Phys. Rev. C 93, 064904 (2016) We report on the measurement of $\rm{J}/\psi$ production in the dielectron
channel at mid-rapidity (|y|<1) in p+p and d+Au collisions at $\sqrt{s_{NN}}$ =
200 GeV from ...the STAR experiment at the Relativistic Heavy Ion Collider. The
transverse momentum $p_{T}$ spectra in p+p for $p_{T}$ < 4 GeV/c and d+Au
collisions for $p_{T}$ < 3 GeV/c are presented. These measurements extend the
STAR coverage for $\rm{J}/\psi$ production in p+p collisions to low $p_{T}$.
The $<p_{T}^{2}>$ from the measured $\rm{J}/\psi$ invariant cross section in
p+p and d+Au collisions are evaluated and compared to similar measurements at
other collision energies. The nuclear modification factor for $\rm{J}/\psi$ is
extracted as a function of $p_{T}$ and collision centrality in d+Au and
compared to model calculations using the modified nuclear Parton Distribution
Function and a final-state $\rm{J}/\psi$ nuclear absorption cross section.
We present measurements of 2$^{nd}$ order azimuthal anisotropy ($v_{2}$) at
mid-rapidity $(|y|<1.0)$ for light nuclei d, t, $^{3}$He (for $\sqrt{s_{NN}}$ =
200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV) ...and anti-nuclei $\bar{\rm d}$
($\sqrt{s_{NN}}$ = 200, 62.4, 39, 27, and 19.6 GeV) and $^{3}\bar{\rm He}$
($\sqrt{s_{NN}}$ = 200 GeV) in the STAR (Solenoidal Tracker at RHIC)
experiment. The $v_{2}$ for these light nuclei produced in heavy-ion collisions
is compared with those for p and $\bar{\rm p}$. We observe mass ordering in
nuclei $v_{2}(p_{T})$ at low transverse momenta ($p_{T}<2.0$ GeV/$c$). We also
find a centrality dependence of $v_{2}$ for d and $\bar{\rm d}$. The magnitude
of $v_{2}$ for t and $^{3}$He agree within statistical errors. Light-nuclei
$v_{2}$ are compared with predictions from a blast wave model. Atomic mass
number ($A$) scaling of light-nuclei $v_{2}(p_{T})$ seems to hold for $p_{T}/A
< 1.5$ GeV/$c$. Results on light-nuclei $v_{2}$ from a
transport-plus-coalescence model are consistent with the experimental
measurements.
Phys. Rev. Lett. 116, 132301 (2016) We present the measurement of the transverse single-spin asymmetry of weak
boson production in transversely polarized proton-proton collisions at
$\sqrt{s} = ...500~\text{GeV}$ by the STAR experiment at RHIC. The measured
observable is sensitive to the Sivers function, one of the transverse momentum
dependent parton distribution functions, which is predicted to have the
opposite sign in proton-proton collisions from that observed in deep inelastic
lepton-proton scattering. These data provide the first experimental
investigation of the non-universality of the Sivers function, fundamental to
our understanding of QCD.
Phys. Rev. Lett. 116, 062301 (2016) We present high precision measurements of elliptic flow near midrapidity
($|y|<1.0$) for multi-strange hadrons and $\phi$ meson as a function of
centrality and ...transverse momentum in Au+Au collisions at center of mass energy
$\sqrt{s_{NN}}=$ 200 GeV. We observe that the transverse momentum dependence of
$\phi$ and $\Omega$ $v_{2}$ is similar to that of $\pi$ and $p$, respectively,
which may indicate that the heavier strange quark flows as strongly as the
lighter up and down quarks. This observation constitutes a clear piece of
evidence for the development of partonic collectivity in heavy-ion collisions
at the top RHIC energy. Number of constituent quark scaling is found to hold
within statistical uncertainty for both 0-30$\%$ and 30-80$\%$ collision
centrality. There is an indication of the breakdown of previously observed mass
ordering between $\phi$ and proton $v_{2}$ at low transverse momentum in the
0-30$\%$ centrality range, possibly indicating late hadronic interactions
affecting the proton $v_{2}$.
Phys. Rev. C 93, 021903 (2016) We present measurements of $\Omega$ and $\phi$ production at mid-rapidity
from Au+Au collisions at nucleon-nucleon center-of-mass energies
$\sqrt{s_{NN}}$ = 7.7, 11.5, ...19.6, 27 and 39 GeV by the STAR experiment at the
Relativistic Heavy Ion Collider (RHIC). Motivated by the coalescence formation
mechanism for these strange hadrons, we study the ratios of
$N(\Omega^{-}+\Omega^{+})/(2N(\phi))$. These ratios as a function of transverse
momentum ($p_T$) fall on a consistent trend at high collision energies, but
start to show deviations in peripheral collisions at $\sqrt{s_{NN}}$ = 19.6, 27
and 39 GeV, and in central collisions at 11.5 GeV in the intermediate $p_T$
region of 2.4-3.6 GeV/c. We further evaluate empirically the strange quark
$p_T$ distributions at hadronization by studying the $\Omega/\phi$ ratios
scaled by the number of constituent quarks. The NCQ-scaled $\Omega/\phi$ ratios
show a suppression of strange quark production in central collisions at 11.5
GeV compared to $\sqrt{s_{NN}} >= 19.6$ GeV. The shapes of the presumably
thermal strange quark distributions in 0-60% most central collisions at 7.7 GeV
show significant deviations from those in 0-10% most central collisions at
higher energies. These features suggest that there is likely a change of the
underlying strange quark dynamics in the transition from quark-matter to
hadronic matter at collision energies below 19.6 GeV.
Phys. Rev. C 92, 024912 (2015) We report on measurements of dielectron ($e^+e^-$) production in Au$+$Au
collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using
the STAR ...detector at RHIC. Systematic measurements of the dielectron yield as a
function of transverse momentum ($p_{\rm T}$) and collision centrality show an
enhancement compared to a cocktail simulation of hadronic sources in the low
invariant-mass region ($M_{ee}<$ 1 GeV/$c^2$). This enhancement cannot be
reproduced by the $\rho$-meson vacuum spectral function. In minimum-bias
collisions, in the invariant-mass range of 0.30 $-$ 0.76 GeV/$c^2$, integrated
over the full $p_{\rm T}$ acceptance, the enhancement factor is 1.76 $\pm$ 0.06
(stat.) $\pm$ 0.26 (sys.) $\pm$ 0.29 (cocktail). The enhancement factor
exhibits weak centrality and $p_{\rm T}$ dependence in STAR's accessible
kinematic regions, while the excess yield in this invariant-mass region as a
function of the number of participating nucleons follows a power-law shape with
a power of 1.44 $\pm$ 0.10. Models that assume an in-medium broadening of the
$\rho$ meson spectral function consistently describe the observed excess in
these measurements. Additionally, we report on measurements of $\omega$ and
$\phi$-meson production through their $e^+e^-$ decay channel. These
measurements show good agreement with Tsallis Blast-Wave model predictions as
well as, in the case of the $\phi$-meson, results through its $K^+K^-$ decay
channel. In the intermediate invariant-mass region (1.1$<M_{ee}<$ 3 GeV/$c^2$),
we investigate the spectral shapes from different collision centralities.
Physics implications for possible in-medium modification of charmed hadron
production and other physics sources are discussed.
Phys. Rev. Lett. 115, 242501 (2015) We report the observation of transverse polarization-dependent azimuthal
correlations in charged pion pair production with the STAR experiment in
$p^\uparrow+p$ ...collisions at RHIC. These correlations directly probe quark
transversity distributions. We measure signals in excess of five standard
deviations at high transverse momenta, at high pseudorapidities eta>0.5, and
for pair masses around the mass of the rho-meson. This is the first direct
transversity measurement in p+p collisions. Comparing the results to data from
lepton-nucleon scattering will test the universality of these spin-dependent
quantities.
Phys. Lett. B 747 (2015) 265 Dihadron angular correlations in $d$+Au collisions at $\sqrt{s_{\rm NN}}=200$
GeV are reported as a function of the measured zero-degree calorimeter neutral
energy and ...the forward charged hadron multiplicity in the Au-beam direction. A
finite correlated yield is observed at large relative pseudorapidity
($\Delta\eta$) on the near side (i.e. relative azimuth $\Delta\phi\sim0$). This
correlated yield as a function of $\Delta\eta$ appears to scale with the
dominant, primarily jet-related, away-side ($\Delta\phi\sim\pi$) yield. The
Fourier coefficients of the $\Delta\phi$ correlation, $V_{n}=\langle\cos
n\Delta\phi\rangle$, have a strong $\Delta\eta$ dependence. In addition, it is
found that $V_{1}$ is approximately inversely proportional to the mid-rapidity
event multiplicity, while $V_{2}$ is independent of it with similar magnitude
in the forward ($d$-going) and backward (Au-going) directions.
Phys. Lett. B 750 (2015) 64 The acceptance-corrected dielectron excess mass spectra, where the known
hadronic sources have been subtracted from the inclusive dielectron mass
spectra, are reported for ...the first time at mid-rapidity $|y_{ee}|<1$ in
minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess
mass spectra are consistently described by a model calculation with a broadened
$\rho$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated
dielectron excess yield at $\sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$
GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has
a value similar to that in In+In collisions at $\sqrt{s_{NN}}$ = 17.3 GeV. For
$\sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is
higher than that at $\sqrt{s_{NN}}$ = 17.3 GeV and increases from peripheral to
central collisions. These measurements indicate that the lifetime of the hot,
dense medium created in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
is longer than those in peripheral collisions and at lower energies.
We report the measurement of the leptonic ($e^{+}e^{-}$) decay channel of the
$\phi$(1020) meson in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV by the
STAR experiment. The transverse momentum ...($p_{\rm T}$) spectrum is measured for
0.1 $\le p_{\rm T} \le 2.5$ GeV/$c$ at mid-rapidity ($|y|\le1$). We obtain the
$p_{\rm T}$-integrated $\phi$(1020) mass $M_{\phi}=1017.7\pm0.8 (\rm {stat.})
\pm0.9 (\rm {sys.})$ MeV/$c^{2}$ and width $\Gamma_{\phi} = 8.0\pm 2.5(\rm
{stat.}) \pm 2.3(\rm {sys.}) \textrm{MeV/}c^{2}$, which are within
1.5\,$\sigma$ and 1.1\,$\sigma$ of the vacuum values, respectively. No
significant difference is observed in the measured $p_{\rm T}$ spectrum,
$dN/dy$, or $\left<p_{\rm T}\right>$ of the $\phi$(1020) meson between the
$e^{+}e^{-}$ and hadronic ($K^{+}K^{-}$) decay channels as measured by the same
experiment. The experimental results are compared to a theoretical model
including medium-modified $\phi$(1020).
