Probing the sensitivity of anisotropic flow coefficients to the initial nuclear structure in pO and OO collisions at the LHC

TL;DR

This work examines how α-clustering in the light nucleus $^{16}$O alters the initial overlap geometry and subsequent hydrodynamic evolution in pO and OO collisions at the LHC. Using a hybrid IP-Glasma+MUSIC+iSS+UrQMD framework, it compares clustered and Woods–Saxon density profiles, extracting initial eccentricities $\epsilon_2$, $\epsilon_3$ and final anisotropic flows $v_2$, $v_3$ through two-particle Q-cumulants. The results show that clustering markedly enhances initial spatial anisotropies and final $v_n$ in OO, with a strong dependence on cluster size, while pO exhibits smaller effects; scaling $v_n$ with $\langle\epsilon_n\rangle$ largely erases the density-profile dependence in OO and reveals a common trend across systems. The findings suggest OO collisions as an effective probe of nuclear geometry, and identify the region around $b/r_{\alpha}\approx 1$ as particularly sensitive to α-clustering signatures, offering a potential route to constrain the oxygen density profile in future experiments.

Abstract

RHIC and LHC have injected $^{16}\rm O$ nuclei in their accelerator complexes with a focus on investigating collectivity and the origin of quark-gluon plasma signatures in small collision systems. The $^{16}\rm O$ nuclei are known to possess clusters of $α$-particles ($^{4}\rm He$) inside the nucleus. This paper attempts to study the clustered-nuclear-geometry dependence of anisotropic flow coefficients such as elliptic flow ($v_2$) and triangular flow ($v_3$), which are sensitive to the nuclear geometry of colliding nuclei. The study is performed in pO and OO collisions at $\sqrt{s_{\rm NN}}=9.61$~TeV and 7~TeV respectively, employing a hybrid model encompassing IP-Glasma + MUSIC + iSS + UrQMD. The results of the clustered nuclear geometry are compared with those of the Woods\,--\,Saxon nuclear profile. Both initial and final state anisotropies are estimated. This study is thus one of its first kind, where the study of anisotropic flow coefficients for pO and OO collisions is presented using a hybrid hydrodynamics model. While the effect of $α$-clustering in pO is found to be small, it is significant for each observable studied in OO collisions. It is also observed that the magnitude of this effect has a noteworthy dependence on the size of the \textsuperscript{4}He.

Figures (8)

• Figure 1: Pictorial representation of $\alpha$-clustered $^{16} \rm O$ nucleus (not to scale).
• Figure 2: Charged particle multiplicity density, $\langle dN_{\rm ch}/d\eta \rangle_{|\eta|<0.5}$, normalized to the average number of participating nucleon pairs, $\langle N_{\rm part} \rangle/2$, plotted as a function of $\langle N_{\rm part} \rangle$ in pO, and OO collisions at $\sqrt{s_{\rm NN}}=9.61$ TeV and $7$ TeV, respectively. Estimated statistical uncertainties are well within the marker sizes.
• Figure 3: Upper panel shows the average eccentricity and triangularity as a function of impact-parameter based centrality class (%) for pO collisions at $\sqrt{s_{\rm NN}}=9.61$ TeV (left) and OO collisions at $\sqrt{s_{\rm NN}}=7$ TeV (right) for Woods--Saxon and $\alpha-$clustered nuclear density profiles, using IP-Glasma. The ratios of $\langle\epsilon_{2}\rangle$ and $\langle\epsilon_{3}\rangle$ from Woods--Saxon to $\alpha-$clustered nuclear density profile are shown in the lower panel. Estimated statistical uncertainties are well within the marker sizes.
• Figure 4: Elliptic and triangular flow calculated using the two-particle Q-cumulant method as a function of collision centrality for pO collisions at $\sqrt{s_{\rm NN}}=9.61$ TeV (left) and OO collisions at $\sqrt{s_{\rm NN}}=7$ TeV (right) from the IP-Glasma + MUSIC + iSS + UrQMD framework. Results include both Woods--Saxon and $\alpha-$clustered type $^{16}\rm O$ nuclei. The ratio of $v_{\rm 2}\{2, |\Delta\eta|>1.0\}$ and $v_{\rm 3}\{2, |\Delta\eta|>1.0\}$ from Woods--Saxon to $\alpha-$cluster density profile is shown in the lower panels.
• Figure 5: $v_{n}\{2, |\Delta\eta|>1.0\}$ (left panel) and $v_{n}\{2, |\Delta\eta|>1.0\}$/$\langle\epsilon_{n}\rangle$ (right panel) as a function of average charged-particle multiplicity ($\langle N_{\rm ch}\rangle_{|\eta|<2.5}$) measured in $|\eta|<2.5$ for pO collisions at $\sqrt{s_{\rm NN}}=9.61$ TeV and OO collisions at $\sqrt{s_{\rm NN}}=7$ TeV using IP-Glasma + MUSIC + iSS + UrQMD model, for Woods--Saxon and $\alpha-$cluster density profiles of $^{16}\rm O$ nuclei. Estimated statistical uncertainties are well within the marker sizes.