At the origin of the Universe, asymmetry between the amount of created matter and antimatter led to the matter-dominated Universe as we know today. The origins of this asymmetry remain not completely understood yet. High-energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter. Much of the created antimatter escapes the rapidly expanding fireball without annihilating, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and study their properties, hoping to shed some light on existing questions on the asymmetry between matter and antimatter. Here we report the first observation of the antimatter hypernucleus \hbox{$^4_{\bar{\Lambda}}\overline{\hbox{H}}$}, composed of a $\bar{\Lambda}$ , an antiproton and two antineutrons. The discovery was made through its two-body decay after production in ultrarelativistic heavy-ion collisions by the STAR experiment at the Relativistic Heavy Ion Collider. In total, 15.6 candidate \hbox{$^4_{\bar{\Lambda}}\overline{\hbox{H}}$} antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei \hbox{$^3_{\bar{\Lambda}}\overline{\hbox{H}}$} and \hbox{$^4_{\bar{\Lambda}}\overline{\hbox{H}}$} are measured and compared with the lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei and (anti)nuclei are also measured and compared with theoretical model predictions, shedding light on their production mechanisms.
Production yield ratios of various particles with the same baryon number. Results combining all collision systems in this work are shown by filled stars. Open stars show results with only U+U and Au+Au collisions, while quadrangular stars show results with only Zr+Zr and Ru+Ru collisions. Statistical uncertainties and systematic uncertainties are shown by vertical bars and boxes, respectively. Previous measurement results and thermal model predictions are also shown for comparison. Ratio Typle Index, 0:$\frac{{}^{3}\bar{He}}{{}^{3}He}$ 1:$\frac{{}^{3}\bar{He}}{{}^{3}He}\times\frac{\bar{p}}{p}$ 2:$\frac{{}^{4}\bar{He}}{{}^{4}He}$ 3:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{{}^{3}_{\Lambda}H}$ 4:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{{}^{3}_{\Lambda}H}\times\frac{\bar{p}}{p}$ 5:$\frac{{}^{4}_{\bar{\Lambda}}\bar{H}}{{}^{4}_{\Lambda}H}$ 6:$\frac{{}^{3}_{\Lambda}H}{{}^{3}He}$ 7:$\frac{{}^{4}_{\Lambda}H}{{}^{4}He}$ 8:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{^{3}\bar{He}}$ 9:$\frac{{}^{4}_{\bar{\Lambda}}\bar{H}}{^{4}\bar{He}}$
Production yield ratios of various particles with the same baryon number. Results combining all collision systems in this work are shown by filled stars. Open stars show results with only U+U and Au+Au collisions, while quadrangular stars show results with only Zr+Zr and Ru+Ru collisions. Statistical uncertainties and systematic uncertainties are shown by vertical bars and boxes, respectively. Previous measurement results and thermal model predictions are also shown for comparison. Ratio Typle Index, 0:$\frac{{}^{3}\bar{He}}{{}^{3}He}$ 1:$\frac{{}^{3}\bar{He}}{{}^{3}He}\times\frac{\bar{p}}{p}$ 2:$\frac{{}^{4}\bar{He}}{{}^{4}He}$ 3:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{{}^{3}_{\Lambda}H}$ 4:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{{}^{3}_{\Lambda}H}\times\frac{\bar{p}}{p}$ 5:$\frac{{}^{4}_{\bar{\Lambda}}\bar{H}}{{}^{4}_{\Lambda}H}$ 6:$\frac{{}^{3}_{\Lambda}H}{{}^{3}He}$ 7:$\frac{{}^{4}_{\Lambda}H}{{}^{4}He}$ 8:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{^{3}\bar{He}}$ 9:$\frac{{}^{4}_{\bar{\Lambda}}\bar{H}}{^{4}\bar{He}}$
Production yield ratios of various particles with the same baryon number. Results combining all collision systems in this work are shown by filled stars. Open stars show results with only U+U and Au+Au collisions, while quadrangular stars show results with only Zr+Zr and Ru+Ru collisions. Statistical uncertainties and systematic uncertainties are shown by vertical bars and boxes, respectively. Previous measurement results and thermal model predictions are also shown for comparison. Ratio Typle Index, 0:$\frac{{}^{3}\bar{He}}{{}^{3}He}$ 1:$\frac{{}^{3}\bar{He}}{{}^{3}He}\times\frac{\bar{p}}{p}$ 2:$\frac{{}^{4}\bar{He}}{{}^{4}He}$ 3:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{{}^{3}_{\Lambda}H}$ 4:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{{}^{3}_{\Lambda}H}\times\frac{\bar{p}}{p}$ 5:$\frac{{}^{4}_{\bar{\Lambda}}\bar{H}}{{}^{4}_{\Lambda}H}$ 6:$\frac{{}^{3}_{\Lambda}H}{{}^{3}He}$ 7:$\frac{{}^{4}_{\Lambda}H}{{}^{4}He}$ 8:$\frac{{}^{3}_{\bar{\Lambda}}\bar{H}}{^{3}\bar{He}}$ 9:$\frac{{}^{4}_{\bar{\Lambda}}\bar{H}}{^{4}\bar{He}}$
We report the triton ($t$) production in mid-rapidity ($|y| <$ 0.5) Au+Au collisions at $\sqrt{s_\mathrm{NN}}$= 7.7--200 GeV measured by the STAR experiment from the first phase of the beam energy scan at the Relativistic Heavy Ion Collider (RHIC). The nuclear compound yield ratio ($\mathrm{N}_t \times \mathrm{N}_p/\mathrm{N}_d^2$), which is predicted to be sensitive to the fluctuation of local neutron density, is observed to decrease monotonically with increasing charged-particle multiplicity ($dN_{ch}/d\eta$) and follows a scaling behavior. The $dN_{ch}/d\eta$ dependence of the yield ratio is compared to calculations from coalescence and thermal models. Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10% most central collisions at 19.6 and 27 GeV, with a significance of 2.3$\sigma$ and 3.4$\sigma$, respectively, giving a combined significance of 4.1$\sigma$. The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation, and decreases with a smaller $p_{T}$ acceptance. The physics implications of these results on the QCD phase structure and the production mechanism of light nuclei in heavy-ion collisions are discussed.
Invariant p_{T} spectra of primordial antiprotons in Au+Au collisions at SQRT(s_NN) = 11.5 GeV at 40-80% centrality
With the STAR experiment at the BNL Relativisic Heavy Ion Collider, we characterize $\sqrt{s_\mathrm{NN}}$ = 200 GeV p+Au collisions by event activity (EA) measured within the pseudorapidity range $eta$$in$ [-5, -3.4] in the Au-going direction and report correlations between this EA and hard- and soft- scale particle production at midrapidity ($\eta$$\in$ [-1, 1]). At the soft scale, charged particle production in low-EA p+Au collisions is comparable to that in p+p collisions and increases monotonically with increasing EA. At the hard scale, we report measurements of high transverse momentum (pT) jets in events of different EAs. In contrast with the soft particle production, high-pT particle production and EA are found to be inversely related. To investigate whether this is a signal of jet quenching in high-EA events, we also report ratios of pT imbalance and azimuthal separation of dijets in high- and low-EA events. Within our measurement precision, no significant differences are observed, disfavoring the presence of jet quenching in the highest 30% EA p+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV.
We measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion Collider. A significant $v_3$ signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at mid-rapidity, $dv_3/dy|_{(y=0)}$, opposite in sign compared to the slope for directed flow. No significant $v_3$ signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.
We report the systematic measurement of protons and light nuclei production in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The transverse momentum ($p_{T}$) spectra of protons ($p$), deuterons ($d$), tritons ($t$), $^{3}\mathrm{He}$, and $^{4}\mathrm{He}$ are measured from mid-rapidity to target rapidity for different collision centralities. We present the rapidity and centrality dependence of particle yields ($dN/dy$), average transverse momentum ($\langle p_{T}\rangle$), yield ratios ($d/p$, $t/p$,$^{3}\mathrm{He}/p$, $^{4}\mathrm{He}/p$), as well as the coalescence parameters ($B_2$, $B_3$). The 4$\pi$ yields for various particles are determined by utilizing the measured rapidity distributions, $dN/dy$. Furthermore, we present the energy, centrality, and rapidity dependence of the compound yield ratios ($N_{p} \times N_{t} / N_{d}^{2}$) and compare them with various model calculations. The physics implications of those results on the production mechanism of light nuclei and on QCD phase structure are discussed.
Density fluctuations near the QCD critical point can be probed via an intermittency analysis in relativistic heavy-ion collisions. We report the first measurement of intermittency in Au$+$Au collisions at $\sqrt{s_\mathrm{_{NN}}}$ = 7.7-200 GeV measured by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The scaled factorial moments of identified charged hadrons are analyzed at mid-rapidity and within the transverse momentum phase space. We observe a power-law behavior of scaled factorial moments in Au$+$Au collisions and a decrease in the extracted scaling exponent ($\nu$) from peripheral to central collisions. The $\nu$ is consistent with a constant for different collisions energies in the mid-central (10-40%) collisions. Moreover, the $\nu$ in the 0-5% most central Au$+$Au collisions exhibits a non-monotonic energy dependence that reaches a possible minimum around $\sqrt{s_\mathrm{_{NN}}}$ = 27 GeV. The physics implications on the QCD phase structure are discussed.
We report multi-differential measurements of strange hadron production ranging from mid- to target-rapidity in Au+Au collisions at a center-of-momentum energy per nucleon pair of $\sqrt{s_{\rm NN}}=3$ GeV with the STAR experiment at RHIC. $K^0_S$ meson and $\Lambda$ hyperon yields are measured via their weak decay channels. Collision centrality and rapidity dependences of the transverse momentum spectra and particle ratios are presented. Particle mass and centrality dependence of the average transverse momenta of $\Lambda$ and $K^0_S$ are compared with other strange particles, providing evidence of the development of hadronic rescattering in such collisions. The 4$\pi$ yields of each of these strange hadrons show a consistent centrality dependence. Discussions on radial flow, the strange hadron production mechanism, and properties of the medium created in such collisions are presented together with results from hadronic transport and thermal model calculations.
Atomic nuclei are self-organized, many-body quantum systems bound by strong nuclear forces within femtometer-scale space. These complex systems manifest a variety of shapes, traditionally explored using non-invasive spectroscopic techniques at low energies. However, at these energies, their instantaneous shapes are obscured by long-timescale quantum fluctuations, making direct observation challenging. Here we introduce the ``collective flow assisted nuclear shape imaging'' method, which images the nuclear global shape by colliding them at ultrarelativistic speeds and analyzing the collective response of outgoing debris. This technique captures a collision-specific snapshot of the spatial matter distribution within the nuclei, which, through the hydrodynamic expansion, imprints patterns on the particle momentum distribution observed in detectors. We benchmark this method in collisions of ground state Uranium-238 nuclei, known for their elongated, axial-symmetric shape. Our findings show a large deformation with a slight deviation from axial symmetry in the nuclear ground state, aligning broadly with previous low-energy experiments. This approach offers a new method for imaging nuclear shapes, enhances our understanding of the initial conditions in high-energy collisions and addresses the important issue of nuclear structure evolution across energy scales.
Data from Figure 3, panel a, 0.2<p_{T}<3 GeV/c
In relativistic heavy-ion collisions, a global spin polarization, $P_\mathrm{H}$, of $\Lambda$ and $\bar{\Lambda}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_\mathrm{H}$ with decreasing $\sqrt{s_{NN}}$. A splitting between $\Lambda$ and $\bar{\Lambda}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{NN}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\bar{\Lambda}}-P_{\Lambda}<0.24$% and $P_{\bar{\Lambda}}-P_{\Lambda}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the na\"ive extraction of the late-stage magnetic field of $B<9.4\cdot10^{12}$ T and $B<1.4\cdot10^{13}$ T at $\sqrt{s_{NN}}=19.6$ and $27$ GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_\mathrm{H}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.
We report new STAR measurements of the single-spin asymmetries $A_L$ for $W^+$ and $W^-$ bosons produced in polarized proton--proton collisions at $\sqrt{s}$ = 510 GeV as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250 pb$^{-1}$. The results are combined with previous results obtained with 86 pb$^{-1}$. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton--proton data suggests a difference between the $\bar{u}$ and $\bar{d}$ quark helicity distributions for $0.05 < x < 0.25$. In addition, we report new results for the double-spin asymmetries $A_{LL}$ for $W^\pm$, as well as $A_L$ for $Z/\gamma^*$ production and subsequent decay into electron--positron pairs.
Distribution of the product of Q, the TPC recon-structed charge-sign, and $E_T/p_T$ in the BEMC region.
Distribution of the product of Q, the TPC recon-structed charge-sign, and $E_T/p_T$ in the EEMC region.
$E_T^e$ distributions of electron candidate events, background contributions, and sum of backgrounds and $W \rightarrow e\nu$ MC signal in the BEMC region for $-1.1 < \eta_e < -0.5$.
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