opposite effect, particularly pronounced in the outer, high transverse momentum $p_\text{t}$ region. Momentum anisotropy also causes differences in the initial momentum mean projection along the beam direction, with larger projections producing more pion mesons. Additionally, significant effects on mean square elliptic flow are observed in non-polarized collisions. We further examine the relationship between the $v_2$-$p_\text{t}$ slope and $\beta_\text{p}$, eliminating systematic errors through the two-system ratio. These findings provide important references for experimentalists in heavy-ion collisions and valuable feedback to theorists regarding nuclear structure.