31 research outputs found
Azimuth Quadrupole Systematics in Au-Au Collisions
We have measured -dependent two-particle number correlations on azimuth
and pseudorapidity for eleven centralities of and 200~GeV
Au-Au collisions at STAR. 2D fits to these angular correlations isolate the
azimuth quadrupole amplitude, denoted , from
localized same-side correlations. Event-plane measurements within
the STAR TPC acceptance can be expressed as a sum of the azimuth quadrupole and
the quadrupole component of the same-side peak. can be
transformed to reveal quadrupole spectra which are approximately
described by a fixed transverse boost and universal L\'evy form nearly
independent of centrality. A parametrization of can be
factored into centrality and -dependent pieces with a simple
dependence above 0.75 GeV/c. Results from STAR are compared to published data
and model predictions.Comment: Conference proceedings for Hot Quarks 201
Measurements of and Production in + Collisions at = 200 GeV
We report measurements of charmed-hadron (, ) production cross
sections at mid-rapidity in + collisions at a center-of-mass energy of
200 GeV by the STAR experiment. Charmed hadrons were reconstructed via the
hadronic decays , and their charge conjugates,
covering the range of 0.62.0 GeV/ and 2.06.0 GeV/ for
and , respectively. From this analysis, the charm-pair production cross
section at mid-rapidity is = 170 45
(stat.) (sys.) b. The extracted charm-pair cross section is
compared to perturbative QCD calculations. The transverse momentum differential
cross section is found to be consistent with the upper bound of a Fixed-Order
Next-to-Leading Logarithm calculation.Comment: 15 pages, 16 figures. Revised version submitted to Phys. Rev.
Observation of the antimatter helium-4 nucleus
High-energy nuclear collisions create an energy density similar to that of
the universe microseconds after the Big Bang, and in both cases, matter and
antimatter are formed with comparable abundance. However, the relatively
short-lived expansion in nuclear collisions allows antimatter to decouple
quickly from matter, and avoid annihilation. Thus, a high energy accelerator of
heavy nuclei is an efficient means of producing and studying antimatter. The
antimatter helium-4 nucleus (), also known as the anti-{\alpha}
(), consists of two antiprotons and two antineutrons (baryon
number B=-4). It has not been observed previously, although the {\alpha}
particle was identified a century ago by Rutherford and is present in cosmic
radiation at the 10% level. Antimatter nuclei with B < -1 have been observed
only as rare products of interactions at particle accelerators, where the rate
of antinucleus production in high-energy collisions decreases by about 1000
with each additional antinucleon. We present the observation of the antimatter
helium-4 nucleus, the heaviest observed antinucleus. In total 18
counts were detected at the STAR experiment at RHIC in 10 recorded Au+Au
collisions at center-of-mass energies of 200 GeV and 62 GeV per nucleon-nucleon
pair. The yield is consistent with expectations from thermodynamic and
coalescent nucleosynthesis models, which has implications beyond nuclear
physics.Comment: 19 pages, 4 figures. Submitted to Nature. Under media embarg
Enhanced strange baryon production in Au+Au collisions compared to p+p at sqrts = 200 GeV
We report on the observed differences in production rates of strange and
multi-strange baryons in Au+Au collisions at sqrts = 200 GeV compared to pp
interactions at the same energy. The strange baryon yields in Au+Au collisions,
then scaled down by the number of participating nucleons, are enhanced relative
to those measured in pp reactions. The enhancement observed increases with the
strangeness content of the baryon, and increases for all strange baryons with
collision centrality. The enhancement is qualitatively similar to that observed
at lower collision energy sqrts =17.3 GeV. The previous observations are for
the bulk production, while at intermediate pT, 1 < pT< 4 GeV/c, the strange
baryons even exceed binary scaling from pp yields.Comment: 7 pages, 4 figures. Printed in PR
Measurements of meson production in relativistic heavy-ion collisions at RHIC
We present results for the measurement of meson production via its
charged kaon decay channel in Au+Au collisions at
, 130, and 200 GeV, and in and +Au collisions
at GeV from the STAR experiment at the BNL Relativistic
Heavy Ion Collider (RHIC). The midrapidity () meson transverse
momentum () spectra in central Au+Au collisions are found to be well
described by a single exponential distribution. On the other hand, the
spectra from , +Au and peripheral Au+Au collisions show power-law tails
at intermediate and high and are described better by Levy
distributions. The constant yield ratio vs beam species, collision
centrality and colliding energy is in contradiction with expectations from
models having kaon coalescence as the dominant mechanism for production
at RHIC. The yield ratio as a function of is consistent
with a model based on the recombination of thermal quarks up to GeV/, but disagrees at higher transverse momenta. The measured nuclear
modification factor, , for the meson increases above unity at
intermediate , similar to that for pions and protons, while is
suppressed due to the energy loss effect in central Au+Au collisions. Number of
constituent quark scaling of both and for the meson
with respect to other hadrons in Au+Au collisions at =200 GeV
at intermediate is observed. These observations support quark
coalescence as being the dominant mechanism of hadronization in the
intermediate region at RHIC.Comment: 22 pages, 21 figures, 4 table
An Experimental Exploration of the QCD Phase Diagram: The Search for the Critical Point and the Onset of De-confinement
The QCD phase diagram lies at the heart of what the RHIC Physics Program is
all about. While RHIC has been operating very successfully at or close to its
maximum energy for almost a decade, it has become clear that this collider can
also be operated at lower energies down to 5 GeV without extensive upgrades. An
exploration of the full region of beam energies available at the RHIC facility
is imperative. The STAR detector, due to its large uniform acceptance and
excellent particle identification capabilities, is uniquely positioned to carry
out this program in depth and detail. The first exploratory beam energy scan
(BES) run at RHIC took place in 2010 (Run 10), since several STAR upgrades,
most importantly a full barrel Time of Flight detector, are now completed which
add new capabilities important for the interesting physics at BES energies. In
this document we discuss current proposed measurements, with estimations of the
accuracy of the measurements given an assumed event count at each beam energy.Comment: 59 pages, 78 figure
Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy ion collisions
Parity-odd domains, corresponding to non-trivial topological solutions of the
QCD vacuum, might be created during relativistic heavy-ion collisions. These
domains are predicted to lead to charge separation of quarks along the orbital
momentum of the system created in non-central collisions. To study this effect,
we investigate a three particle mixed harmonics azimuthal correlator which is a
\P-even observable, but directly sensitive to the charge separation effect. We
report measurements of this observable using the STAR detector in Au+Au and
Cu+Cu collisions at =200 and 62~GeV. The results are presented
as a function of collision centrality, particle separation in rapidity, and
particle transverse momentum. A signal consistent with several of the
theoretical expectations is detected in all four data sets. We compare our
results to the predictions of existing event generators, and discuss in detail
possible contributions from other effects that are not related to parity
violation.Comment: 17 pages, 14 figures, as accepted for publication in Physical Review
C
Strangelet Search in AuAu Collisions at 200 GeV
We have searched for strangelets in a triggered sample of 61 million central
(top 4%) Au+Au collisions at \sNN = 200 GeV near beam rapidities at the STAR
detector. We have sensitivity to metastable strangelets with lifetimes of order
, in contrast to limits over ten times longer in AGS studies and
longer still at the SPS. Upper limits of a few 10^{-6} to 10^{-7} per central
Au+Au collision are set for strangelets with mass GeV/c^{2}.Comment: As publishe
Studying Parton Energy Loss in Heavy-Ion Collisions via Direct-Photon and Charged-Particle Azimuthal Correlations
Charged-particle spectra associated with direct photon () and
are measured in + and Au+Au collisions at center-of-mass energy
GeV with the STAR detector at RHIC. A hower-shape
analysis is used to partially discriminate between and .
Assuming no associated charged particles in the direction (near
side) and small contribution from fragmentation photons (), the
associated charged-particle yields opposite to (away side) are
extracted. At mid-rapidity () in central Au+Au collisions,
charged-particle yields associated with and at high
transverse momentum ( GeV/) are suppressed by a factor
of 3-5 compared with + collisions. The observed suppression of the
associated charged particles, in the kinematic range and GeV/, is similar for and , and
independent of the energy within uncertainties. These
measurements indicate that the parton energy loss, in the covered kinematic
range, is insensitive to the parton path length.Comment: submitted to Phys. Rev. Lett, 6 pages, 4 figure
Hadronic resonance production in +Au collisions at = 200 GeV at RHIC
We present the first measurements of the , (892),
(1232), (1385), and (1520) resonances in +Au
collisions at = 200 GeV, reconstructed via their hadronic
decay channels using the STAR detector at RHIC. The masses and widths of these
resonances are studied as a function of transverse momentum (). We observe
that the resonance spectra follow a generalized scaling law with the transverse
mass (). The of , , and . The , ,
, , and ratios in
+Au collisions are compared to the measurements in minimum bias
interactions, where we observe that both measurements are comparable. The
nuclear modification factors () of the , , and
scale with the number of binary collisions () for 1.2 GeV/.Comment: STAR Collaboration. Submitted to PR