26 research outputs found
Search for the Chiral Magnetic Effect in Au+Au collisions at GeV with the STAR forward Event Plane Detectors
A decisive experimental test of the Chiral Magnetic Effect (CME) is
considered one of the major scientific goals at the Relativistic Heavy-Ion
Collider (RHIC) towards understanding the nontrivial topological fluctuations
of the Quantum Chromodynamics vacuum. In heavy-ion collisions, the CME is
expected to result in a charge separation phenomenon across the reaction plane,
whose strength could be strongly energy dependent. The previous CME searches
have been focused on top RHIC energy collisions. In this Letter, we present a
low energy search for the CME in Au+Au collisions at
GeV. We measure elliptic flow scaled charge-dependent correlators relative to
the event planes that are defined at both mid-rapidity and at
forward rapidity . We compare the results based on the
directed flow plane () at forward rapidity and the elliptic flow plane
() at both central and forward rapidity. The CME scenario is expected
to result in a larger correlation relative to than to , while
a flow driven background scenario would lead to a consistent result for both
event planes[1,2]. In 10-50\% centrality, results using three different event
planes are found to be consistent within experimental uncertainties, suggesting
a flow driven background scenario dominating the measurement. We obtain an
upper limit on the deviation from a flow driven background scenario at the 95\%
confidence level. This work opens up a possible road map towards future CME
search with the high statistics data from the RHIC Beam Energy Scan Phase-II.Comment: main: 8 pages, 5 figures; supplementary material: 2 pages, 1 figur
Measurement of and binding energy in Au+Au collisions at = 3 GeV
Measurements of mass and binding energy of and
in Au+Au collisions at GeV are
presented, with an aim to address the charge symmetry breaking (CSB) problem in
hypernuclei systems with atomic number A = 4. The binding energies
are measured to be MeV and MeV for and , respectively. The measured binding-energy difference
is MeV for ground states. Combined with
the -ray transition energies, the binding-energy difference for excited
states is MeV, which is negative and
comparable to the value of the ground states within uncertainties. These new
measurements on the binding-energy difference in A = 4 hypernuclei
systems are consistent with the theoretical calculations that result in
and present a new method for the study of CSB effect using relativistic
heavy-ion collisions.Comment: 8 pages, 5 figure
Observation of Global Spin Alignment of and Vector Mesons in Nuclear Collisions
The strong force, as one of the four fundamental forces at work in the
universe, governs interactions of quarks and gluons, and binds together the
atomic nucleus. 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 distance scales 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 and , emerge with a surprising pattern of global
spin alignment. In particular, the global spin alignment for is
unexpectedly large, while that for is consistent with zero. The
observed spin-alignment pattern and magnitude for the cannot be
explained by conventional mechanisms, while a model with strong force fields
accommodates the current data. This is the first time that the strong force
field is experimentally supported as a key mechanism that leads to global spin
alignment. We extract a quantity proportional to the intensity of the field of
the strong force. Within the framework of the Standard Model, where the strong
force is typically described in the quark and gluon language of Quantum
Chromodynamics, the field being considered here is an effective proxy
description. This is a qualitatively new class of measurement, which opens a
new avenue for studying the behaviour of strong force fields via their imprint
on spin alignment
Tomography of Ultra-relativistic Nuclei with Polarized Photon-gluon Collisions
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. ). In this experiment, the polarization was
utilized in diffractive photoproduction to observe a unique spin interference
pattern in the angular distribution of decays.
The observed interference is a result of an overlap of two wave functions at a
distance an order of magnitude larger than the travel distance
within its lifetime. The strong-interaction nuclear radii were extracted from
these diffractive interactions, and found to be fm () and fm (), larger than the nuclear charge
radii. The observable is demonstrated to be sensitive to the nuclear geometry
and quantum interference of non-identical particles
Measurement of electrons from open heavy-flavor hadron decays in Au+Au collisions at GeV with the STAR detector
We report a new measurement of the production of electrons from open
heavy-flavor hadron decays (HFEs) at mid-rapidity ( 0.7) in Au+Au
collisions at GeV. Invariant yields of HFEs are
measured for the transverse momentum range of GeV/ in
various configurations of the collision geometry. The HFE yields in head-on
Au+Au collisions are suppressed by approximately a factor of 2 compared to that
in + collisions scaled by the average number of binary collisions,
indicating strong interactions between heavy quarks and the hot and dense
medium created in heavy-ion collisions. Comparison of these results with models
provides additional tests of theoretical calculations of heavy quark energy
loss in the quark-gluon plasma
Elliptic Flow of Heavy-Flavor Decay Electrons in Au+Au Collisions at = 27 and 54.4 GeV at RHIC
We report on new measurements of elliptic flow () of electrons from
heavy-flavor hadron decays at mid-rapidity () in Au+Au collisions at
= 27 and 54.4 GeV from the STAR experiment. Heavy-flavor
decay electrons () in Au+Au collisions at =
54.4 GeV exhibit a non-zero in the transverse momentum ()
region of 2 GeV/ with the magnitude comparable to that at
GeV. The measured at 54.4 GeV is
also consistent with the expectation of their parent charm hadron
following number-of-constituent-quark scaling as other light and strange flavor
hadrons at this energy. These suggest that charm quarks gain significant
collectivity through the evolution of the QCD medium and may reach local
thermal equilibrium in Au+Au collisions at GeV. The
measured in Au+Au collisions at 27
GeV is consistent with zero within large uncertainties. The energy dependence
of for different flavor particles () shows an
indication of quark mass hierarchy in reaching thermalization in high-energy
nuclear collisions.Comment: 12 pages, 7 figures, 1 tabl
Search for the chiral magnetic effect via charge-dependent azimuthal correlations relative to spectator and participant planes in Au+Au collisions at = 200 GeV
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 with a significance of 1--3 standard
deviations in mid-central (intermediate impact parameter) collisions.
Significant residual background effects may, however, still be present.Comment: 8 pages, 3 figure