Session L15: Mini-Symposium: Results from RHIC Beam Energy Scan II (Part 2)

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The first beam energy scan I (BES-I) at the Relativistic Heavy Ion Collider (RHIC) scanned a range of energies from $\sqrt{ s_{NN} } $ = 7.7 to 62.4 GeV, which ended in 2014. The success of the BES-I results justified a new energy scan (BES-II) with higher statistics and detector upgrades. The first collider energy from BES-II, 27 GeV, was run in 2018. This analysis will address the spectra and yields of $\pi$, $K$, and p as a function of rapidity and centrality. Only mid-rapidity spectra for $\pi$, $K$, and p have been published from the BES-I energies. The transverse mass spectra of these particles are crucial to pin down the collision's location on the QCD phase diagram. A look into the relative particle yields as a function of rapidity shows how the chemical freeze-out temperature and chemical potentials vary with rapidity. These measurements that extend beyond mid-rapidity are compared to experimental results extracted from previous mid-rapidity particle yields.   

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Directed flow ($v_{1}$) and elliptic flow ($v_{2}$) are important observables in the relativistic heavy-ion collisions, as they are established during the early stage of the system evolution, which can allow us to access the collective properties of the expanding system. This is an important part of our program for studying the QCD phase structure at RHIC. In this talk, we will present the centrality dependence of identified particle ($\pi^{\pm}$, $K^{\pm}$, p) $v_{1}$ and $v_{2}$ in Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV with the fixed-target mode (beam energy of 3.85 GeV/A) at STAR. The transverse momentum and rapidity dependence of identified particle $v_{1}$ and $v_{2}$ will be discussed. We will also discuss the number of constituent quark (NCQ) scaling in $v_{2}$ and energy dependence of $v_{1}$ and $v_{2}$. These results will be compared to those from STAR BES-I data. In addition, model calculations of $v_{1}$ and $v_{2}$ for those identified hadrons will also be compared to our results.   

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The $\phi$-meson is the lightest bound state of strange quarks($s\overline{s}$). It has a relatively small hadronic interaction cross-section, therefore $\phi$-meson is considered to be a sensitive probe of the earlier dynamics in the heavy-ion collision. Recent measurements by HADES and FOPI on subthreshold $\phi$-meson production show a larger $\phi/K^{-}$ ratio compared to the results at higher energies[1,2], and this larger $\phi/K^{-}$ ratio cannot be described by thermal model calculations with Grand Canonical Ensemble for strangeness. In this presentation, we will report on our first measurements of $\phi$-meson production in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 3$\ $GeV of energy just above the NN threshold. The data were taken in 2018 by the STAR experiment with Fixed Target configuration. $\phi$-mesons are measured through their hadronic decay channel, $\phi\rightarrow K^-+K^+$. After being corrected for the detector acceptance and tracking efficiencies, invariant yields and directed flow of $\phi$-mesons as well as $\phi/K^{-}$ ratio are presented in several centrality intervals and the results will be compared to model calculations. [1], HADES, Phys. Let. B 778, (2018) 403-407. [2], FOPI, Eur. Phys. J. A 52, (2016) 177.   

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The \(\phi\) meson is composed of strange quarks (\(s\overline s\)) and has a small hadron scattering cross section which reduces the influence of rescattering in the later stage of heavy-ion collisions. Thus anisotropic flows of \(\phi\) mesons are sensitive to the early stages of the collisions and are important observables for the study of QCD phase diagram at RHIC. In this talk, we will present measurements of anisotropic flows of \(\phi\) mesons in Au+Au collisions from the STAR fixed-target program (FXT). \(\phi\) mesons are reconstructed through the decay channel \(\phi\rightarrow K^{+}+ K^{-}\). We will compare our new results with STAR Beam Energy Scan I (BES-I) results.   

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Heavy-ion collisions at small beam energies have the potential to reveal the rich phase structure of QCD at large densities. Among the possible phases of dense QCD matter are inhomogeneous regimes, which feature periodic modulations of the spatial structure. We argue that if the matter created in heavy-ion collisions traverses such a regime, it can leave imprints in the correlations of particles. Spatial inhomogeneities result in a characteristic momentum dependence of particle number correlations, which therefore is an experimentally accessible signature of an inhomogeneous regime. As an explicit example, we consider an inhomogeneous pion quantum spin liquid phase which can occur at large density. In this phase we compute net-proton number fluctuations in a large-N limit, as well as momentum-dependent particle number correlations on the freeze-out surface. These results can serve as a first guideline for a systematic search for inhomogeneous phases with heavy-ion collisions.   

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Azimuthal anisotropy scaling functions are presented for identified particle species, spanning beam energies ($\sqrt{s_{NN}}$) from RHIC to the LHC. The scaling functions, which clarify the respective influence of initial-state eccentricity, expansion dynamics, and final-state viscous attenuation, indicate characteristic signatures for the specific viscosity's dependence on the temperature ($T$) and the baryon ($\mu_{B}$), strangeness ($\mu_{S}$), and isospin ($\mu_{I}$) chemical potentials. The extracted scaling coefficients provide new and unique constraints for the detailed characterization of both the phase structure of the QCD phase diagram and its respective phases' transport properties.   

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Study of light nuclei production in heavy-ion collisions, which will reflect their production mechanism and the underlying collision dynamics, is of particular interest for both theoretical and experimental efforts[1]. Comprehensive measurements of light nuclei collectivity $v_1$ and $v_2$ provide valuable information on the nucleon coalescence sum rule and will lead to better understanding of light nuclei production mechanism in such collisions. Particularly, in the collision energy regime of several GeV, the relatively long passing time of the colliding two nuclei naturally leads to cross talk between the spectator matter and the fireball. The light nuclei flow pattern may be strongly affected by the spectator fragments. In this talk, we will present new precise measurements of $v_1$ and $v_2$ for deuterons, tritons, $^3$He, and $^4$He in Au+Au collisions at 3 GeV by the STAR experiment at RHIC. The particle rapidity and transverse momentum dependence of $v_1$ and $v_2$ for these particles will be presented. These results will also be discussed within the framework of nucleon coalescence and compared with available model calculations. [1], Peter Braun-Munzinger and Benjamin Dönigus, Nuclear Physics A. 98, 114 (2019)   

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Light nuclei, such as deuteron and triton, are loosely bound objects. Their yields are expected to be sensitive to the baryon density fluctuations and can be used to probe the signature of the first order phase transition and/or a critical point in heavy-ion collisions.\\ Since 2018, RHIC has started the second phase of beam energy scan program (BES-II), focusing on the energies below 27 GeV. From 2018 to 2020, STAR experiment has taken the data of high statistics Au+Au collisions at 9.2, 11.5, 14.6, 19.6 and 27 GeV (collider mode) and 3.0 - 7.7 GeV (fixed-target mode).\\ In this talk, we will present measurements of light nuclei production in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV measured in 2018 by STAR experiment under Fixed-target mode. We will show the transverse momentum spectra of proton, deuteron, triton, $^{3}\mathrm{He}$, $^{4}\mathrm{He}$ at various rapidity slices. The rapidity and centrality dependence of coalescence parameters $B_2(d)$, $B_3(t)$ and $B_3({^{3}\mathrm{He})$, particle ratios ($d/p$, $t/p$, $t/d$, $^{3}\mathrm{He}/p$ and $^{4}\mathrm{He}/p$), and yield ratios of $N_{p}N_{t}/N_{d}^{2}$, $N_{^{4}He}N_{p}/N_{^{3}He}N_{d}$ and $N_{^{4}He}N_{t}N_{p}^{2}/N_{^{3}He}N_{d}^{3}$ will be also presented. Their physics implications will be discussed.   

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The study of hyperon-nucleon(Y-N) interaction is of great interest in recent years because of its relation to high-density matter systems such as neutron stars. The presence of hyperons inside neutron stars would soften the equation of state, inhibiting the formation of large mass neutron stars. Hypernuclei, bound states of nucleons and hyperons, serve as a probe to study the Y-N interaction. Precise measurements of the lifetime can provide direct information on the Y-N interaction. The data from fixed target Au+Au collisions at $\sqrt{s_{\rm{NN}}}=3$ GeV, taken in 2018 by the STAR detector, is ideal for studying the properties of light hypernuclei, such as ${}^{3}_{\Lambda}$H and ${}^{4}_{\Lambda}$H, due to the large statistics and high production yield. In this talk, lifetime measurements of ${}^{3}_{\Lambda}$H and ${}^{4}_{\Lambda}$H in Au+Au collisions at $\sqrt{s_{\rm{NN}}}=3$ GeV will be presented. The new results will be compared to previous measurements, and physics implications will be discussed.