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STAR focus: Energy Dependence of Intermittency for Charged Hadrons in Au+Au Collisions at RHIC

Relativistic heavy-ion collision experiments serve as one of the primary avenues for studying strong interactions, aiming to explore the properties of Quark-Gluon Plasma (QGP) and the phase structure of Quantum Chromodynamics (QCD), including the search for first-order phase transitions in nuclear matter and the QCD critical point. Based on the 3D Ising-QCD calculations, large density fluctuations are developed near the QCD critical point in the heavy-ion collisions due to the power-law or self-similar structure of the density-density correlation function. Such fluctuations manifest itself as critical intermittency and can be probed via the framework of intermittency analysis by utilizing the scaled factorial moments (SFMs). The energy dependence of intermittency index and scaling exponent, extracted from the power-law scaling of SFMs, is proposed for the search of the QCD critical point.

Recently, the STAR Collaboration published significant results titled “Energy Dependence of Intermittency for Charged Hadrons in Au+Au Collisions at RHIC” in Physics Letters B 845, 138165 (2023). The paper 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). 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 minimum around $\sqrt{s_\mathrm{_{NN}}}$ = 27 GeV. The observed non-monotonic energy dependence of $\nu$ in the most central collisions may be due to the signal of density fluctuations induced by the QCD critical point. These systematic measurements of intermittency for charged hadrons over a broad energy range provide important insights into our understanding of the QCD phase diagram.

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FIG. 1. Energy dependence of the scaling exponent ($\nu$) for identified charged hadrons ($h^{\pm}$ in Au$+$Au collisions at $\sqrt{s_\mathrm{_{NN}}}$ = 7.7-200 GeV. Red circles and blue squares represent $\nu$ in the most central collisions (0-5$\%$) and the mid-central collisions (10-40$\%$), respectively. The statistical and systematic errors are shown in bars and brackets, respectively.

Posted Nov 13, 2023

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STAR focus: Hyperon polarization along the beam direction relative to the second and third harmonics event planes in isobar collisions at √sNN = 200 GeV

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 √sNN = 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 (SIPBBP and SIPLY).

Posted Jan 7, 2024

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STAR focus: Beam Energy Dependence of Triton Production and Yield Ratio (Nt × Np/Nd2) in Au + Au Collisions at RHIC

Relativistic heavy-ion collision experiments serve as one of the primary avenues for studying strong interactions, aiming to explore the properties of Quark-Gluon Plasma (QGP) and the phase structure of Quantum Chromodynamics (QCD), including the search for first-order phase transitions in nuclear matter and the QCD critical point, which marks the endpoint of the phase boundary. Light nuclei are among the main products in the final stages of heavy-ion collision experiments, characterized by a small binding energy and finite radius, making them effective probes for exploring phase boundaries and critical points in the QCD phase diagram. Theoretical studies propose that the compound yield ratio of light nuclei based on the nucleon coalescence model (Nt × Np / Nd2) is directly correlated with local neutron density fluctuations in the system, providing a sensitive measurement for the search of the QCD critical point and first-order phase transitions.

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Figure: Collision energy, centrality, and pT dependence of the yield ratio Nt × Np / Nd2 in Au + Au collisions at RHIC. Solid circles are the results from 0%–10% central (left panel) and 40%–80% peripheral (right panel) collisions. Colored bands in panel (a) denote pT acceptance dependence, for which the statistical and systematic uncertainties are added in quadrature. Red solid circles are the final results with extrapolation to the full pT range. Statistical and systematic uncertainties are shown as bars and brackets, respectively. Red vertical bands on the right side of panels represent the common systematic uncertainties. Dashed lines are the coalescence baselines obtained from the coalescence-inspired fit. Shaded areas denote the calculations from hadronic transport AMPT and MUSIC + UrQMD hybrid models.

Recently, the STAR Collaboration published significant results titled "Beam Energy Dependence of Triton Production and Yield Ratio (Nt × Np / Nd2) in Au + Au Collisions at RHIC" in PHYSICAL REVIEW LETTERS 130, 202301 (2023). The paper focuses on the measurement of triton yields, the extraction of primordial proton yields, and the results of light nuclei compound yield ratio in the STAR BES-I. We observed that the     Nt × Np / Nd2 ratio exhibits scaling behavior with the system volume, and most importantly, a significant deviation from the model baseline at 4.1σ was observed in the 0%-10% central collisions at $\sqrt{s_{\mathrm{NN}}}$ = 19.6 and 27 GeV, which may be due to the enhanced baryon density fluctuations induced by the critical point or first-order phase transition in heavy-ion collisions. These systematic measurements of triton yields and yield ratios over a broad energy range provide important insights into the production dynamics of light nuclei and our understanding of the QCD phase diagram.

Posted June 17, 2023

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STAR focus: Measurement of electrons from open heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with the STAR detector

Studying the properties of the Quark Gluon Plasma (QGP) created in heavy-ion collisions is a main goal of the RHIC physics program. Heavy quarks, i.e., charm and bottom quarks, have emerged as essential probes of the QGP as they are produced predominantly at the initial stage of the heavy-ion collisions and subsequently experience the entire evolution of the QGP. In particular, heavy quarks lose energy through interactions with the QGP via both collisional and radiative processes. These interactions modify the momentum distributions of heavy quarks in heavy-ion collisions compared to that in p+p collisions, and measurements of such modifications provide important insights into the properties of the QGP. Experimentally, nuclear modification factor, $R_{\rm{AA}}$, is measured to study the parton energy loss.

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Figure: HFE $R_{\rm{AA}}$ (red circles) as a function of $p_{\rm{T}}$ in different centrality intervals of Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, compared with STAR (yellow stars) and PHENIX (green squares) published results, and Duke (blue line) and PHSD (orange line) model calculations. Vertical bars and boxes around data points represent combined statistical and systematic uncertainties from both Au+Au and p+p measurements, respectively. Boxes at unity show the global uncertainties, which for this analysis include the 8% global uncertainty on p+p reference and the $N_{coll.}$ uncertainties. The left box is for PHENIX and the right one for STAR.

Recently, the STAR Collaboration published "Measurement of electrons from open heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with the STAR detector " in JHEP 06 (2023) 176. In this publication, the new measurements of $R_{\rm AA}$ for inclusive heavy flavor-decay electrons in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV are reported. We find the HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in p + p collisions scaled by the average number of binary collisions ($N_{coll.}$), indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the QGP. Furthermore, these results provide an improved reference for $R_{\rm{AA}}$ measurements of charm- and bottom-hadron decayed electrons in heavy-ion collisions.

Posted Jul 7, 2023

Previous STAR Focus Features

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March 13, 2024
Please join us in congratulating Dr. Jagbir Singh from Panjab University in Chandigarh, who, on February 16, 2024, successfully defended his Ph.D. thesis, "Investigation Of Elliptic Flow And Chiral Magnetic Effect With The Star Detector.” His supervisors were Drs. M.M. Aggarwal and A.K. Bhati. Jabgbir plans on continuing in academia, and we wish him all the best. Congratulations, and many thanks to Dr. M.M. Aggarwal and Dr. A.K. Bhati. for their continued contribution to educating the next generation of physicists.

March 13, 2024
Please join us in congratulating Dr. Yiding Han from Rice University (Houston, TX), who, on March 5, 2024, successfully defended his Ph.D. thesis, "Thermal dielectron measurements in Au+Au collisions at sqrt(sNN) = 14.6, and 19.6 GeV with the STAR experiment." His supervisor was Prof. Frank Geurts (Rice University). Yiding will continue his career in medical physics at Baylor College of Medicine. We wish him all the best in his future career.

March 4, 2024
Please join us in congratulating Dr. Lukas Kramarik from Czech Technical University in Prague, who, on February 28, 2024, successfully defended his Ph.D. thesis, "Open Charm Production at STAR." His supervisor was Prof. Jaroslav Bielcik and co-advisor Xin Dong (LBNL). Congratulations, and many thanks to Prof. Bielcik and Dr. Dong for their continued contribution to educating the next generation of physicists. Lukas will continue his career in the private sector. We wish him all the best in his future career.

November 25, 2023
Please join us in congratulating Dr. Pawel Szymanski from Warsaw University of Technology, who successfully defended his Ph.D. thesis “Femtoscopy of meson-meson and meson-baryon systems obtained in heavy ion collisions in the Beam Energy Scan program in the STAR experiment.” His supervisor was Prof. Hanna Zbroszczyk. Pawel is moving to industry and we wish him all the best in his future career.

November 14, 2023
Please join us in congratulating Dr. Tong Liu from Yale University who last Tuesday successfully defended his Ph.D. thesis titled “Inclusive Hadron Yield Analysis in Small and Mid-sized Collision Systems at 200 GeV at STAR.” His supervisor was Prof. Helen Caines. They are preparing these results for publication. Tong has accepted a position with Squarepoint Capital. We wish him all the best in his future career.


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