Phys. Rev. D 103, 091103 (2021) We report high-precision measurements of the longitudinal double-spin
asymmetry, $A_{LL}$, for midrapidity inclusive jet and dijet production in
polarized $pp$ ...collisions at a center-of-mass energy of
$\sqrt{s}=200\,\mathrm{GeV}$. The new inclusive jet data are sensitive to the
gluon helicity distribution, $\Delta g(x,Q^2)$, for gluon momentum fractions in
the range from $x \simeq 0.05$ to $x \simeq 0.5$, while the new dijet data
provide further constraints on the $x$ dependence of $\Delta g(x,Q^2)$. The
results are in good agreement with previous measurements at
$\sqrt{s}=200\,\mathrm{GeV}$ and with recent theoretical evaluations of prior
world data. Our new results have better precision and thus strengthen the
evidence that $\Delta g(x,Q^2)$ is positive for $x > 0.05$.
Phys. Rev. Lett. 127, 092301 (2021) We report on the first measurement of charm-strange meson $D_s^{\pm}$
production at midrapidity in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200
GeV from the ...STAR experiment. The yield ratio between strange ($D_{s}^{\pm}$)
and non-strange ($D^{0}$) open-charm mesons is presented and compared to model
calculations. A significant enhancement, relative to a PYTHIA simulation of
$p$+$p$ collisions, is observed in the $D_{s}^{\pm}/D^0$ yield ratio in Au+Au
collisions over a large range of collision centralities. Model calculations
incorporating abundant strange-quark production in the quark-gluon plasma (QGP)
and coalescence hadronization qualitatively reproduce the data. The
transverse-momentum integrated yield ratio of $D_{s}^{\pm}/D^0$ at midrapidity
is consistent with a prediction from a statistical hadronization model with the
parameters constrained by the yields of light and strange hadrons measured at
the same collision energy. These results suggest that the coalescence of charm
quarks with strange quarks in the QGP plays an important role in $D_{s}^{\pm}$
meson production in heavy-ion collisions.
Phys. Rev. Lett. 126, 162301 (2021) Global polarization of $\Xi$ and $\Omega$ hyperons has been measured for the
first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The
measurements of ...the $\Xi^-$ and $\bar{\Xi}^+$ hyperon polarization have been
performed by two independent methods, via analysis of the angular distribution
of the daughter particles in the parity violating weak decay
$\Xi\rightarrow\Lambda+\pi$, as well as by measuring the polarization of the
daughter $\Lambda$-hyperon, polarized via polarization transfer from its
parent. The polarization, obtained by combining the results from the two
methods and averaged over $\Xi^-$ and $\bar{\Xi}^+$, is measured to be $\langle
P_\Xi \rangle = 0.47\pm0.10~({\rm stat.})\pm0.23~({\rm syst.})\,\%$ for the
collision centrality 20\%-80\%. The $\langle P_\Xi \rangle$ is found to be
slightly larger than the inclusive $\Lambda$ polarization and in reasonable
agreement with a multi-phase transport model (AMPT). The $\langle P_\Xi
\rangle$ is found to follow the centrality dependence of the vorticity
predicted in the model, increasing toward more peripheral collisions. The
global polarization of $\Omega$, $\langle P_\Omega \rangle = 1.11\pm0.87~({\rm
stat.})\pm1.97~({\rm syst.})\,\%$ was obtained by measuring the polarization of
daughter $\Lambda$ in the decay $\Omega \rightarrow \Lambda + K$, assuming the
polarization transfer factor $C_{\Omega\Lambda}=1$.
Phys. Rev. D 103, 092009 (2021) The STAR Collaboration reports measurements of the transverse single-spin
asymmetry (TSSA) of inclusive $\pi^0$ at center-of-mass energies ($\sqrt s$) of
200 GeV and ...500 GeV in transversely polarized proton-proton collisions in the
pseudo-rapidity region 2.7 to 4.0. The results at the two different energies
show a continuous increase of the TSSA with Feynman-$x$, and, when compared to
previous measurements, no dependence on $\sqrt s$ from 19.4 GeV to 500 GeV is
found. To investigate the underlying physics leading to this large TSSA,
different topologies have been studied. $\pi^0$ with no nearby particles tend
to have a higher TSSA than inclusive $\pi^0$. The TSSA for inclusive
electromagnetic jets, sensitive to the Sivers effect in the initial state, is
substantially smaller, but shows the same behavior as the inclusive $\pi^0$
asymmetry as a function of Feynman-$x$. To investigate final-state effects, the
Collins asymmetry of $\pi^0$ inside electromagnetic jets has been measured. The
Collins asymmetry is analyzed for its dependence on the $\pi^0$ momentum
transverse to the jet thrust axis and its dependence on the fraction of jet
energy carried by the $\pi^0$. The asymmetry was found to be small in each case
for both center-of-mass energies. All the measurements are compared to
QCD-based theoretical calculations for transverse-momentum-dependent parton
distribution functions and fragmentation functions. Some discrepancies are
found, which indicates new mechanisms might be involved.
Phys. Rev. D 103, 072005 (2021) The STAR Collaboration reports a measurement of the transverse single-spin
asymmetries, $A_{N}$, for neutral pions produced in polarized proton collisions
with protons ...($pp$), with aluminum nuclei ($p\rm{Al}$) and with gold nuclei
($p\rm{Au}$) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral
pions are observed in the forward direction relative to the transversely
polarized proton beam, in the pseudo-rapidity region $2.7<\eta<3.8$. Results
are presented for $\pi^0$s observed in the STAR FMS electromagnetic calorimeter
in narrow Feynman x ($x_F$) and transverse momentum ($p_T$) bins, spanning the
range $0.17<x_F<0.81$ and $1.7<p_{T}<6.0$ GeV/$c$. For fixed $x_F<0.47$, the
asymmetries are found to rise with increasing transverse momentum. For larger
$x_F$, the asymmetry flattens or falls as ${p_T}$ increases. Parametrizing the
ratio $r(A) \equiv A_N(pA)/A_N(pp)=A^P$ over the kinematic range, the ratio
$r(A)$ is found to depend only weakly on $A$, with ${\langle}P{\rangle} =
-0.027 \pm 0.005$. No significant difference in $P$ is observed between the
low-$p_T$ region, $p_T<2.5$ GeV/$c$, where gluon saturation effects may play a
role, and the high-$p_T$ region, $p_T>2.5$ GeV/$c$. It is further observed that
the value of $A_N$ is significantly larger for events with a large-$p_T$
isolated $\pi^0$ than for events with a non-isolated $\pi^0$ accompanied by
additional jet-like fragments. The nuclear dependence $r(A)$ is similar for
isolated and non-isolated $\pi^0$ events.
Phys. Rev. D 103, 012001 (2021) We report on the $W$ and $Z/\gamma^*$ differential and total cross sections
as well as the $W^+$/$W^-$ and $(W^+ + W^-)$/$(Z/\gamma^*)$ cross-section
ratios measured ...by the STAR experiment at RHIC in $p+p$ collisions at $\sqrt{s}
= 500$ GeV and $510$ GeV. The cross sections and their ratios are sensitive to
quark and antiquark parton distribution functions. In particular, at leading
order, the $W$ cross-section ratio is sensitive to the $\bar{d}/\bar{u}$ ratio.
These measurements were taken at high $Q^2 \sim M_W^2,M_Z^2$ and can serve as
input into global analyses to provide constraints on the sea quark
distributions. The results presented here combine three STAR data sets from
2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb$^{-1}$.
We also assess the expected impact that our $W^+/W^-$ cross-section ratios will
have on various quark distributions, and find sensitivity to the
$\bar{u}-\bar{d}$ and $\bar{d}/\bar{u}$ distributions.
Phys. Rev. C 103, 034908 (2021) The Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider
(RHIC) was extended to energies below $\sqrt{\textit{s}_{NN}}$ = 7.7 GeV in
2015 by ...successful implementation of the fixed-target mode of operation in the
STAR (Solenoidal Track At RHIC) experiment. In the fixed-target mode, ions
circulate in one ring of the collider and interact with a stationary target at
the entrance of the STAR Time Projection Chamber. The first results for Au+Au
collisions at $\sqrt{\textit{s}_{NN}}$ = 4.5 GeV are presented, including
directed and elliptic flow of identified hadrons, and radii from pion
femtoscopy. The proton flow and pion femtoscopy results agree quantitatively
with earlier measurements by Alternating Gradient Synchrotron experiments at
similar energies. This validates running the STAR experiment in the
fixed-target configuration. Pion directed and elliptic flow are presented for
the first time at this beam energy. Pion and proton elliptic flow show behavior
which hints at constituent quark scaling, but large error bars preclude
reliable conclusions. The ongoing second phase of BES (BES-II) will provide
fixed-target data sets with 100 times more events at each of several energies
down to $\sqrt{\textit{s}_{NN}}$ = 3.0 GeV.
Phys. Rev. D 102, 092009 (2020) We report on new measurements of inclusive J/$\psi$ polarization at
mid-rapidity in p+p collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment
at RHIC. The ...polarization parameters, $\lambda_\theta$, $\lambda_\phi$, and
$\lambda_{\theta\phi}$, are measured as a function of transverse momentum
($p_T$) in both the Helicity and Collins-Soper (CS) reference frames within
$p_T< 10$ GeV/$C$. Except for $\lambda_\theta$ in the CS frame at the highest
measured $p_T$, all three polarization parameters are consistent with 0 in both
reference frames without any strong $p_T$ dependence. Several model
calculations are compared with data, and the one using the Color Glass
Condensate effective field theory coupled with non-relativistic QCD gives the
best overall description of the experimental results, even though other models
cannot be ruled out due to experimental uncertainties.
Flow harmonics ($\textit{v}_{n}$) of the Fourier expansion for the azimuthal
distributions of hadrons are commonly employed to quantify the azimuthal
anisotropy of particle production relative to the ...collision symmetry planes.
While lower order Fourier coefficients ($\textit{v}_{2}$ and $\textit{v}_{3}$)
are more directly related to the corresponding eccentricities of the initial
state, the higher-order flow harmonics ($\textit{v}_{n>3}$) can be induced by a
mode-coupled response to the lower-order anisotropies, in addition to a linear
response to the same-order anisotropies. These higher-order flow harmonics and
their linear and mode-coupled contributions can be used to more precisely
constrain the initial conditions and the transport properties of the medium in
theoretical models. The multiparticle azimuthal cumulant method is used to
measure the linear and mode-coupled contributions in the higher-order
anisotropic flow, the mode-coupled response coefficients, and the correlations
of the event plane angles for charged particles as functions of centrality and
transverse momentum in Au+Au collisions at nucleon-nucleon center-of-mass
energy $\sqrt{\textit{s}_{NN}}$ = 200 GeV. The results are compared to similar
LHC measurements as well as to several viscous hydrodynamic calculations with
varying initial conditions.
Phys. Rev. C 106, 034908 (2022) Quark interactions with topological gluon configurations can induce local
chirality imbalance and parity violation in quantum chromodynamics, which can
lead to the ...chiral magnetic effect (CME) -- an electric charge separation along
the strong magnetic field in relativistic heavy-ion collisions. The
CME-sensitive azimuthal correlator observable ($\Delta\gamma$) is contaminated
by background arising, in part, from resonance decays coupled with elliptic
anisotropy ($v_{2}$). We report here differential measurements of the
correlator as a function of the pair invariant mass ($m_{\rm inv}$) in 20-50\%
centrality Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$= 200 GeV by the STAR
experiment at RHIC. Strong resonance background contributions to $\Delta\gamma$
are observed. At large $m_{\rm inv}$ where this background is significantly
reduced, the $\Delta\gamma$ value is found to be significantly smaller. An
event-shape-engineering technique is deployed to determine the $v_{2}$
background shape as a function of $m_{\rm inv}$. We extract a $v_2$-independent
and $m_{\rm inv}$-averaged signal $\Delta\gamma_{\rm sig}$ = (0.03 $\pm$ 0.06
$\pm$ 0.08) $\times10^{-4}$, or $(2\pm4\pm5)\%$ of the inclusive
$\Delta\gamma(m_{\rm inv}>0.4$ GeV/$c^2$)$ =(1.58 \pm 0.02 \pm 0.02)
\times10^{-4}$, within pion $p_{T}$ = 0.2 - 0.8~\gevc and averaged over
pseudorapidity ranges of $-1 < \eta < -0.05$ and $0.05 < \eta < 1$. This
represents an upper limit of $0.23\times10^{-4}$, or $15\%$ of the inclusive
result, at $95\%$ confidence level for the $m_{\rm inv}$-integrated CME
contribution.
