Evidence of Quadrupole and Octupole Deformations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Zr</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>96</mml:mn></mml:mrow></mml:mmultiscripts><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>Zr</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>96</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Ru</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>96</mml:mn></mml:mrow></mml:mmultiscripts><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mrow><mml:mi>Ru</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>96</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math> Collisions at Ultrarelativistic Energies
Chunjian Zhang, Jiangyong Jia
Abstract
In the hydrodynamic model description of heavy-ion collisions, the elliptic flow ${v}_{2}$ and triangular flow ${v}_{3}$ are sensitive to the quadrupole deformation ${\ensuremath{\beta}}_{2}$ and octupole deformation ${\ensuremath{\beta}}_{3}$ of the colliding nuclei. The relations between ${v}_{n}$ and ${\ensuremath{\beta}}_{n}$ have recently been clarified and were found to follow a simple parametric form. The STAR Collaboration has just published precision ${v}_{n}$ data from isobaric $^{96}\mathrm{Ru}+^{96}\mathrm{Ru}$ and $^{96}\mathrm{Zr}+^{96}\mathrm{Zr}$ collisions, where they observe large differences in central collisions ${v}_{2,\mathrm{Ru}}>{v}_{2,\mathrm{Zr}}$ and ${v}_{3,\mathrm{Ru}}<{v}_{3,\mathrm{Zr}}$. Using a transport model simulation, we show that these orderings are a natural consequence of ${\ensuremath{\beta}}_{2,\mathrm{Ru}}\ensuremath{\gg}{\ensuremath{\beta}}_{2,\mathrm{Zr}}$ and ${\ensuremath{\beta}}_{3,\mathrm{Ru}}\ensuremath{\ll}{\ensuremath{\beta}}_{3,\mathrm{Zr}}$. We reproduce the centrality dependence of the ${v}_{2}$ ratio qualitatively and ${v}_{3}$ ratio quantitatively and extract values of ${\ensuremath{\beta}}_{2}$ and ${\ensuremath{\beta}}_{3}$ that are consistent with those measured at low-energy nuclear structure experiments. STAR data provide the first direct evidence of strong octupole correlations in the ground state of $^{96}\mathrm{Zr}$ in heavy-ion collisions. Our analysis demonstrates that flow measurements in high-energy, heavy-ion collisions, especially using isobaric systems, are a new precision tool to study nuclear structure physics.