Matter-antimatter asymmetry is a research topic of fundamental interest, as it is the basis for the observed dominance of matter over antimatter in the universe. 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 antimatter created escapes the rapidly expanding fireball without annihilation, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and study their properties. In this work, we report the first observation of the antimatter hypernucleus $^4_{\bar{\Lambda}}\overline{\hbox{H}}$, composed of a Λ, an antiproton and two antineutrons. This discovery was made through its two-body decay channel $^4_{\bar{\Lambda}}\overline{\hbox{H}} {\rightarrow}^4\overline{\hbox{He}}+\pi^+$ in the STAR experiment at the Relativistic Heavy Ion Collider. We use Kalman Filter algorithm to analyse the data totaling 6.6 billion collision events in the Au+Au, Ru+Ru, and Zr+Zr collisions with the collision energy $\sqrt{s_{NN}}=200$GeV, as well as U+U collisions with the collision energy of $\sqrt{s_{NN}}=193$GeV. We obtained 15.6 signal candidates for the antimatter hypernucleus ${}^{4}_{\bar{\Lambda}}\overline{\hbox{H}}$, with the estimated background counts 6.4 and significance of 4.7. We also obtained 941 ${}^{3}_{\Lambda}\hbox{H}$ signal candidates, 637 ${}^{3}_{\bar{\Lambda}}\overline{\hbox{H}}$ signal candidates and 24.4 $^4_\Lambda\hbox{H}$ signal candidates using the same method.

Figure: Invariant-mass distributions of $^3\hbox{He}+\pi^-$ (A), $^3\overline{\hbox{He}}+\pi^+$ (B), $^4\hbox{He}+\pi^-$ (C) and $^4\overline{\hbox{He}}+\pi^+$ (D).} The solid bands mark the signal invariant-mass regions. The obtained signal count ($N_{\rm Sig}$), background count ($N_{\rm Bg}$), and signal significances ($Z_{\rm count}$ and $Z_{\rm shape}$) are listed in each panel.

Comparing lifetimes between a particle and its corrsponding antiparticle is an important experimental way to test the $CPT$ symmetry and to search for new mechanisms that cause this matter and antimatter asymmetry. In this work, we measured the lifetimes of ${}^{3}_{\Lambda}\hbox{H}$, ${}^{3}_{\bar{\Lambda}}\overline{\hbox{H}}$, ${}^{4}_{\Lambda}\hbox{H}$, and ${}^{4}_{\bar{\Lambda}}\overline{\hbox{H}}$. The lifetime difference between the hypernuclei and antihypernuclei was compared and we haven't found the CPT symmetry broken within the measurement uncertainty. Various production yield ratios among (anti)hypernuclei and (anti)nuclei are measured. We found $\left(^3\overline{\hbox{He}}/^3\hbox{He}\right)\times\left(\bar{p}/p\right)\approx ^4\overline{\hbox{He}}/^4\hbox{He}$ and $\left(^3_{\bar{\Lambda}}\overline{\hbox{H}}/^3_{\Lambda}\hbox{H}\right)\times\left(\bar{p}/p\right) \approx ^4_{\bar{\Lambda}}\overline{\hbox{H}}/^4_{\Lambda}\hbox{H}$, these agree with the thermal model and coalescence picture predictions. And $^4_{\Lambda}\hbox{H}/^4\hbox{He}$ and $^4_{\bar{\Lambda}}\overline{\hbox{H}}/^4\overline{\hbox{He}}$ is expected to be about 4 times higher than $^3_{\Lambda}\hbox{H}/^3\hbox{He}$ and $^3_{\bar{\Lambda}}\overline{\hbox{H}}/^3\overline{\hbox{He}}$, respectively, hinting that $^4_{\Lambda}\hbox{H}$ has an excited state with spin 1. These measurements shed light on their production mechanism. This work has been published by NATURE in August 2024.

Posted Aug 26, 2024

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Polarization phenomena in heavy-ion collisions has drawn great attention since the observation of hyperon global polarization, the non-zero average spin polarization of produced particles along the orbital angular momentum of colliding nuclei. The STAR Collaboration measurement of the polarization along the beam direction relative to the second harmonic event plane, indicated the local vorticity due to the elliptic flow (stronger collective expansion in in-plane than in out-of-plane directions). These experimental results being in contradiction to the hydrodynamical and transport model predictions based on thermal vorticity, posed the "spin sign puzzle" in heavy-ion collisions. The contribution from thermal shear is recently found to be important to explain the data but the intense discussion is still ongoing to solve the puzzle.

Recently, the STAR Collaboration published a paper entitled "Hyperon polarization along the beam direction relative to the second and third harmonics event planes in isobar collisions at √s_NN = 200 GeV" in Physical Review Letter131, 202301 (2023). Our new study extended the polarization measurements due to vorticity originating in the triangular flow, thus providing new independent information. As shown in the figure below, Λ polarization along the beam direction relative to the triangluar flow plane (third harmonic event plane) is found to be similar in phase to the result with the second harmonic event plane, with magnitude exhibiting increasing trend towards peripheral collisions. These results indicate that complex vortical structures are created in heavy-ion collisions. Our results are compared with hydrodynamic model calculations with two different implementations of the thermal shear leading to the opposite sign of polarization and require further investigation. Our new data provide important inputs for resolving the spin puzzle and better understanding of the spin dynamics.

Figure: Centrality dependence of Λ hyperon polarization along the beam direction relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at 200 GeV. Solid bands show hydrodynamic model calculations with contribution from the shear-induced-polarization in two different implementations (SIP_BBP and SIP_LY).

Posted Jan 7, 2024

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One of the primary objectives of the Beam Energy Scan II (BES II), undertaken by the STAR collaboration, is to identify and study the transition from hadronic matter to the Quark Gluon Plasma (QGP). A critical aspect of this research is to discern how nuclear matter transforms from a state of high baryon density nucleons in low-energy heavy ion collisions to a QGP in higher energy collisions. BES II allows us to study this transition by measuring anisotropic flow at various collision energies and observing how the expansion of the medium produced by the collisions changes. Anisotropic flow describes the shape and direction of expansion of the medium produced in heavy-ion collisions, and triangular flow ($v_{3}$) describes the strength of a triangular component during that expansion. In past reports from energies above $\sqrt{s_{\mathrm{NN}}}=27$ GeV, $v_3$ was determined to arise from a random triangular arrangement during some collisions. This description implies that $v_3$ will have no correlation to the reaction plane of collisions as the orientation of the triangle fluctuates with no preferred angle.

STAR has recently published measurements of $v_3$ from Au+Au collisions at the lowest STAR energy of $\sqrt{s_{\mathrm{NN}}}=3.0$ GeV. It was found that protons exhibit a non-zero $v_3$ signal that is correlated to the reaction plane, revealing a stark difference between $v_3$ at low and high energies. This report also includes comparisons to predictions of $v_3$ by the JAM simulation in order to explain the source of this signal. A new triangular arrangement was found at this low energy, and it was discovered that there is a strong connection between this low energy $v_3$ and the equation of state for the medium produced in collisions. This new form of $v_3$ could be a useful tool for studying the equation of state and the transition between hadronic matter and the QGP.

Posted April 22, 2024

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Nuclear science focuses on the origin and structure of the nucleus and the nucleons within it, which account for essentially all the mass of the visible universe. Half a century of investigations has revealed that nucleons themselves are composed of quarks, bound together by gluons, and have led to the development of the fundamental theory of Quantum Chromo-Dynamics (QCD). Recent-generation colliders have precisely measured the collinear parton distributions (1 dimension) inside the nucleon along the longitudinal direction, while the investigation of nucleon structure in transverse momentum and space has been limited. From transverse momentum dependent parton distributions (TMDs) we can obtain an “image” of the proton structure in transverse as well as in longitudinal momentum space (2+1 dimensions).

Figure:Differential cross section of the Z boson as a function of its transverse momentum (left) and transverse single spin asymmetry of the Z boson (right).

The STAR Collaboration has recently published “Measurements of the Z boson cross section and transverse single spin asymmetry in 510 GeV p+p collisions” in Phys. Lett. B 854 (2024) 138715 . In this Letter, we report the first measurement of the Z boson differential cross section as a function of its transverse momentum in p+p collisions at a center-of-mass energy of 510 GeV, shown in the left panel of the above figure. It provides important constraints on the evolution of the TMDs in phase space. We also report the precision measurement of the Z boson transverse single spin asymmetry (TSSA) in transversely polarized p+p collisions at 510 GeV, shown in the right panel of the above figure. The TSSA of the Z boson is sensitive to one of the polarized TMDs, the Sivers function, which is predicted to have the opposite sign in p+p → W/Z + X compared to that which enters in semi-inclusive deep inelastic scattering. This non-universality of the Sivers function is a fundamental prediction from the gauge invariance of QCD. The experimental verification of this sign change hypothesis is a crucial measurement in hadronic physics and provides an important test of QCD factorization. So far, the STAR result cannot conclusively verify the prediction with the current statistics (340 $pb^{-1}$ ). The precision of the TSSA measurement will be improved using an additional 400 $pb^{−1}$ sample of p+p data at 508 GeV that STAR collected in 2022.

Posted May 24, 2024

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