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.
Phys.Rev.C 102 (2020) 5, 054913 The STAR Collaboration at the Relativistic Heavy Ion Collider reports the
first measurement of inclusive jet production in peripheral and central Au+Au
collisions at ...$\sqrt{s_{NN}}$=200 GeV. Jets are reconstructed with the
anti-k$_{T}$ algorithm using charged tracks with pseudorapidity $|\eta|<1.0$
and transverse momentum $0.2<p_{T,jet}^{ch}<30$ GeV/$c$, with jet resolution
parameter $R$=0.2, 0.3, and 0.4. The large background yield uncorrelated with
the jet signal is observed to be dominated by statistical phase space,
consistent with a previous coincidence measurement. This background is
suppressed by requiring a high-transverse-momentum (high-$p_T$) leading hadron
in accepted jet candidates. The bias imposed by this requirement is assessed,
and the $p_T$ region in which the bias is small is identified. Inclusive
charged-particle jet distributions are reported in peripheral and central Au+Au
collisions for $5<p_{T,jet}^{ch}<25$ GeV/$c$ and $5<p_{T,jet}^{ch}<30$ GeV/$c$,
respectively. The charged-particle jet inclusive yield is suppressed for
central Au+Au collisions, compared to both the peripheral Au+Au yield from this
measurement and to the $pp$ yield calculated using the PYTHIA event generator.
The magnitude of the suppression is consistent with that of inclusive hadron
production at high $p_T$, and that of semi-inclusive recoil jet yield when
expressed in terms of energy loss due to medium-induced energy transport.
Comparison of inclusive charged-particle jet yields for different values of $R$
exhibits no significant evidence for medium-induced broadening of the
transverse jet profile for $R<0.4$ in central Au+Au collisions. The measured
distributions are consistent with theoretical model calculations that
incorporate jet quenching.
JHEP 07 (2020) 178 We report on the measurement of the Central Exclusive Production of charged
particle pairs $h^{+}h^{-}$ ($h = \pi, K, p$) with the STAR detector at RHIC in
proton-proton collisions ...at $\sqrt{s} = 200$ GeV. The charged particle pairs
produced in the reaction $pp\to p^\prime+h^{+}h^{-}+p^\prime$ are reconstructed
from the tracks in the central detector, while the forward-scattered protons
are measured in the Roman Pot system. Differential cross sections are measured
in the fiducial region, which roughly corresponds to the square of the
four-momentum transfers at the proton vertices in the range $0.04~\mbox{GeV}^2
< -t_1 , -t_2 < 0.2~\mbox{GeV}^2$, invariant masses of the charged particle
pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in
the range $|\eta|<0.7$. The measured cross sections are compared to
phenomenological predictions based on the Double Pomeron Exchange (DPE) model.
Structures observed in the mass spectra of $\pi^{+}\pi^{-}$ and $K^{+}K^{-}$
pairs are consistent with the DPE model, while angular distributions of pions
suggest a dominant spin-0 contribution to $\pi^{+}\pi^{-}$ production. The
fiducial $\pi^+\pi^-$ cross section is extrapolated to the Lorentz-invariant
region, which allows decomposition of the invariant mass spectrum into
continuum and resonant contributions. The extrapolated cross section is well
described by the continuum production and at least three resonances, the
$f_0(980)$, $f_2(1270)$ and $f_0(1500)$, with a possible small contribution
from the $f_0(1370)$. Fits to the extrapolated differential cross section as a
function of $t_1$ and $t_2$ enable extraction of the exponential slope
parameters in several bins of the invariant mass of $\pi^+\pi^-$ pairs. These
parameters are sensitive to the size of the interaction region.
We report results on the total and elastic cross sections in proton-proton
collisions at $\sqrt{s}=200$ GeV obtained with the Roman Pot setup of the STAR
experiment at the Relativistic Heavy Ion ...Collider (RHIC). The elastic
differential cross section was measured in the squared four-momentum transfer
range $0.045 \leq -t \leq 0.135$ GeV$^2$. The value of the exponential slope
parameter $B$ of the elastic differential cross section $d\sigma/dt \sim
e^{-Bt}$ in the measured $-t$ range was found to be $B = 14.32 \pm 0.09
(stat.)^{\scriptstyle +0.13}_{\scriptstyle -0.28} (syst.)$ GeV$^{-2}$. The
total cross section $\sigma_{tot}$, obtained from extrapolation of the
$d\sigma/dt$ to the optical point at $-t = 0$, is $\sigma_{tot} = 54.67 \pm
0.21 (stat.) ^{\scriptstyle +1.28}_{\scriptstyle -1.38} (syst.)$ mb. We also
present the values of the elastic cross section $\sigma_{el} = 10.85 \pm 0.03
(stat.) ^{\scriptstyle +0.49}_{\scriptstyle -0.41}(syst.)$ mb, the elastic
cross section integrated within the STAR $t$-range $\sigma^{det}_{el} = 4.05
\pm 0.01 (stat.) ^{\scriptstyle+0.18}_{\scriptstyle -0.17}(syst.)$ mb, and the
inelastic cross section $\sigma_{inel} = 43.82 \pm 0.21 (stat.) ^{\scriptstyle
+1.37}_{\scriptstyle -1.44} (syst.)$ mb. The results are compared with the
world data.
Phys.Lett.B 811 (2020) In this letter, measurements of the shared momentum fraction ($z_{\rm{g}}$)
and the groomed jet radius ($R_{\rm{g}}$), as defined in the SoftDrop algorihm,
are reported in \pp ...collisions at $\sqrt{s} = 200$ GeV collected by the STAR
experiment. These substructure observables are differentially measured for jets
of varying resolution parameters from $R = 0.2 - 0.6$ in the transverse
momentum range $15 < p_{\rm{T, jet}} < 60$ GeV$/c$. These studies show that, in
the $p_{\rm{T, jet}}$ range accessible at $\sqrt{s} = 200$ GeV and with
increasing jet resolution parameter and jet transverse momentum, the
$z_{\rm{g}}$ distribution asymptotically converges to the DGLAP splitting
kernel for a quark radiating a gluon. The groomed jet radius measurements
reflect a momentum-dependent narrowing of the jet structure for jets of a given
resolution parameter, i.e., the larger the $p_{\rm{T, jet}}$, the narrower the
first splitting. For the first time, these fully corrected measurements are
compared to Monte Carlo generators with leading order QCD matrix elements and
leading log in the parton shower, and to state-of-the-art theoretical
calculations at next-to-leading-log accuracy. We observe that PYTHIA 6 with
parameters tuned to reproduce RHIC measurements is able to quantitatively
describe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, are
unable to provide a simultaneous description of both $z_{\rm{g}}$ and
$R_{\rm{g}}$, resulting in opportunities for fine parameter tuning of these
models for \pp collisions at RHIC energies. We also find that the theoretical
calculations without non-perturbative corrections are able to qualitatively
describe the trend in data for jets of large resolution parameters at high
$p_{\rm{T, jet}}$, but fail at small jet resolution parameters and low jet
transverse momenta.
Phys. Rev. C 102, 024903 (2020) The measurements of particle multiplicity distributions have generated
considerable interest in understanding the fluctuations of conserved quantum
numbers in the ...Quantum Chromodynamics (QCD) hadronization regime, in particular
near a possible critical point and near the chemical freeze-out. We report the
measurement of efficiency and centrality bin width corrected cumulant ratios
($C_{2}/C_{1}$, $C_{3}/C_{2}$) of net-$\Lambda$ distributions, in the context
of both strangeness and baryon number conservation, as a function of collision
energy, centrality and rapidity. The results are for Au + Au collisions at five
beam energies ($\sqrt{s_{NN}}$ = 19.6, 27, 39, 62.4 and 200 GeV) recorded with
the Solenoidal Tracker at RHIC (STAR). We compare our results to the Poisson
and negative binomial (NBD) expectations, as well as to Ultra-relativistic
Quantum Molecular Dynamics (UrQMD) and Hadron Resonance Gas (HRG) model
predictions. Both NBD and Poisson baselines agree with data within the
statistical and systematic uncertainties. The ratios of the measured cumulants
show no features of critical fluctuations. The chemical freeze-out temperatures
extracted from a recent HRG calculation, which was successfully used to
describe the net-proton, net-kaon and net-charge data, indicate $\Lambda$
freeze-out conditions similar to those of kaons. However, large deviations are
found when comparing to temperatures obtained from net-proton fluctuations. The
net-$\Lambda$ cumulants show a weak, but finite, dependence on the rapidity
coverage in the acceptance of the detector, which can be attributed to quantum
number conservation.
Phys. Rev. D 101, 052004 (2020) Particle production sensitive to non-factorizable and non-perturbative
processes that contribute to the underlying event associated with a high
transverse momentum ...($p_{T}$) jet in proton+proton collisions at $\sqrt{s}$=200
GeV is studied with the STAR detector. Each event is divided into three regions
based on the azimuthal angle with respect to the highest-$p_{T}$ jet direction:
in the leading jet direction ("Toward"), opposite to the leading jet ("Away"),
and perpendicular to the leading jet ("Transverse"). In the Transverse region,
the average charged particle density is found to be between 0.4 and 0.6 and the
mean transverse momentum, $\langle p_{T}\rangle$, between 0.5-0.7 GeV/$c$ for
particles with $p_{T}$$>$0.2 GeV/$c$ at mid-pseudorapidity ($|\eta|$$<$1) and
jet $p_{T}$$>$15 GeV/$c$. Both average particle density and $\langle
p_{T}\rangle$ depend weakly on the leading jet $p_{T}$. Closer inspection of
the Transverse region hints that contributions to the underlying event from
initial- and final-state radiation are significantly smaller in these
collisions than at the higher energies, up to 13 TeV, recorded at the LHC.
Underlying event measurements associated with a high-$p_{T}$ jet will
contribute to our understanding of QCD processes at hard and soft scales at
RHIC energies, as well as provide constraints to modeling of underlying event
dynamics.
