Narrow Results

The directed flow of identified particles can serve as a sensitive tool for investigating the interactions during initial and final states in heavy ion collisions. This study examines the rapidity distribution ($dN∕dy$), rapidity-odd directed flow ($v_1$) and its slope ($d{v}_1∕dy$) for ${\pi }^{\pm }$, $K^{\pm }$, p, and $\overline{\mathrm{\text{p}}}$ in Au+Au collisions at different collision centralities and beam energies ($\sqrt{s_{\mathrm{\text{NN}}}}=7.7$, 11.5, 14.5, 19.6, 27, and 39GeV) using the UrQMD model. We investigate the impact of late-stage hadronic interactions on charge-dependent $v_1(y)$ and its slope by modifying the duration of the hadronic cascade lifetime ($\tau$). The energy dependence of $d{v}_1∕dy$ for p ($\overline{\mathrm{\text{p}}}$) exhibits distinct pattern compared to ${\pi }^{\pm }$ and $K^{\pm }$. Notably, we observe a change in the sign reversal position of proton $d{v}_1∕dy$ at different beam energies with varying $\tau$ in central and mid-central collisions. Moreover, the difference in $d{v}_1∕dy$ between positively and negatively charged hadrons ($\mathrm{\Delta }d{v}_1∕dy$) demonstrates a stark centrality dependence for different particle species. The deuteron displays a significant increase in $d{v}_1∕dy$ with increasing $\tau$ compared to p and n. This investigation underscores the importance of considering the temporal evolution and duration of the hadronic phase when interpreting the sign reversal, charge splitting of $v_1$ and light nuclei formation at lower RHIC energies.

We describe azimuth structure commonly associated with elliptic and directed flow in the context of 2D angular autocorrelations for the purpose of precise separation of so-called nonflow (mainly minijets) from flow. We extend the Fourier-transform description of azimuth structure to include power spectra and autocorrelations related by the Wiener–Khintchine theorem. We analyze several examples of conventional flow analysis in that context and question the relevance of reaction plane estimation to flow analysis. We introduce the 2D angular autocorrelation with examples from data analysis and describe a simulation exercise which demonstrates precise separation of flow and nonflow using the 2D autocorrelation method. We show that an alternative correlation measure based on Pearson's normalized covariance provides a more intuitive measure of azimuth structure.

Rapidity-odd directed flow ($v_1$) of multi-strange baryons ($\mathrm{\Xi }$ and $\mathrm{\Omega }$) at mid-rapidity is reported for Au+Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC). We focus on particle species where all constituent quarks are produced, such as $K^{-}(ūs)$, $\bar{p}(ūū\bar{d})$, $\bar{\mathrm{\Lambda }}(ū\bar{d}\bar{s})$, $\varphi (s\bar{s})$, ${\overline{\mathrm{\Xi }}}^{+}(\bar{d}\bar{s}\bar{s})$, ${\mathrm{\Omega }}^{-}(sss)$ and ${\overline{\mathrm{\Omega }}}^{+}(\bar{s}\bar{s}\bar{s})$, and demonstrate using a novel analysis method that the coalescence sum rule holds in a common kinematic region for hadron combinations with identical quark content. We examine the coalescence sum rule as a function of rapidity for nonidentical quark content having the same quark-level mass but different strangeness ($\mathrm{\Delta }S$) and electric charge ($\mathrm{\Delta }q$). The difference in the directed flow of different quark and anti-quark combinations, e.g., $v_1({\mathrm{\Omega }}^{-}(sss))-v_1({\bar{\mathrm{\Omega }}}^{+}(\bar{s}\bar{s}\bar{s}))$, is a measure of coalescence sum rule violation, and we call it directed flow splitting ($\mathrm{\Delta }{v}_1$). We measure $\mathrm{\Delta }S$ and $\mathrm{\Delta }q$ dependence of the $\mathrm{\Delta }{v}_1$ slope ($d\mathrm{\Delta }{v}_1∕dy$) between produced quarks and anti-quarks in Au+Au collisions at $\sqrt{s_{NN}}=27$GeV and 200GeV. Measurements have been compared with A Multi-Phase Transport (AMPT) and Parton-Hadron String Dynamics (PHSD) models with electromagnetic field calculations.

Using an updated Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, the balance energies of free protons, free neutrons, and Z=1 particles (including free protons, deuterons and tritons) from mass symmetric heavy-ion collisions with isobars (A=132) are studied. We investigated the initial isospin and nuclear symmetry energy effects on the balance energy, and found that the balance energies of free neutrons and Z=1 particles are influenced by these effects, while the balance energy of free protons is not.