4,251 research outputs found
The Degrees of Freedom of Partial Least Squares Regression
The derivation of statistical properties for Partial Least Squares regression
can be a challenging task. The reason is that the construction of latent
components from the predictor variables also depends on the response variable.
While this typically leads to good performance and interpretable models in
practice, it makes the statistical analysis more involved. In this work, we
study the intrinsic complexity of Partial Least Squares Regression. Our
contribution is an unbiased estimate of its Degrees of Freedom. It is defined
as the trace of the first derivative of the fitted values, seen as a function
of the response. We establish two equivalent representations that rely on the
close connection of Partial Least Squares to matrix decompositions and Krylov
subspace techniques. We show that the Degrees of Freedom depend on the
collinearity of the predictor variables: The lower the collinearity is, the
higher the Degrees of Freedom are. In particular, they are typically higher
than the naive approach that defines the Degrees of Freedom as the number of
components. Further, we illustrate how the Degrees of Freedom approach can be
used for the comparison of different regression methods. In the experimental
section, we show that our Degrees of Freedom estimate in combination with
information criteria is useful for model selection.Comment: to appear in the Journal of the American Statistical Associatio
Higgs radiation off quarks in supersymmetric theories at e^+e^- colliders
Yukawa couplings between Higgs bosons and quarks in supersymmetric theories
can be measured in the processes e^+e^- -> Q Qbar + Higgs. We have determined
the cross sections of these processes in the minimal supersymmetric model
including the complete set of next-to-leading order QCD corrections for all
channels.Comment: 12 pages, latex, 3 figure
Bounds on R-parity violating supersymmetric couplings from leptonic and semi-leptonic meson decays
We present a comprehensive update of the bounds on R-Parity violating
supersymmetric couplings from lepton-flavour- and lepton-number-violating decay
processes. We consider tau and mu decays as well as leptonic and semi-leptonic
decays of mesons. We present several new bounds resulting from tau, eta and
Kaon decays and correct some results in the literature concerning B-meson
decays.Comment: 30 pages; changed title, updated some bounds from the literature from
different references, added reference
Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers
The occurrence of high, persistent ice supersaturation inside and outside cold cirrus in the tropical tropopause layer (TTL) remains an enigma that is intensely debated as the "ice supersaturation puzzle". However, it was recently confirmed that observed supersaturations are consistent with very low ice crystal concentrations, which is incompatible with the idea that homogeneous freezing is the major method of ice formation in the TTL. Thus, the tropical tropopause "ice supersaturation puzzle" has become an "ice nucleation puzzle". To explain the low ice crystal concentrations, a number of mainly heterogeneous freezing methods have been proposed. Here, we reproduce in situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamic conditions in the TTL, namely the superposition of slow large-scale updraughts with high-frequency short waves. From the simulations, it follows that the full range of observed ice crystal concentrations can be explained when the model results are composed from scenarios with consecutive heterogeneous and homogeneous ice formation and scenarios with pure homogeneous ice formation occurring in very slow (< 1 cm s<sup>â1</sup>) and faster (> 1 cm s<sup>â1</sup>) large-scale updraughts, respectively. This statistical analysis shows that about 80% of TTL cirrus can be explained by "classical" homogeneous ice nucleation, while the remaining 20% stem from heterogeneous and homogeneous freezing occurring within the same environment. The mechanism limiting ice crystal production via homogeneous freezing in an environment full of gravity waves is the shortness of the gravity waves, which stalls freezing events before a higher ice crystal concentration can be formed
Quantum Gravitational Contributions to the CMB Anisotropy Spectrum
We derive the primordial power spectrum of density fluctuations in the
framework of quantum cosmology. For this purpose we perform a Born-Oppenheimer
approximation to the Wheeler-DeWitt equation for an inflationary universe with
a scalar field. In this way we first recover the scale-invariant power spectrum
that is found as an approximation in the simplest inflationary models. We then
obtain quantum gravitational corrections to this spectrum and discuss whether
they lead to measurable signatures in the CMB anisotropy spectrum. The
non-observation so far of such corrections translates into an upper bound on
the energy scale of inflation.Comment: 4 pages, v3: sign error in Eq. (5) and its consequences correcte
U(1)-Symmetry breaking and violation of axial symmetry in TlCuCl3 and other insulating spin systems
We describe the Bose-Einstein condensate of magnetic bosonic quasiparticles
in insulating spin systems using a phenomenological standard functional method
for T = 0. We show that results that are already known from advanced
computational techniques immediately follow. The inclusion of a perturbative
anisotropy term that violates the axial symmetry allows us to remarkably well
explain a number of experimental features of the dimerized spin-1/2 system
TlCuCl3. Based on an energetic argument we predict a general intrinsic
instability of an axially symmetric magnetic condensate towards a violation of
this symmetry, which leads to the spontaneous formation of an anisotropy gap in
the energy spectrum above the critical field. We, therefore, expect that a true
Goldstone mode in insulating spin systems, i.e., a strictly linear
energy-dispersion relation down to arbitrarily small excitations energies,
cannot be observed in any real material.Comment: 6 pages, 3 figure
TMDlib and TMDplotter: library and plotting tools for transverse-momentum-dependent parton distributions
Transverse-momentum-dependent distributions (TMDs) are central in high-energy
physics from both theoretical and phenomenological points of view. In this
manual we introduce the library, TMDlib, of fits and parameterisations for
transverse-momentum-dependent parton distribution functions (TMD PDFs) and
fragmentation functions (TMD FFs) together with an online plotting tool,
TMDplotter. We provide a description of the program components and of the
different physical frameworks the user can access via the available
parameterisations.Comment: version 2, referring to TMDlib 1.0.2 - comments and references adde
Color-Octet Contributions to Photoproduction
We have calculated the leading color-octet contributions to the production of
particles in photon-proton collisions. Using the values for the
color-octet matrix elements extracted from fits to prompt data at the
Tevatron, we demonstrate that distinctive color-octet signatures should be
visible in photoproduction. However, these predictions appear at
variance with recent experimental data obtained at HERA, indicating that the
phenomenological importance of the color-octet contributions is smaller than
expected from theoretical considerations and suggested by the Tevatron fits.Comment: 10 pages, LaTeX, epsfig, 4 figure
Description of non-specific DNA-protein interaction and facilitated diffusion with a dynamical model
We propose a dynamical model for non-specific DNA-protein interaction, which
is based on the 'bead-spring' model previously developed by other groups, and
investigate its properties using Brownian Dynamics simulations. We show that
the model successfully reproduces some of the observed properties of real
systems and predictions of kinetic models. For example, sampling of the DNA
sequence by the protein proceeds via a succession of 3d motion in the solvent,
1d sliding along the sequence, short hops between neighboring sites, and
intersegmental transfers. Moreover, facilitated diffusion takes place in a
certain range of values of the protein effective charge, that is, the
combination of 1d sliding and 3d motion leads to faster DNA sampling than pure
3d motion. At last, the number of base pairs visited during a sliding event is
comparable to the values deduced from single-molecule experiments. We also
point out and discuss some discrepancies between the predictions of this model
and some recent experimental results as well as some hypotheses and predictions
of kinetic models
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