Title: Observation of the nuclear octupole collectivity in relativistic isobar collisions at STAR
PA:  Jiangyong Jia, Shengli Huang, Chunjian Zhang

Target Journal: Nature Physics
Paper Draft:

In preparation 

Analysis Note:

In preparation 

Abstract:

In this letter, we report measurements for the anisotropic flow ($v_n$, n=2, 3) ratio and the mean transverse momentum ($\left\langle p_T \right\rangle$) variance ratio between $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr as a function of corrected multiplicity at $\sqrt {s_{NN}}$ = 200 GeV from STAR collaboration.  Both $v_n$ and $\left\langle p_T \right\rangle$ are measured with the charged particle tracks in $|\eta| < 1$ and 0.2 $< p_{T} <$ 2 GeV/c. The nonmonotonic behavior in $v_2$ ratio has been found indicating large quadrupole deformation in $^{96}_{44}$Ru. While the $v_3$ ratio shows continuous decrease from peripheral to central collisons implying the clear octupole deformation in $^{96}_{40}$Zr nuclei. Meanwhile, Ru+Ru over Zr+Zr ratio of the $\left\langle\delta \mathrm{p}_{T}^{2}\right\rangle /\left\langle p_{T}\right\rangle^{2}$ also shows consistent nuclear deformation effect in the whole centrality regions.

Figure1: 

The ratio of Ru+Ru to Zr+Zr for elliptic anisotropy $v_2$ (left) and the triangular anisotropy $v_3$ (right) as a function of corrected multiplicity in $|\eta|<0.5$. The statistical and systematic uncertainties are shown as vertical error bars and bands, respectively. Comparisons with AMPT are also shown with open markers.

Figure2:

Ru+Ru over Zr+Zr ratio of the $\left\langle\delta \mathrm{p}_{T}^{2}\right\rangle /\left\langle p_{T}\right\rangle^{2}$ as a function of corrected multiplicity in $|\eta|<0.5$. The statistical and systematic uncertainties are shown as vertical error bars and bands, respectively. Comparisons with Monte CarloGlauber are also shown with open markers.

Conclusions:

In summary, the measurement of the anisotropic flow ($v_n$, n=2, 3) ratio and the mean transverse momentum ($\left\langle p_T \right\rangle$) variance ratio between $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr as a function of corrected multiplicity at $\sqrt {s_{NN}}$ = 200 GeV from STAR collaboration. The nonmonotonic trend in $v_2$ ratio indicates large quadrupole deformation in $^{96}_{44}$Ru and possible radial structure in $^{96}_{40}$Zr. The $v_3$ ratio shows continuous decrease implies the clear octupole deformation in $^{96}_{40}$Zr nuclei. Meanwhile, Ru+Ru over Zr+Zr ratio of the $\left\langle\delta \mathrm{p}_{T}^{2}\right\rangle /\left\langle p_{T}\right\rangle^{2}$ also shows consistent nuclear deformation effect in the whole centrality regions. This is the direct observation of $^{96}_{40}$Zr octupole deformation using heavy-ion collisions. Isobar collision is a new tool to study the nuclear structure. It opens up new opportunity to study nuclear structure by picking any isobar pairs with the same mass number along the stable nuclei island at a very short time scale ($\sim 10^{-24}$s) through heavy-ion collisions.

Relevant presentations:

•Bulk-corr:
    Chunjian Zhang (Sep. 29 2021) https://drupal.star.bnl.gov/STAR/system/files/isobar_nuclear_deformation_0929_Collmeeting_czhang_v3.pdf 
    Chunjian Zhang (Sep. 8 2021) https://drupal.star.bnl.gov/STAR/system/files/isobar_nuclear_deformation_0908_bulkcorr_czhang.pdf
    Chunjian Zhang (Aug. 4 2021) https://drupal.star.bnl.gov/STAR/system/files/isobar_nuclear_deformation_0804_bulkcorr_czhang.pdf
    Chunjian Zhang (July 9 2021) https://drupal.star.bnl.gov/STAR/system/files/isobar_nuclear_deformation_0608_bulkcorr_czhang.pdf
    Shengli Huang (Feb. 24 2021) https://drupal.star.bnl.gov/STAR/blog/slhuang/vn-pt-correlation-isobar-0
    Chunjian Zhang ( Jan. 13 2021) https://drupal.star.bnl.gov/STAR/system/files/isobar_bulk_corr_0113_ChunjianZhang.pdf