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 are expected lose energy in the QGP via (quasi-)elastic scatterings with the medium constituents and induced gluon radiation, and QCD predicts that heavy quarks lose less energy than light quarks due to the so-called “dead cone” effect. Therefore, parton energy loss in the QGP is expected to follow a hierarchy ordered by parton color charge and mass. Experimentally, nuclear modification factors, R_AA and R_CP, are measured to study the parton energy loss.

Recently, the STAR Collaboration published "Evidence of Mass Ordering of Charm and Bottom Quark Energy Loss in Au+Au Collisions at RHIC" in Eur. Phys. J. C 82 (2022)1150. In this publication, the new measurements of R_AA for inclusive heavy flavor-decay electrons and separately for bottom- and charm-decay electrons, as well as double ratios of bottom- and charm-decay electron R_AA and R_CP in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV are reported. We find the bottom-decay electron R_AA and R_CP to be significantly higher than those of charm-decay electrons. Duke and PHSD model calculations, including mass-dependent parton energy loss in a strongly coupled medium, are found to be consistent with data. These observations provide strong evidence of the mass ordering of charm and bottom quark energy loss in the QGP.

Figure: (a) The R_AA ratio of bottom- to charm-decay electrons in intervals of electron p_T in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. (b) The R_CP ratios of bottom-decay electrons to that of charm-decay electrons in intervals of electron p_T in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. The red diamonds show the ratios of R_CP (0−20%/40−80%), and the blue circles show the ratios of R_CP (0−20%/20−40%). In all panels the error bars and the brackets show statistical and systematic uncertainties, respectively. The Duke and PHSD models are shown as the various lines. The null hypothesis calculations are shown as the shaded bands.

Posted Mar 8, 2023

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A primary goal of the RHIC physics program is to study the properties of the Quark-Gluon Plasma (QGP), a novel state of QCD matter consisting of deconfined quarks and gluons. Among various probes used, Υ mesons, bound states of bottom and anti-bottom quark pairs, play a unique role as they are believed to be produced before the QGP formation, and then get destroyed in the QGP due to the color-screening of the potential between the bottom and anti-bottom quarks as well as scatterings with medium constituents. There are three Υ states (Υ(1S), Υ(2S), Υ(3S)), which possess different amounts of binding energies, with Υ(1S) being bounded the strongest while Υ(3S) the weakest. Consequently, they are expected to experience different levels of yield suppression in the QGP depending on the interplay between the medium temperature and their binding energies. Measurement of such sequential suppression for the three Y states can be used to study the modification of the QCD force in the medium and the QGP’s thermodynamic properties.

Recently, the STAR experiment published the first measurement of yield suppression for the three Y states separately in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, as shown in the figure below (PRL 130 (2023) 112301). The level of suppression is quantified using the nuclear modification factor (R_AA), a ratio between the Y yield in Au+Au collisions to that in p+p collisions scaled properly to account for the trivial geometric difference between nucleon+nucleon and nucleus+nucleus collisions. In the 0-60% centrality class, R_AA for all three Y states are much smaller than 1, indicating large losses of yields in the QGP. It is worth noting that a large fraction of the measured Y(1S) suppression arises from the decreased Y(2S) and Y(3S) yields that contribute to the Y(1S) production through decays. Comparing among different Y states, Y(3S) is significantly more suppressed than Y(1S) while Y(2S) lies in between. The result is consistent with the expected sequential suppression for the Y family in heavy-ion collisions, and can be used to provide additional constraints to model calculations, a necessary step to infer QGP properties from these measurements. With the large sample of Au+Au collisions to be collected by the STAR experiment in 2023 and 2025, a significant improvement in the measurement precision, especially for the excited Y(2S) and Y(3S) states, is foreseen.

Figure: Left: Y(1S) (circles) and Y(2S) (squares) R_AA as a function of N_part for p_T < 10 GeV/c. Data points for Y(2S) are displaced horizontally for better visibility. The vertical bars on data points indicate statistical errors, while the systematic uncertainties are shown as boxes. Shadowed bands around each marker depict the systematic uncertainties from N_coll . The bands at unity indicate the global uncertainties. Right: R_AA for various Y states, including the 95% upper limit for Y(3S), in 0-60% Au+Au collisions.

Posted Mar 15, 2023

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The STAR Collaboration recently published the “Pattern of Global Spin Alignment of φ and K^*0 mesons in Heavy- Ion Collisions” in Nature.

Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely φ and K^*0, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for φ is unexpectedly large, while that for K^*0 is consistent with zero. The ρ₀₀ for φ mesons, averaged over beam energies between 11.5 and 62 GeV is 0.3512 ± 0.0017 (stat.) ± 0.0017 (syst.). Taking the total uncertainty as the sum in quadrature of statistical and systematic uncertainties, our results indicate that the φ-meson ρ₀₀ is above 1/3 with a significance of 7.4σ. Th ρ₀₀ for K^*0, averaged over beam energies of 54.4 GeV and below is 0.3356 ± 0.0034 (stat.) ± 0.0043 (syst.).

Figure: Global spin alignment of φ and K^*0 vector mesons in heavy-ion collisions. The measured matrix element ρ₀₀ as a function of beam energy for the φ and K^*0 vector mesons within the indicated windows of centrality, transverse momentum (p_T ) and rapidity (y). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV. The red solid curve is a fit to data in the range of $\sqrt{s_{\mathrm{NN}}}$ = 19.6 to 200 GeV, based on a theoretical calculation with a φ-meson field. Th red dashed line is an extension of the solid curve with the fitted parameter G_s^(y). The black dashed line represents ρ₀₀ = 1/3.

Measurements of the global spin alignment of vector mesons is argued to provide new knowledge about the vector meson fields. The vector meson fields are an essential part of the nuclear force that binds nucleons inside atomic nuclei, and are also pivotal in describing properties of nuclear structure and nuclear matter. The ρ₀₀ for the φ meson has a desirable feature in that all contributions depend on squares of field amplitudes; it can be regarded as a field analyzer which makes it possible to extract the imprint of the φ-meson field even if the field fluctuates strongly in space-time. Another important feature worthy of mention is that an essential contribution to the φ-meson ρ₀₀ is from the term $~S\cdot(\mathrm{E}_{\phi}\times \mathrm{p})$, where E_φ is the electric part of the φ-meson field induced by the local, net strangenes current density, and S and p are the spin and momentum of the strange (anti)quarks, respectively. Such a term is nothing but the quark version of the spin-orbit force which, at the nucleon level, plays a key role in the nuclear shell structure. Our measurements of a signal based on global spin alignment for vector mesons reveal a surprising pattern and a value for φ meson that is orders of magnitude larger than can be explained by conventional effects. This work provides a potential new avenue for understanding the strong interaction at work at the sub-nucleon level.

Posted January 19, 2023

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Experimental observations at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have confirmed the existence of Quark-Gluon Plasma (QGP) in high-energy heavy-ion collisions at μ_B ~ 0. The transition from hadronic matter to QGP is believed to be a crossover based on Lattice QCD calculations, but there may be a first-order phase transition and a critical point at finite baryon chemical potential in the QCD phase diagram. Higher-order cumulants of conserved quantities (Baryon number, Electric charge, and Strangeness) have been suggested as sensitive observables to search for the critical point and phase boundary in the QCD phase diagram. The fourth-order cumulant ratio (C₄/C₂) of conserved quantities is predicted to exhibit a non-monotonic energy dependence due the critical point. A recent paper on net-proton cumulants from the RHIC beam energy scan program has demonstrated a non-monotonic trend, but with large statistical uncertainty.

Figure: Collision energy dependence of the ratios of cumulants, C₄/C₂, for proton (squares) and net-proton (red circles) from top 5% Au+Au collisions at RHIC. The points for protons are shifted horizontally for clarity. The new result for proton from the $\sqrt{s_{\rm NN}}$ = 3 GeV collisions is shown as a cyan filled square. HADES data of 2.4 GeV top 10% collisions is also shown. The vertical black and gray bars are the statistical and systematic uncertainties, respectively. In addition, results from the transport model UrQMD are presented.

As a longer version of the recent published paper Phys. Rev. Lett. 128, 202303, the paper Phys. Rev. C 107, 024908 [Editors’ Suggestion] summarizes in detail the analysis of proton cumulant and correlation function measurements of STAR fixed-target $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collision data. The paper discusses the effect of pileup which is important in fixed-target data analysis and gives a correction method in higher-moments measurements. In addition, at low collision energy, due to limited reference multiplicity to define centrality, a large initial volume fluctuation effect is seen. A model dependent method to suppress the volume fluctuation effect is tested in this work and shows that the most central centrality is least affected by the effect. However the method is not ideal due to its model dependency.

The above figure shows the net-proton and proton fourth-order cumulant ratio C₄/C₂ from the top 5% most central Au+Au collisions. As one can see there is a clear non-monotonic energy dependence in the data but a monotonic dependence in the model calculation. The cyan filled square at $\sqrt{s_{\rm NN}}$ = 3 GeV represents the new measurement. The consistency of data with UrQMD calculation (yellow cross) implies that the collisions occurred at 3 GeV are dominated by hadronic interactions. It is expected that with more data from the RHIC beam energy scan phase II, the uncertainty will be reduced.

Posted Feb 23, 2023

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