The polarization of Λ and Λover ¯ hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at sqrts_{NN}=200 ...GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild p_{T} dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagrees with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and p_{T} dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.
The elliptic (v_{2}) and triangular (v_{3}) azimuthal anisotropy coefficients in central ^{3}He+Au, d+Au, and p+Au collisions at sqrts_{NN}=200 GeV are measured as a function of transverse momentum ...(p_{T}) at midrapidity (|η|<0.9), via the azimuthal angular correlation between two particles both at |η|<0.9. While the v_{2}(p_{T}) values depend on the colliding systems, the v_{3}(p_{T}) values are system independent within the uncertainties, suggesting an influence on eccentricity from subnucleonic fluctuations in these small-sized systems. These results also provide stringent constraints for the hydrodynamic modeling of these systems.
We report here the first observation of directed flow (v_{1}) of the hypernuclei _{Λ}^{3}H and _{Λ}^{4}H in mid-central Au+Au collisions at sqrts_{NN}=3 GeV at RHIC. These data are taken as part of ...the beam energy scan program carried out by the STAR experiment. From 165×10^{6} events in 5%-40% centrality, about 8400 _{Λ}^{3}H and 5200 _{Λ}^{4}H candidates are reconstructed through two- and three-body decay channels. We observe that these hypernuclei exhibit significant directed flow. Comparing to that of light nuclei, it is found that the midrapidity v_{1} slopes of _{Λ}^{3}H and _{Λ}^{4}H follow baryon number scaling, implying that the coalescence is the dominant mechanism for these hypernuclei production in the 3 GeV Au+Au collisions.
Phys. Rev. C 110, 014905 (2024) For the search of the chiral magnetic effect (CME), STAR previously presented
the results from isobar collisions
(${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$,
...${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) obtained through a blind
analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the
CME-sensitive charge-dependent azimuthal correlator ($\Delta\gamma$),
normalized by elliptic anisotropy ($v_{2}$), was observed to be close to but
systematically larger than the inverse multiplicity ratio. The background
baseline for the isobar ratio, $Y =
\frac{(\Delta\gamma/v_{2})^{\text{Ru}}}{(\Delta\gamma/v_{2})^{\text{Zr}}}$, is
naively expected to be $\frac{(1/N)^{\text{Ru}}}{(1/N)^{\text{Zr}}}$; however,
genuine two- and three-particle correlations are expected to alter it. We
estimate the contributions to $Y$ from those correlations, utilizing both the
isobar data and HIJING simulations. After including those contributions, we
arrive at a final background baseline for $Y$, which is consistent with the
isobar data. We extract an upper limit for the CME fraction in the
$\Delta\gamma$ measurement of approximately $10\%$ at a $95\%$ confidence level
on in isobar collisions at $\sqrt{s_{\text{NN}}} = 200$ GeV, with an expected
$15\%$ difference in their squared magnetic fields.
At the origin of the Universe, asymmetry between the amount of created matter
and antimatter led to the matter-dominated Universe as we know today. The
origins of this asymmetry remain not completely ...understood yet. High-energy
nuclear collisions create conditions similar to the Universe microseconds after
the Big Bang, with comparable amounts of matter and antimatter. Much of the
created antimatter escapes the rapidly expanding fireball without annihilating,
making such collisions an effective experimental tool to create heavy
antimatter nuclear objects and study their properties, hoping to shed some
light on existing questions on the asymmetry between matter and antimatter.
Here we report the first observation of the antimatter hypernucleus
\hbox{$^4_{\bar{\Lambda}}\overline{\hbox{H}}$}, composed of a $\bar{\Lambda}$ ,
an antiproton and two antineutrons. The discovery was made through its two-body
decay after production in ultrarelativistic heavy-ion collisions by the STAR
experiment at the Relativistic Heavy Ion Collider. In total, 15.6 candidate
\hbox{$^4_{\bar{\Lambda}}\overline{\hbox{H}}$} antimatter hypernuclei are
obtained with an estimated background count of 6.4. The lifetimes of the
antihypernuclei \hbox{$^3_{\bar{\Lambda}}\overline{\hbox{H}}$} and
\hbox{$^4_{\bar{\Lambda}}\overline{\hbox{H}}$} are measured and compared with
the lifetimes of their corresponding hypernuclei, testing the symmetry between
matter and antimatter. Various production yield ratios among (anti)hypernuclei
and (anti)nuclei are also measured and compared with theoretical model
predictions, shedding light on their production mechanisms.
Physics Letters B, Volume 852, May 2024, 138601 We report results on an elastic cross section measurement in proton-proton
collisions at a center-of-mass energy $\sqrt{s}=510$ GeV, obtained with the
...Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider
(RHIC). The elastic differential cross section is measured in the four-momentum
transfer squared range $0.23 \leq -t \leq 0.67$ GeV$^2$. We find that a
constant slope $B$ does not fit the data in the aforementioned $t$ range, and
we obtain a much better fit using a second-order polynomial for $B(t)$. The $t$
dependence of $B$ is determined using six subintervals of $t$ in the STAR
measured $t$ range, and is in good agreement with the phenomenological models.
The measured elastic differential cross section $\mathrm{d}\sigma/\mathrm{dt}$
agrees well with the results obtained at $\sqrt{s} = 546$ GeV for
proton--antiproton collisions by the UA4 experiment. We also determine that the
integrated elastic cross section within the STAR $t$-range is
$\sigma^\mathrm{fid}_\mathrm{el} = 462.1 \pm 0.9 (\mathrm{stat.}) \pm 1.1
(\mathrm {syst.}) \pm 11.6 (\mathrm {scale})$~$\mu\mathrm{b}$.
The longitudinal and transverse spin transfers to $\Lambda$
($\overline{\Lambda}$) hyperons in polarized proton-proton collisions are
expected to be sensitive to the helicity and transversity ...distributions,
respectively, of (anti-)strange quarks in the proton, and to the corresponding
polarized fragmentation functions. We report improved measurements of the
longitudinal spin transfer coefficient, $D_{LL}$, and the transverse spin
transfer coefficient, $D_{TT}$, to $\Lambda$ and $\overline{\Lambda}$ in
polarized proton-proton collisions at $\sqrt{s}$ = 200 GeV by the STAR
experiment at RHIC. The data set includes longitudinally polarized
proton-proton collisions with an integrated luminosity of 52 pb$^{-1}$, and
transversely polarized proton-proton collisions with a similar integrated
luminosity. Both data sets have about twice the statistics of previous results
and cover a kinematic range of $|\eta_{\Lambda(\overline{\Lambda})}|$ $<$ 1.2
and transverse momentum $p_{T,{\Lambda(\overline{\Lambda})}}$ up to 8 GeV/$c$.
We also report the first measurements of the hyperon spin transfer coefficients
$D_{LL}$ and $D_{TT}$ as a function of the fractional jet momentum $z$ carried
by the hyperon, which can provide more direct constraints on the polarized
fragmentation functions.
Phys. Rev. C 109, 044914 (2024) We measure triangular flow relative to the reaction plane at 3 GeV
center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion
Collider. A significant ...$v_3$ signal for protons is observed, which increases
for higher rapidity, higher transverse momentum, and more peripheral
collisions. The triangular flow is essentially rapidity-odd with a slope at
mid-rapidity, $dv_3/dy|_{(y=0)}$, opposite in sign compared to the slope for
directed flow. No significant $v_3$ signal is observed for charged pions and
kaons. Comparisons with models suggest that a mean field potential is required
to describe these results, and that the triangular shape of the participant
nucleons is the result of stopping and nuclear geometry.
Angular distributions of charged particles relative to jet axes are studied
in $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV Au+Au collisions as a function of the jet
orientation with respect to the event ...plane. This differential study tests the
expected path-length dependence of energy loss experienced by a hard-scattered
parton as it traverses the hot and dense medium formed in heavy-ion collisions.
A second-order event plane is used in the analysis as an experimental estimate
of the reaction plane formed by the collision impact parameter and the beam
direction. Charged-particle jets with $15 < p_{\rm T, jet} <$ 20 and $20 <
p_{\rm T, jet} <$ 40 GeV/$c$ were reconstructed with the anti-$k_{\rm T}$
algorithm with radius parameter setting of (R=0.4) in the 20-50\% centrality
bin to maximize the initial-state eccentricity of the interaction region. The
reaction plane fit method is implemented to remove the flow-modulated
background with better precision than prior methods. Yields and widths of
jet-associated charged-hadron distributions are extracted in three angular bins
between the jet axis and the event plane. The event-plane (EP) dependence is
further quantified by ratios of the associated yields in different EP bins. No
dependence on orientation of the jet axis with respect to the event plane is
seen within the uncertainties in the kinematic regime studied. This finding is
consistent with a similar experimental observation by ALICE in
$\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV Pb+Pb collision data.
Phys. Rev. C 108, 014909 (2023) Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been
observed in non-central heavy-ion collisions. The strong magnetic field
primarily created ...by the spectator protons in such collisions would split the
$\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} -
P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts
topological charge fluctuations in vacuum, resulting in a chirality imbalance
or parity violation in a local domain. This would give rise to an imbalance
($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} +
N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$
($\bar{\Lambda}$) as well as a charge separation along the magnetic field,
referred to as the chiral magnetic effect (CME). This charge separation can be
characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and
parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of
$\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive
conclusions concerning the CME or the magnetic field, and $\Delta n$ has not
been measured previously. Correlations among these observables may reveal new
insights. This paper reports measurements of correlation between $\Delta n$ and
$\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation
between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au
collisions at 27 GeV. For both measurements, no correlations have been observed
beyond statistical fluctuations.
