Phys. Rev. C 106, 014908 (2022) Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects;
however, the collective behavior observed in these collisions shows a hint of
...hot-nuclear-matter effects. The identified-particle spectra, especially the
$\phi$ mesons which contain strange and antistrange quarks and have a
relatively small hadronic-interaction cross section, are a good tool to study
these effects. The PHENIX experiment has measured $\phi$ mesons in a specific
set of small collision systems $p$$+$Al, $p$$+$Au, and $^3$He$+$Au, as well as
$d$$+$Au Phys. Rev. C {\bf 83}, 024909 (2011), at $\sqrt{s_{_{NN}}}=200$ GeV.
The transverse-momentum spectra and nuclear-modification factors are presented
and compared to theoretical-model predictions. The comparisons with different
calculations suggest that quark-gluon plasma may be formed in these small
collision systems at $\sqrt{s_{_{NN}}}=200$ GeV. However, the volume and the
lifetime of the produced medium may be insufficient for observing
strangeness-enhancement and jet-quenching effects. Comparison with calculations
suggests that the main production mechanisms of $\phi$ mesons at midrapidity
may be different in $p$$+$Al versus $p/d/$$^3$He$+$Au collisions at
$\sqrt{s_{_{NN}}}=200$ GeV. While thermal quark recombination seems to dominate
in $p/d/$$^3$He$+$Au collisions, fragmentation seems to be the main production
mechanism in $p$$+$Al collisions.
Two-particle correlation measurements projected onto two-dimensional,
transverse rapidity coordinates ($y_{T1},y_{T2}$), allow access to dynamical
properties of the QCD medium produced in ...relativistic heavy-ion collisions that
angular correlation measurements are not sensitive to. We report non-identified
charged-particle correlations for Au + Au minimum-bias collisions at
$\sqrt{s_{\rm NN}}$ = 200 GeV taken by the STAR experiment at the Relativistic
Heavy-Ion Collider (RHIC). Correlations are presented as 2D functions of
transverse rapidity for like-sign, unlike-sign and all charged-particle pairs,
as well as for particle pairs whose relative azimuthal angles lie on the
near-side, the away-side, or at all relative azimuth. The correlations are
constructed using charged particles with transverse momentum $p_T \geq 0.15$
GeV/$c$, pseudorapidity from $-$1 to 1, and azimuthal angles from $-\pi$ to
$\pi$. The significant correlation structures that are observed evolve smoothly
with collision centrality. The major correlation features include a saddle
shape plus a broad peak with maximum near $y_T \approx 3$, corresponding to
$p_T \approx$ 1.5 GeV/$c$. The broad peak is observed in both like- and
unlike-sign charge combinations and in near- and away-side relative azimuthal
angles. The all-charge, all-azimuth correlation measurements are compared with
the theoretical predictions of {\sc hijing} and {\sc epos}. The results
indicate that the correlations for peripheral to mid-central collisions can be
approximately described as a superposition of nucleon + nucleon collisions with
minimal effects from the QCD medium. Strong medium effects are indicated in
mid- to most-central collisions.
Phys. Rev. C 107, 024912 (2023) Azimuthal anisotropy of produced particles is one of the most important
observables used to access the collective properties of the expanding medium
created in ...relativistic heavy-ion collisions. In this paper, we present second
($v_{2}$) and third ($v_{3}$) order azimuthal anisotropies of $K_{S}^{0}$,
$\phi$, $\Lambda$, $\Xi$ and $\Omega$ at mid-rapidity ($|y|<$1) in Au+Au
collisions at $\sqrt{s_{\text{NN}}}$ = 54.4 GeV measured by the STAR detector.
The $v_{2}$ and $v_{3}$ are measured as a function of transverse momentum and
centrality. Their energy dependence is also studied. $v_{3}$ is found to be
more sensitive to the change in the center-of-mass energy than $v_{2}$. Scaling
by constituent quark number is found to hold for $v_{2}$ within 10%. This
observation could be evidence for the development of partonic collectivity in
54.4 GeV Au+Au collisions. Differences in $v_{2}$ and $v_{3}$ between baryons
and anti-baryons are presented, and ratios of $v_{3}$/$v_{2}^{3/2}$ are studied
and motivated by hydrodynamical calculations. The ratio of $v_{2}$ of $\phi$
mesons to that of anti-protons ($v_{2}(\phi)/v_{2}(\bar{p})$) shows centrality
dependence at low transverse momentum, presumably resulting from the larger
effects from hadronic interactions on anti-proton $v_{2}$.
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 $\phi$
and $K^{*0}$, emerge with a surprising pattern of global spin alignment. In
particular, the global spin alignment for $\phi$ is unexpectedly large, while
that for $K^{*0}$ is consistent with zero. The observed spin-alignment pattern
and magnitude for the $\phi$ cannot be explained by conventional mechanisms,
while a model with a connection to strong force fields, i.e. an effective proxy
description within the Standard Model and Quantum Chromodynamics, accommodates
the current data. This connection, if fully established, will open a potential
new avenue for studying the behaviour of strong force fields.
There is strong evidence for the formation of small droplets of quark-gluon
plasma in $p/d/^{3}$He+Au collisions at the Relativistic Heavy Ion Collider
(RHIC) and in $p$+$p$/Pb collisions at the ...Large Hadron Collider. In
particular, the analysis of data at RHIC for different geometries obtained by
varying the projectile size and shape has proven insightful. In the present
analysis, we find excellent agreement with the previously published PHENIX at
RHIC results on elliptical and triangular flow with an independent analysis via
the two-particle correlation method, which has quite different systematic
uncertainties and an independent code base. In addition, the results are
extended to other detector combinations with different kinematic
(pseudorapidity) coverage. These results provide additional constraints on
contributions from nonflow and longitudinal decorrelations.
Phys. Rev. Lett. 128, 092301 (2022) The chiral magnetic effect (CME) refers to charge separation along a strong
magnetic field due to imbalanced chirality of quarks in local parity and
charge-parity ...violating domains in quantum chromodynamics. The experimental
measurement of the charge separation is made difficult by the presence of a
major background from elliptic azimuthal anisotropy. This background and the
CME signal have different sensitivities to the spectator and participant
planes, and could thus be determined by measurements with respect to these
planes. We report such measurements in Au+Au collisions at a nucleon-nucleon
center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is
found that the charge separation, with the flow background removed, is
consistent with zero in peripheral (large impact parameter) collisions. Some
indication of finite CME signals is seen in mid-central (intermediate impact
parameter) collisions. Significant residual background effects may, however,
still be present.
According to first principle Lattice QCD calculations, the transition from
quark-gluon plasma to hadronic matter is a smooth crossover in the region
$\mu_{\rm B}\leq T_{c}$. In this range the ratio, ...$C_{6}/C_{2}$, of net-baryon
distributions are predicted to be negative. In this paper, we report the first
measurement of the midrapidity net-proton $C_{6}/C_{2}$ from 27, 54.4 and 200
GeV Au+Au collisions at RHIC. The dependence on collision centrality and
kinematic acceptance in ($p_{T}$, $y$) are analyzed. While for 27 and 54.4 GeV
collisions the $C_{6}/C_{2}$ values are close to zero within uncertainties, it
is observed that for 200 GeV collisions, the $C_{6}/C_{2}$ ratio becomes
progressively negative from peripheral to central collisions. Transport model
calculations without critical dynamics predict mostly positive values except
for the most central collisions within uncertainties. These observations seem
to favor a smooth crossover in the high energy nuclear collisions at top RHIC
energy.
Phys. Rev. D 104, 052007 (2021) We present the first inclusive measurements of the invariant and SoftDrop jet
mass in proton-proton collisions at $\sqrt{s}=200$ GeV at STAR. The
measurements are ...fully corrected for detector effects, and reported
differentially in both the jet transverse momentum and jet radius parameter. We
compare the measurements to established leading-order Monte Carlo event
generators and find that STAR-tuned PYTHIA-6 reproduces the data, while LHC
tunes of PYTHIA-8 and HERWIG-7 do not agree with the data, providing further
constraints on parameter tuning. Finally, we observe that SoftDrop grooming,
for which the contribution of wide-angle non-perturbative radiation is
suppressed, shifts the jet mass distributions into closer agreement with the
partonic jet mass as determined by both PYTHIA-8 and a
next-to-leading-logarithmic accuracy perturbative QCD calculation. These
measurements complement recent LHC measurements in a different kinematic
region, as well as establish a baseline for future jet mass measurements in
heavy-ion collisions at RHIC.
Phys. Rev. C 103, 064907 (2021) We present systematic measurements of azimuthal anisotropy for strange and
multistrange hadrons ($K^{0}_{s}$, $\Lambda$, $\Xi$, and $\Omega$) and $\phi$
mesons at ...midrapidity ($|y| <$ 1.0) in collisions of U + U nuclei at
$\sqrt{s_{NN}} = 193$ GeV, recorded by the STAR detector at the Relativistic
Heavy Ion Collider. Transverse momentum ($p_{\text{T}}$) dependence of flow
coefficients ($v_{2}$, $v_{3}$, and $v_{4}$) is presented for minimum bias
collisions and three different centrality intervals. Number of constituent
quark scaling of the measured flow coefficients in U + U collisions is
discussed. We also present the ratio of $v_{n}$ scaled by the participant
eccentricity ($\varepsilon_{n}\left\lbrace 2 \right\rbrace$) to explore system
size dependence and collectivity in U + U collisions. The magnitude of
$v_{2}/\varepsilon_{2}$ is found to be smaller in U + U collisions than that in
central Au + Au collisions contradicting naive eccentricity scaling.
Furthermore, the ratios between various flow harmonics ($v_{3}/v_{2}^{3/2}$,
$v_{4}/v_{2}^{4/2}$) are studied and compared with hydrodynamic and transport
model calculations.
