22,085 research outputs found
Fluctuations and Correlations in STAR
The study of correlations and fluctuations can provide evidence for the
production of the quark-gluon plasma (QGP) in relativistic heavy ion
collisions. Various theories predict that the production of a QGP phase in
relativistic heavy ion collisions could produce significant event-by-event
correlations and fluctuations in, transverse momentum, multiplicity, etc. Some
of the recent results using STAR at RHIC will be presented along with results
from other experiments at RHIC. The focus is on forward-backward multiplicity
correlations, balance function, charge and transverse momentum fluctuations,
and correlations.Comment: 6 pages, 13 figures, Invited talk at QNP06, Madrid, Spain,June 200
Helioseismology of Pre-Emerging Active Regions II: Average Emergence Properties
We report on average subsurface properties of pre-emerging active regions as
compared to areas where no active region emergence was detected. Helioseismic
holography is applied to samples of the two populations (pre-emergence and
without emergence), each sample having over 100 members, which were selected to
minimize systematic bias, as described in Leka et al. We find that there are
statistically significant signatures (i.e., difference in the means of more
than a few standard errors) in the average subsurface flows and the apparent
wave speed that precede the formation of an active region. The measurements
here rule out spatially extended flows of more than about 15 m/s in the top 20
Mm below the photosphere over the course of the day preceding the start of
visible emergence. These measurements place strong constraints on models of
active region formation.Comment: 15 pages, 10 figures, ApJ (published
Validating Forward Modeling and Inversions of Helioseismic Holography Measurements
Here we use synthetic data to explore the performance of forward models and
inverse methods for helioseismic holography. Specifically, this work presents
the first comprehensive test of inverse modeling for flows using
lateral-vantage (deep-focus) holography. We derive sensitivity functions in the
Born approximation. We then use these sensitivity functions in a series of
forward models and inversions of flows from a publicly available
magnetohydrodynamic quiet-Sun simulation. The forward travel times computed
using the kernels generally compare favorably with measurements obtained by
applying holography, in a lateral-vantage configuration, on a 15-hour time
series of artificial Dopplergrams extracted from the simulation. Inversions for
the horizontal flow components are able to reproduce the flows in the upper 3Mm
of the domain, but are compromised by noise at greater depths.Comment: accepted for publication by the Astrophysical
Fluctuations and the QCD phase diagram
In this contribution the role of quantum fluctuations for the QCD phase
diagram is discussed. This concerns in particular the importance of the matter
back-reaction to the gluonic sector. The impact of these fluctuations on the
location of the confinement/deconfinement and the chiral transition lines as
well as their interrelation are investigated. Consequences of our findings for
the size of a possible quarkyonic phase and location of a critical endpoint in
the phase diagram are drawn.Comment: 7 pages, 3 figures, to appear in Physics of Atomic Nucle
Is there a unique thermal source of dileptons in Pb(158 AGeV) + Au, Pb reactions?
An analysis of the dilepton measurements in the reactions Pb(158 AGeV)
+ Au, Pb points to a unique thermal source contributing to the invariant mass
and transverse momentum spectra. Effects of the flow pattern are discussed.Comment: 7 LaTeX pages including eps figure
Parallel updating cellular automaton models of driven diffusive Frenkel-Kontorova-type systems
Three cellular automaton (CA) models of increasing complexity are introduced
to model driven diffusive systems related to the generalized Frenkel-Kontorova
(FK) models recently proposed by Braun [Phys.Rev.E58, 1311 (1998)]. The models
are defined in terms of parallel updating rules. Simulation results are
presented for these models. The features are qualitatively similar to those
models defined previously in terms of sequentially updating rules. Essential
features of the FK model such as phase transitions, jamming due to atoms in the
immobile state, and hysteresis in the relationship between the fraction of
atoms in the running state and the bias field are captured. Formulating in
terms of parallel updating rules has the advantage that the models can be
treated analytically by following the time evolution of the occupation on every
site of the lattice. Results of this analytical approach are given for the two
simpler models. The steady state properties are found by studying the stable
fixed points of a closed set of dynamical equations obtained within the
approximation of retaining spatial correlations only upto two nearest
neighboring sites. Results are found to be in good agreement with numerical
data.Comment: 26 pages, 4 eps figure
Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice
Quantum spin liquids are materials that feature quantum entangled spin
correlations and avoid magnetic long-range order at T = 0 K. Particularly
interesting are two-dimensional honeycomb spin lattices where a plethora of
exotic quantum spin liquids have been predicted. Here, we experimentally study
an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and
next-nearest neighbor interactions. We demonstrate that YbBr avoids order
down to at least T=100 mK and features a dynamic spin-spin correlation function
with broad continuum scattering typical of quantum spin liquids near a quantum
critical point. The continuum in the spin spectrum is consistent with plaquette
type fluctuations predicted by theory. Our study is the experimental
demonstration that strong quantum fluctuations can exist on the honeycomb
lattice even in the absence of Kitaev-type interactions, and opens a new
perspective on quantum spin liquids.Comment: 32 pages, 7 Figure
Energy Dependence of Short and Long-Range Multiplicity Correlations in Au+Au Collisions from STAR
A general overview of the measurement of long-range multiplicity correlations
measured by the STAR experiment in Au+Au collisions at RHIC is presented. The
presence of long-range correlations can provide insight into the early stages,
and the type of matter produced in, these collisions. These measurements have
been made in Au+Au collisions at = 200 and 62.4 GeV. These
results indicate a relatively large long-range correlation is produced in Au+Au
collisions compared to a {\it pp} baseline at = 200 GeV. A
weaker long-range correlation is seen as a function of incident energy.
Further, comparison of the onset of the long-range correlation to the
calculated percolation density parameter at = 200 GeV is
presented.Comment: 8 pages, 6 figures, Conference Proceedings for the XI International
Workshop on Correlation and Fluctuation in Multiparticle Production,
Hangzhou, China, November 21-25, 200
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