The STAR Collaboration has recently published, “Measurement of the longitudinal spin asymmetries for weak boson production in proton-proton collisions at √s = 510 GeV,” in Physical Review D 99, 051102(R).

This paper reports on measurements of the parity-violating asymmetry in the production of positrons and electrons from decays of weak bosons from collisions with one of the proton beams polarized longitudinally. In 510 GeV center-of-mass proton-proton collisions at RHIC, W⁺ bosons are produced primarily in the interactions of up quarks and down antiquarks, whereas W^- bosons originate from down quarks and up antiquarks. The spin-asymmetry measurements of the decay positrons thus provide sensitivity to the up quark and down antiquark helicities in the proton, whereas the decay electrons do so for the down quark and up antiquark helicities. Combined, they make it possible to delineate the light quark and antiquark polarizations in the proton by flavor.

Left: Longitudinal single-spin asymmetries, A_L, for W^± production as a function of the positron or electron pseudorapidity, η_e, for the combined STAR 2011, 2012 and 2013 data samples for 25 < E_T < 50GeV (points) in comparison to theory expectations (curves and bands). Right: The difference of the light sea-quark polarizations as a function of x at a scale of Q² = 10(GeV/c)². The green band shows the NNPDFpol1.1 results and the blue hatched band shows the corresponding distribution after the STAR 2013 W^± data are included by reweighting.

These measurements provided one of the two initial motivations for the spin-physics program at RHIC and the data, shown in the left panel of the figure above, are the final data from STAR on this topic. Shown are the results from previously published data obtained in 2011 and 2012 combined with new results from the dedicated RHIC run in 2013. As seen from the right panel, the data have now reached a level of precision that makes it possible, for the first time, to conclude that the polarization of up antiquarks is larger than that of the down antiquarks.

Posted March 27, 2019

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The STAR Collaboration has recently published “Centrality and transverse momentum dependence of D⁰-meson production at mid-rapidity in Au + Au collisions at √s_NN = 200 GeV,” in Physical Review C 99, 034908 (2019).

This paper presents a new measurement of D⁰ meson production enabled by the Heavy Flavor Tracker (HFT) high-resolution silicon detector system in Au + Au collisions at √s_NN = 200 GeV. Compared to the previous measurement with the TPC only, the new data contain greatly improved precision which allows us to systematically investigate the D⁰ meson production in wide centrality and transverse momentum regions. The new improved data are expected to further constrain our understanding of the charm medium interaction as well as to better determine the medium transport parameters together with the previous publication on D⁰ elliptic flow measurement.

Left: D⁰ RCP for different centrality classes with the 40–60% spectrum as the reference compared to that of other light and strange mesons (π, Κ_s, and φ) as well as the model calculations. Right: Integrated D⁰ cross section per nucleon-nucleon collision at mid-rapidity for p_T > 0 (a) and p_T > 4 GeV/c (b) as a function of centrality Npart. The green boxes on the data points depict the overall normalization uncertainties in p + p and Au + Au data respectively.

The nuclear modification factors RCP of D⁰ meson are shown in the left figure, and show significantly suppression at high p_T and the suppression level is comparable to that of light hadrons at p_T > 5 GeV/c, indicates that charm quarks lose significant energy when traversing through the hot QCD medium. Right figure shows the p_T-integrated D⁰ production cross section per nucleon-nucleon collisions in Au + Au collisions together with the measurement in p + p. The full-p_T integrated cross section in Au + Au collisions seem to be smaller than that in p + p collisions by 1.5σ, indicating that cold nuclear matter effects (CNM) and/or hadronization through quark coalescence may play an important role in Au + Au collisions. While for p_T > 4GeV/c, it shows a clear decreasing trend from peripheral to mid-central and central collisions.

Posted March 27, 2019

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The STAR Collaboration recently has had the paper “Collision-energy dependence of pt correlations in Au+Au collisions at energies available at the BNL Relativistic Heavy Ion Collider” has published in Phys. Rev. C 99, 044918.

The study of event-by-event correlations and fluctuations in global quantities can provide insight into the properties of the hot and dense matter created in Au+Au collisions at RHIC. Correlations of transverse momentum, p_t, have been proposed as a measure of thermalization and as a probe for the critical point of QCD. A detailed study of the of dependence of two-particle p_t correlations on collision energy and centrality may demonstrate the effects of thermalization. If the matter produced in collisions at RHIC passes through the QMD critical point, the fluctuations are predicted to increase with respect to a baseline of uncorrelated emission. A possible signature of the critical point could be non-monotonic behavior of the two-particle correlations as a function of the collision energy in central collisions.

This paper reports two-particle transverse-momentum correlations from Au+Au collisions taken during the RHIC Beam Energy Scan at center of mass energies ranging from 7.7 GeV to 200 GeV. These measurements are compared to previous measurements from the CERES Collaboration at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization.

Left: The relative dynamical correlation for 7.7 GeV and 200 GeV Au+Au collisions compare with similar results fron 2.76 TeV Pb+Pb collision. The dashed line represents a fit to the data at 200 GeV given by 22.3%/(N_part)^1/2. Statistical and systematic errors are shown. Right: The relative dynamical correlation for Au+Au collisions as a function of collision energy for the 0-5% centrality bin along with results for Pb+Pb collisions from ALICE along with UrQMD calculations and results from Boltzmann-Langevin model calculations. The solid line is drawn to guide the eye. Statistical and systematic errors are shown.

Posted April 29, 2018

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The STAR Collaboration has recently published, Azimuthal Harmonics in Small and Large Collision Systems at RHIC Top Energies, in Physical Review Letter, 122, 172301(2019).

This publication reports and compares recent integral and differential measurements (obtained with the STAR TPC), of the flow harmonics ($v_n, n=1-3$) for charged hadrons produced in U+U collisions ($\sqrt{s_{{NN}}}$ = 193 GeV) and Au+Au, Cu+Au, Cu+Cu, d+Au, and p+Au collisions ($\sqrt{s_{{NN}}}$ = 200 GeV). The measurements for these disparate collision-systems, allow systematic variations of the initial-state eccentricity and its fluctuations, as well as the size of the produced fireball at approximately the same collision energy. All are expected to influence the magnitude of $v_n$. The comparisons between the measurements for these small, medium and large collision systems, give unique insights into the role of final-state interactions as the collision-system size is varied. Similarly, the measurements provide more discerning constraints which can aid extraction of the temperature-dependent specific shear viscosity of the hot and dense media created in the collisions.

Two-particle azimuthal correlation functions (a-f) and four-particle cumulants (g) for pT-integrated track pairs. Results are shown for U+U (a) collisions (193 GeV) and Au+Au (b), Cu+Au (c), Cu+Cu (d), d+Au (e) and p+Au (f) collisions (200 GeV) for the same charged particle multiplicity $N_{ch}$. Panel (g) shows the four-particle second-order cumulant vs. $N_{ch}$, obtained with the three sub-events method from the same data sets.

The measurements exploit both the two- and multi-particle correlation techniques to extract $v_n$ as a function of the transverse momentum ($p_T$) and mean charged particle multiplicity $N_{ch}$. The first figure shows representative correlation functions (a-f) and four particle cumulants (g) which accentuate the qualitative similarity between the charged particle azimuthal distributions obtained for the full range of collision-system sizes. Further quantitative study reveals that for a fixed value of $N_{ch}$, the $v^{even}_{1}$ and $v_{3}$ coefficients are essentially independent of the colliding species, indicating that for a given $N_{ch}$, the fluctuation-driven initial-state eccentricities, $\varepsilon_{1}$ and $\varepsilon_{3}$, are system independent. By contrast, the $v_{2}$ coefficients indicate sizeable variations (for fixed $N_{ch}$) with the colliding species, showing the strong collision-system dependence of the shape-driven eccentricity $\varepsilon_{2}$.

Posted May 31, 2018

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