Phys. Rev. C. 107. 034907 (2023) We report the measurement of $K^{*0}$ meson at midrapidity ($|y|<$ 1.0) in
Au+Au collisions at $\sqrt{s_{\rm NN}}$~=~7.7, 11.5, 14.5, 19.6, 27 and 39 GeV
collected by ...the STAR experiment during the RHIC beam energy scan (BES)
program. The transverse momentum spectra, yield, and average transverse
momentum of $K^{*0}$ are presented as functions of collision centrality and
beam energy. The $K^{*0}/K$ yield ratios are presented for different collision
centrality intervals and beam energies. The $K^{*0}/K$ ratio in heavy-ion
collisions are observed to be smaller than that in small system collisions (e+e
and p+p). The $K^{*0}/K$ ratio follows a similar centrality dependence to that
observed in previous RHIC and LHC measurements. The data favor the scenario of
the dominance of hadronic re-scattering over regeneration for $K^{*0}$
production in the hadronic phase of the medium.
Phys. Rev. C 107, 024908(2023) We report a measurement of cumulants and correlation functions of
event-by-event proton multiplicity distributions from fixed-target Au+Au
collisions at $\sqrt{s_{\rm ...NN}}$ = 3 GeV measured by the STAR experiment.
Protons are identified within the rapidity ($y$) and transverse momentum
($p_{\rm T}$) region $-0.9 < y<0$ and $0.4 < p_{\rm T} <2.0 $ GeV/$c$ in the
center-of-mass frame. A systematic analysis of the proton cumulants and
correlation functions up to sixth-order as well as the corresponding ratios as
a function of the collision centrality, $p_{\rm T}$, and $y$ are presented. The
effect of pileup and initial volume fluctuations on these observables and the
respective corrections are discussed in detail. The results are compared to
calculations from the hadronic transport UrQMD model as well as a hydrodynamic
model. In the most central 5\% collisions, the value of proton cumulant ratio
$C_4/C_2$ is negative, drastically different from the values observed in Au+Au
collisions at higher energies. Compared to model calculations including Lattice
QCD, a hadronic transport model, and a hydrodynamic model, the strong
suppression in the ratio of $C_4/C_2$ at 3 GeV Au+Au collisions indicates an
energy regime dominated by hadronic interactions.
M. Abdallah et. al., STAR Collaboration, Physics Letters B 834
(2022) 137449 Measurements of mass and $\Lambda$ binding energy of $\rm ^4_{\Lambda}H$ and
$\rm ^4_{\Lambda}He$ in Au+Au collisions at ...$\sqrt{s_{_{\rm NN}}}=3$ GeV are
presented, with an aim to address the charge symmetry breaking (CSB) problem in
hypernuclei systems with atomic number A = 4. The $\Lambda$ binding energies
are measured to be $\rm 2.22\pm0.06(stat.) \pm0.14(syst.)$ MeV and $\rm
2.38\pm0.13(stat.) \pm0.12(syst.)$ MeV for $\rm ^4_{\Lambda}H$ and $\rm
^4_{\Lambda}He$, respectively. The measured $\Lambda$ binding-energy difference
is $\rm 0.16\pm0.14(stat.)\pm0.10(syst.)$ MeV for ground states. Combined with
the $\gamma$-ray transition energies, the binding-energy difference for excited
states is $\rm -0.16\pm0.14(stat.)\pm0.10(syst.)$ MeV, which is negative and
comparable to the value of the ground states within uncertainties. These new
measurements on the $\Lambda$ binding-energy difference in A = 4 hypernuclei
systems are consistent with the theoretical calculations that result in $\rm
\Delta B_{\Lambda}^4(1_{exc}^{+})\approx -\Delta B_{\Lambda}^4(0_{g.s.}^{+})<0$
and present a new method for the study of CSB effect using relativistic
heavy-ion collisions.
Phys. Rev. C 107 (2023) 024901 We present the first measurements of transverse momentum spectra of
$\pi^{\pm}$, $K^{\pm}$, $p(\bar{p})$ at midrapidity ($|y| < 0.1$) in U+U
collisions at ...$\sqrt{s_{NN}}$ = 193 GeV with the STAR detector at the
Relativistic Heavy Ion Collider (RHIC). The centrality dependence of particle
yields, average transverse momenta, particle ratios and kinetic freeze-out
parameters are discussed. The results are compared with the published results
from Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV in STAR. The results are
also compared to those from A Multi Phase Transport (AMPT) model.
The STAR Collaboration reports measurements of the transverse single-spin
asymmetries, $A_N$, for inclusive jets and identified `hadrons within jets'
production at midrapidity from transversely ...polarized $pp$ collisions at
$\sqrt{s}$ = 200 GeV, based on data recorded in 2012 and 2015. The inclusive
jet asymmetry measurements include $A_N$ for inclusive jets and $A_N$ for jets
containing a charged pion carrying a momentum fraction $z>0.3$ of the jet
momentum. The identified hadron within jet asymmetry measurements include the
Collins effect for charged pions, kaons and protons, and the Collins-like
effect for charged pions. The measured asymmetries are determined for several
distinct kinematic regions, characterized by the jet transverse momentum
$p_{T}$ and pseudorapidity $\eta$, as well as the hadron momentum fraction $z$
and momentum transverse to the jet axis $j_{T}$. These results probe higher
momentum scales ($Q^{2}$ up to $\sim$\,900 GeV$^{2}$) than current,
semi-inclusive deep inelastic scattering measurements, and they provide new
constraints on quark transversity in the proton and enable tests of evolution,
universality and factorization breaking in the transverse-momentum-dependent
formalism.
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 $v_{2}$.
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.
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.
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.
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