978 research outputs found
Recommended from our members
Improving the condition number of estimated covariance matrices
High dimensional error covariance matrices and their inverses are used to weight the
contribution of observation and background information in data assimilation procedures. As
observation error covariance matrices are often obtained by sampling methods, estimates are
often degenerate or ill-conditioned, making it impossible to invert an observation error
covariance matrix without the use of techniques to reduce its condition number. In this paper
we present new theory for two existing methods that can be used to ‘recondition’ any covariance
matrix: ridge regression, and the minimum eigenvalue method. We compare these methods
with multiplicative variance inflation, which cannot alter the condition number of a matrix, but
is often used to account for neglected correlation information. We investigate the impact of
reconditioning on variances and correlations of a general covariance matrix in both a theoretical
and practical setting. Improved theoretical understanding provides guidance to users regarding
method selection, and choice of target condition number. The new theory shows that, for the
same target condition number, both methods increase variances compared to the original
matrix, with larger increases for ridge regression than the minimum eigenvalue method. We
prove that the ridge regression method strictly decreases the absolute value of off-diagonal
correlations. Theoretical comparison of the impact of reconditioning and multiplicative
variance inflation on the data assimilation objective function shows that variance inflation alters
information across all scales uniformly, whereas reconditioning has a larger effect on scales
corresponding to smaller eigenvalues. We then consider two examples: a general correlation
function, and an observation error covariance matrix arising from interchannel correlations. The
minimum eigenvalue method results in smaller overall changes to the correlation matrix than
ridge regression, but can increase off-diagonal correlations. Data assimilation experiments reveal
that reconditioning corrects spurious noise in the analysis but underestimates the true signal
compared to multiplicative variance inflation
Hadronization of massive quark matter
We present a fast hadronization model for the constituent quark plasma (CQP)
produced in relativistic heavy ion collisions at SPS. The model is based on
rate equations and on an equation of state inspired by the string
phenomenology. This equation of state has a confining character. We display the
time evolution of the relevant physical quantities during the hadronization
process and the final hadron multiplicities. The results indicate that the
hadronization of CQP is fast.Comment: 12 pages, Latex, 2 EPS figures, contribution to the Proceedings of
the 4th International Conference on Strangeness in Quark Matter (SQM'98),
Padova, Italy, 20-24 July 199
Estimation of properties of low-lying excited states of Hubbard models : a multi-configurational symmetrized projector quantum Monte Carlo approach
We present in detail the recently developed multi-configurational symmetrized
projector quantum Monte Carlo (MSPQMC) method for excited states of the Hubbard
model. We describe the implementation of the Monte Carlo method for a
multi-configurational trial wavefunction. We give a detailed discussion of
issues related to the symmetry of the projection procedure which validates our
Monte Carlo procedure for excited states and leads naturally to the idea of
symmetrized sampling for correlation functions, developed earlier in the
context of ground state simulations. It also leads to three possible averaging
schemes. We have analyzed the errors incurred in these various averaging
procedures and discuss and detail the preferred averaging procedure for
correlations that do not have the full symmetry of the Hamiltonian. We study
the energies and correlation functions of the low-lying excited states of the
half-filled Hubbard model in 1-D. We have used this technique to study the
pair-binding energies of two holes in and systems, which compare
well the Bethe ansatz data of Fye, Martins and Scalettar. We have also studied
small clusters amenable to exact diagonalization studies in 2-D and have
reproduced their energies and correlation functions by the MSPQMC method. We
identify two ways in which a multiconfigurational trial wavefunction can lead
to a negative sign problem. We observe that this effect is not severe in 1-D
and tends to vanish with increasing system size. We also note that this does
not enhance the severity of the sign problem in two dimensions.Comment: 29 pages, 2 figures available on request, submitted to Phys. Rev.
The climate sensitivity of carbon, timber, and species richness covaries with forest age in boreal–temperate North America
Climate change threatens the provisioning of forest ecosystem services and biodiversity (ESB). The climate sensitivity of ESB may vary with forest development from young to old-growth conditions as structure and composition shift over time and space. This study addresses knowledge gaps hindering implementation of adaptive forest management strategies to sustain ESB. We focused on a number of ESB indicators to (a) analyze associations among carbon storage, timber growth rate, and species richness along a forest development gradient; (b) test the sensitivity of these associations to climatic changes; and (c) identify hotspots of climate sensitivity across the boreal–temperate forests of eastern North America. From pre-existing databases and literature, we compiled a unique dataset of 18,507 forest plots. We used a full Bayesian framework to quantify responses of nine ESB indicators. The Bayesian models were used to assess the sensitivity of these indicators and their associations to projected increases in temperature and precipitation. We found the strongest association among the investigated ESB indicators in old forests (\u3e170 years). These forests simultaneously support high levels of carbon storage, timber growth, and species richness. Older forests also exhibit low climate sensitivity of associations among ESB indicators as compared to younger forests. While regions with a currently low combined ESB performance benefitted from climate change, regions with a high ESB performance were particularly vulnerable to climate change. In particular, climate sensitivity was highest east and southeast of the Great Lakes, signaling potential priority areas for adaptive management. Our findings suggest that strategies aimed at enhancing the representation of older forest conditions at landscape scales will help sustain ESB in a changing world
Phase Transitions Driven by Vortices in 2D Superfluids and Superconductors: From Kosterlitz-Thouless to 1st Order
The Landau-Ginzburg-Wilson hamiltonian is studied for different values of the
parameter which multiplies the quartic term (it turns out that this
is equivalent to consider different values of the coherence length in
units of the lattice spacing ). It is observed that amplitude fluctuations
can change dramatically the nature of the phase transition: for small values of
(), instead of the smooth Kosterlitz-Thouless transition
there is a {\em first order} transition with a discontinuous jump in the vortex
density and a larger non-universal drop in the helicity modulus. In
particular, for sufficiently small (), the density of
bound pairs of vortex-antivortex below is so low that, drops to zero
almost for all temperature .Comment: 8 pages, 5 .eps figure
First Order Transition in the Ginzburg-Landau Model
The d-dimensional complex Ginzburg-Landau (GL) model is solved according to a
variational method by separating phase and amplitude. The GL transition becomes
first order for high superfluid density because of effects of phase
fluctuations. We discuss its origin with various arguments showing that, in
particular for d = 3, the validity of our approach lies precisely in the first
order domain.Comment: 4 pages including 2 figure
Attosecond control of electrons emitted from a nanoscale metal tip
Attosecond science is based on steering of electrons with the electric field
of well-controlled femtosecond laser pulses. It has led to, for example, the
generation of XUV light pulses with a duration in the sub-100-attosecond
regime, to the measurement of intra-molecular dynamics by diffraction of an
electron taken from the molecule under scrutiny, and to novel ultrafast
electron holography. All these effects have been observed with atoms or
molecules in the gas phase. Although predicted to occur, a strong light-phase
sensitivity of electrons liberated by few-cycle laser pulses from solids has
hitherto been elusive. Here we show a carrier-envelope (C-E) phase-dependent
current modulation of up to 100% recorded in spectra of electrons laser-emitted
from a nanometric tungsten tip. Controlled by the C-E phase, electrons
originate from either one or two sub-500as long instances within the 6-fs laser
pulse, leading to the presence or absence of spectral interference. We also
show that coherent elastic re-scattering of liberated electrons takes place at
the metal surface. Due to field enhancement at the tip, a simple laser
oscillator suffices to reach the required peak electric field strengths,
allowing attosecond science experiments to be performed at the 100-Megahertz
repetition rate level and rendering complex amplified laser systems
dispensable. Practically, this work represents a simple, exquisitely sensitive
C-E phase sensor device, which can be shrunk in volume down to ~ 1cm3. The
results indicate that the above-mentioned novel attosecond science techniques
developed with and for atoms and molecules can also be employed with solids. In
particular, we foresee sub-femtosecond (sub-) nanometre probing of (collective)
electron dynamics, such as plasmon polaritons, in solid-state systems ranging
in size from mesoscopic solids via clusters to single protruding atoms.Comment: Final manuscript version submitted to Natur
Thermal analysis of hadron multiplicities from relativistic quantum molecular dynamics
Some questions arising in the application of the thermal model to hadron
production in heavy ion collisions are studied. We do so by applying the
thermal model of hadron production to particle yields calculated by the
microscopic transport model RQMD(v2.3). We study the bias of incomplete
information about the final hadronic state on the extraction of thermal
parameters.It is found that the subset of particles measured typically in the
experiments looks more thermal than the complete set of stable particles. The
hadrons which show the largest deviations from thermal behaviour in RQMD(v2.3)
are the multistrange baryons and antibaryons. We also looked at the influence
of rapidity cuts on the extraction of thermal parameters and found that they
lead to different thermal parameters and larger disagreement between the RQMD
yields and the thermal model.Comment: 12 pages, 2 figures, uses REVTEX, only misprint and stylistic
corrections, to appear in Physical Review
Coulomb Effect: A Possible Probe for the Evolution of Hadronic Matter
Electromagnetic field produced in high-energy heavy-ion collisions contains
much useful information, because the field can be directly related to the
motion of the matter in the whole stage of the reaction. One can divide the
total electromagnetic field into three parts, i.e., the contributions from the
incident nuclei, non-participating nucleons and charged fluid, the latter
consisting of strongly interacting hadrons or quarks. Parametrizing the
space-time evolution of the charged fluid based on hydrodynamic model, we study
the development of the electromagnetic field which accompanies the high-energy
heavy-ion collisions. We found that the incident nuclei bring a rather strong
electromagnetic field to the interaction region of hadrons or quarks over a few
fm after the collision. On the other hand, the observed charged hadrons'
spectra are mostly affected (Coulomb effect) by the field of the charged fluid.
We compare the result of our model with experimental data and found that the
model reproduces them well. The pion yield ratio pi^-/pi+ at a RHIC energy,
Au+Au 100+100 GeV/nucleon, is also predicted.Comment: 23 pages, RevTex, 19 eps figures, revised versio
Strange hyperon and antihyperon production from quark and string-rope matter
Hyperon and antihyperon production is investigated using two microscopical
models: {\bf (1)} the fast hadronization of quark matter as given by the ALCOR
model; {\bf (2)} string formation and fragmentation as in the HIJING/B model.
We calculate the particle numbers and momentum distributions for Pb+Pb
collisions at CERN SPS energies in order to compare the two models with each
other and with the available experimental data. We show that these two
theoretical approaches give similar yields for the hyperons, but strongly
differ for antihyperons.Comment: 11 pages, Latex, 3 EPS figures, contribution to the Proceedings of
the 4th International Conference on Strangeness in Quark Matter (SQM'98),
Padova, Italy, 20-24 July 199
- …