STAR: The STAR Collaboration

STAR focus: Evidence of Mass Ordering of Charm and Bottom Quark Energy Loss in Au+Au Collisions at RHIC

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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STAR focus: Measurement of sequential Upsilon suppression in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with the STAR experiment

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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STAR focus: Beam Energy Dependence of Triton Production and Yield Ratio (N_t × N_p/N_d²) 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 (N_t × N_p / N_d²) 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.

Figure: Collision energy, centrality, and pT dependence of the yield ratio N_t × N_p / N_d² 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 p_T 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 p_T 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 (N_t × N_p / N_d²) 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 N_t × N_p / N_d² 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 Previous STAR Focus Features

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.
		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