The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-\(\pi^0\)s produced at forward pseudorapidities (\(2.6<\eta<4.0\)) in \(p\)+\(p\), \(p+\)Al, and \(p+\)Au ...collisions at a center-of-mass energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back \(\pi^0\) pairs in \(p+\)Al and \(p+\)Au collisions compared to the \(p\)+\(p\) data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in \(p+\)Au relative to \(p+\)Al collisions exhibits a dependence of the saturation scale, \(Q_s^2\), on the mass number, \(A\). A linear scaling of the suppression with \(A^{1/3}\) is observed with a slope of \(-0.09\) \(\pm\) \(0.01\).
Measurements of the production of forward pi0 mesons from p + p and d + Au collisions at square root sNN=200 GeV are reported. The p + p yield generally agrees with next-to-leading order perturbative ...QCD calculations. The d + Au yield per binary collision is suppressed as eta increases, decreasing to approximately 30% of the p + p yield at eta =4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward pi0 with charged hadrons at eta approximately 0 show a recoil peak in p + p that is suppressed in d + Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (\(y\)) ...and transverse momentum (\(p_{\rm T}\)) within \(-0.5 < y<0\) and \(0.4 < p_{\rm T} <2.0 \) GeV/\(c\). In the most central 0--5\% collisions, a proton cumulant ratio is measured to be \(C_4/C_2=-0.85 \pm 0.09 ~(\rm stat.) \pm 0.82 ~(\rm syst.)\), which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our \(C_4/C_2\) in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in \(C_4/C_2\) is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.
A linearly polarized photon can be quantized from the Lorentz-boosted electromagnetic field of a nucleus traveling at ultra-relativistic speed. When two relativistic heavy nuclei pass one another at ...a distance of a few nuclear radii, the photon from one nucleus may interact through a virtual quark-antiquark pair with gluons from the other nucleus forming a short-lived vector meson (e.g. \({\rho^0}\)). In this experiment, the polarization was utilized in diffractive photoproduction to observe a unique spin interference pattern in the angular distribution of \({\rho^0\rightarrow\pi^+\pi^-}\) decays. The observed interference is a result of an overlap of two wave functions at a distance an order of magnitude larger than the \({\rho^0}\) travel distance within its lifetime. The strong-interaction nuclear radii were extracted from these diffractive interactions, and found to be \(6.53\pm 0.06\) fm (\(^{197} {\rm Au }\)) and \(7.29\pm 0.08\) fm (\(^{238} {\rm U}\)), larger than the nuclear charge radii. The observable is demonstrated to be sensitive to the nuclear geometry and quantum interference of non-identical particles.
We present high-precision measurements of elliptic, triangular, and
quadrangular flow $v_{2}$, $v_{3}$, and $v_{4}$, respectively, at midrapidity
($|\eta|<1.0$) for identified hadrons $\pi$, $p$, ...$K$, $\varphi$, $K_s$,
$\Lambda$ as a function of centrality and transverse momentum in Au+Au
collisions at the center-of-mass energy $\sqrt{s_{\rm NN}}=$ 200 GeV. We
observe similar $v_{n}$ trends between light and strange mesons which indicates
that the heavier strange quarks flow as strongly as the lighter up and down
quarks. The number-of-constituent-quark scaling for $v_{2}$, $v_{3}$, and
$v_{4}$ is found to hold within statistical uncertainty for 0-10$\%$, 10-40$\%$
and 40-80$\%$ collision centrality intervals. The results are compared to
several viscous hydrodynamic calculations with varying initial conditions, and
could serve as an additional constraint to the development of hydrodynamic
models.
We report the first measurements of a complete second-order cumulant matrix of net-charge, net-proton, and net-kaon multiplicity distributions for the first phase of the beam energy scan program at ...RHIC. This includes the centrality and, for the first time, the pseudorapidity window dependence of both diagonal and off-diagonal cumulants in Au+Au collisions at \sNN~= 7.7-200 GeV. Within the available acceptance of \(|\eta|<0.5\), the cumulants grow linearly with the pseudorapidity window. Relative to the corresponding measurements in peripheral collisions, the ratio of off-diagonal over diagonal cumulants in central collisions indicates an excess correlation between net-charge and net-kaon, as well as between net-charge and net-proton. The strength of such excess correlation increases with the collision energy. The correlation between net-proton and net-kaon multiplicity distributions is observed to be negative at \sNN~= 200 GeV and change to positive at the lowest collision energy. Model calculations based on non-thermal (UrQMD) and thermal (HRG) production of hadrons cannot explain the data. These measurements will help map the QCD phase diagram, constrain hadron resonance gas model calculations, and provide new insights on the energy dependence of baryon-strangeness correlations. An erratum has been added to address the issue of self-correlation in the previously considered efficiency correction for off-diagonal cumulant measurement. Previously considered unidentified (net-)charge correlation results (\(\sigma^{11}_{Q,p}\) and \(\sigma^{11}_{Q,k})\) are now replaced with identified (net-)charge correlation (\(\sigma^{11}_{Q^{PID},p}\) and \(\sigma^{11}_{Q^{PID},k}\))
In high-energy heavy-ion collisions, partonic collectivity is evidenced by
the constituent quark number scaling of elliptic flow anisotropy for identified
hadrons. A breaking of this scaling and ...dominance of baryonic interactions is
found for identified hadron collective flow measurements in $\sqrt{s_{\rm NN}}$
= 3 GeV Au+Au collisions. In this paper, we report measurements of the first-
and second-order azimuthal anisotropic parameters, $v_1$ and $v_2$, of light
nuclei ($d$, $t$, $^{3}$He, $^{4}$He) produced in $\sqrt{s_{\rm NN}}$ = 3 GeV
Au+Au collisions at the STAR experiment. An atomic mass number scaling is found
in the measured $v_1$ slopes of light nuclei at mid-rapidity. For the measured
$v_2$ magnitude, a strong rapidity dependence is observed. Unlike $v_2$ at
higher collision energies, the $v_2$ values at mid-rapidity for all light
nuclei are negative and no scaling is observed with the atomic mass number.
Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic
mean-field plus nucleon coalescence, are in good agreement with our
observations, implying baryonic interactions dominate the collective dynamics
in 3 GeV Au+Au collisions at RHIC.
Non-monotonic variation with collision energy (\(\sqrt{s_{\rm NN}}\)) of the moments of the net-baryon number distribution in heavy-ion collisions, related to the correlation length and the ...susceptibilities of the system, is suggested as a signature for the Quantum Chromodynamics (QCD) critical point. We report the first evidence of a non-monotonic variation in kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of \rootsnn with 3.1\(\sigma\) significance, for head-on (central) gold-on-gold (Au+Au) collisions measured using the STAR detector at RHIC. Data in non-central Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of \(\sqrt{s_{\rm NN}}\).
We report measurements of the longitudinal double-spin asymmetry, $A_{LL}$,
for inclusive jet and dijet production in polarized proton-proton collisions at
midrapidity and center-of-mass energy ...$\sqrt{s}$ = 510 GeV, using the high
luminosity data sample collected by the STAR experiment in 2013. These
measurements complement and improve the precision of previous STAR measurements
at the same center-of-mass energy that probe the polarized gluon distribution
function at partonic momentum fraction 0.015 $\lesssim x \lesssim$ 0.25. The
dijet asymmetries are separated into four jet-pair topologies, which provide
further constraints on the $x$ dependence of the polarized gluon distribution
function. These measurements are in agreement with previous STAR measurements
and with predictions from current next-to-leading order global analyses. They
provide more precise data at low dijet invariant mass that will better
constraint the shape of the polarized gluon distribution function of the
proton.
Phys. Rev. Lett. 128, 202301 (2022) We report precision measurements of hypernuclei ^3_\Lambda \rm{H}$ and
^4_\Lambda \rm{H}$ lifetimes obtained from Au+Au collisions at \snn =
3.0\,GeV and 7.2\,GeV ...collected by the STAR experiment at RHIC, and the first
measurement of ^3_\Lambda \rm{H}$ and ^4_\Lambda \rm{H}$ mid-rapidity
yields in Au+Au collisions at \snn = 3.0\,GeV. ^3_\Lambda \rm{H}$ and
^4_\Lambda \rm{H}$, being the two simplest bound states composed of hyperons
and nucleons, are cornerstones in the field of hypernuclear physics. Their
lifetimes are measured to be $221\pm15(\rm stat.)\pm19(\rm syst.)$\,ps for
^3_\Lambda \rm{H}$ and $218\pm6(\rm stat.)\pm13(\rm syst.)$\,ps for
^4_\Lambda \rm{H}$. The $p_T$-integrated yields of ^3_\Lambda \rm{H}$ and
^4_\Lambda \rm{H}$ are presented in different centrality and rapidity
intervals. It is observed that the shape of the rapidity distribution of
^4_\Lambda \rm{H}$ is different for 0--10\% and 10--50\% centrality
collisions. Thermal model calculations, using the canonical ensemble for
strangeness, describes the ^3_\Lambda \rm{H}$ yield well, while
underestimating the ^4_\Lambda \rm{H}$ yield. Transport models, combining
baryonic mean-field and coalescence (JAM) or utilizing dynamical cluster
formation via baryonic interactions (PHQMD) for light nuclei and hypernuclei
production, approximately describe the measured ^3_\Lambda \rm{H}$ and
^4_\Lambda \rm{H}$ yields. Our measurements provide means to precisely
assess our understanding of the fundamental baryonic interactions with strange
quarks, which can impact our understanding of more complicated systems
involving hyperons, such as the interior of neutron stars or exotic
hypernuclei.
