361 research outputs found
Candecomp/Parafac with zero constraints at arbitrary positions in a loading matrix
When one interprets Candecomp/Parafac (CP) solutions for analyzing three-way data, small loadings are often ignored, that is, considered to be zero. Rather than just considering them zero, it seems better to actually model such values as zero. This can be done by successive modeling approaches as well as by a simultaneous modeling approach. This paper offers algorithms for three such approaches, and compares them on the basis of empirical data and a simulation study. The conclusion of the latter was that, under realistic circumstances, all approaches recovered the underlying structure well, when the number of values to constrain to zero was given. Whereas the simultaneous modeling approach seemed to perform slightly better, differences were very small and not substantial. Given that the simultaneous approach is far more time consuming than the successive approaches, the present study suggests that for practical purposes successive approaches for modeling zeros in the CP model seem to be indicated
Adaptation and enslavement in endosymbiont-host associations
The evolutionary persistence of symbiotic associations is a puzzle.
Adaptation should eliminate cooperative traits if it is possible to enjoy the
advantages of cooperation without reciprocating - a facet of cooperation known
in game theory as the Prisoner's Dilemma. Despite this barrier, symbioses are
widespread, and may have been necessary for the evolution of complex life. The
discovery of strategies such as tit-for-tat has been presented as a general
solution to the problem of cooperation. However, this only holds for
within-species cooperation, where a single strategy will come to dominate the
population. In a symbiotic association each species may have a different
strategy, and the theoretical analysis of the single species problem is no
guide to the outcome. We present basic analysis of two-species cooperation and
show that a species with a fast adaptation rate is enslaved by a slowly
evolving one. Paradoxically, the rapidly evolving species becomes highly
cooperative, whereas the slowly evolving one gives little in return. This helps
understand the occurrence of endosymbioses where the host benefits, but the
symbionts appear to gain little from the association.Comment: v2: Correction made to equations 5 & 6 v3: Revised version accepted
in Phys. Rev. E; New figure adde
Direct CP violation in radiative b decays in and beyond the Standard Model
We consider the partial rate asymmetry in the inclusive decay modes b to s
gamma and b to d gamma, concentrating on non-standard models with new charged
Higgs interactions. We find that the charged Higgs contribution to the
asymmetry for b to s gamma is small in such models due to a universal
cancellation mechanism. The asymmetry is therefore difficult to distinguish
experimentally from the Standard Model (SM) value, which is also small. The
cancellation mechanism is found to be rendered inoperative in supersymmetry due
to the presence of chargino loops. Unlike b to s gamma, the rate asymmetry for
b to d gamma in Higgs models can be quite different from its SM value,
generally ranging from -20% to +20%. Specific model calculations are performed
for the Three-Higgs Doublet Model and the ``Top'' Two-Higgs Doublet Model to
serve as illustrations. We also offer some suggestions that may be helpful to
experimentalists in the detection of the inclusive mode b to d gamma.Comment: RevTex, 24 pages, 6 figures, minor changes, version to appear in PR
Direct CP-asymmetry in Inclusive Rare B-decays in 2HDM
The direct CP-asymmetry in the inclusive and decays is investigated in the two-Higgs doublet extension of the
Standard Model (2HDM). The investigation is performed in the lowest
non-vanishing order of the perturbation theory using the existing restrictions
on the 2HDM parameters space. It is shown that the direct CP-asymmetry in the
decay can deviate significantly from the Standard Model
predictions. In the presence of only one source of CP-violation (the CKM matrix
weak phase) can have the sign opposite to that in
the SM. The new source of CP-violation can make
arbitrary small (unlike the SM case) and hence unmeasurable. Quantitatively,
the obtained results suffer from the uncertainty of the choice of
renormalization scale. As for the rate asymmetry, its
renormalization scale dependence in the lowest non-vanishing order does not
allow to conclude if this quantity is efficient for testing New Physics beyond
the Standard Model.Comment: 16 pages including 2 figure
Finite-size effects on multibody neutrino exchange
The effect of multibody massless neutrino exchanges between neutrons inside a
finite-size neutron star is studied. We use an effective Lagrangian, which
incorporates the effect of the neutrons on the neutrinos. Following Schwinger,
it is shown that the total interaction energy density is computed by comparing
the zero point energy of the neutrino sea with and without the star. It has
already been shown that in an infinite-size star the total energy due to
neutrino exchange vanishes exactly. The opposite claim that massless neutrino
exchange would produce a huge energy is due to an improper summation of an
infrared-divergent quantity. The same vanishing of the total energy has been
proved exactly in the case of a finite star in a one-dimensional toy model.
Here we study the three-dimensional case. We first consider the effect of a
sharp star border, assumed to be a plane. We find that there is a non-
vanishing of the zero point energy density difference between the inside and
the outside due to the refraction index at the border and the consequent
non-penetrating waves. An analytical and numerical calculation for the case of
a spherical star with a sharp border confirms that the preceding border effect
is the dominant one. The total result is shown to be infrared-safe, thus
confirming that there is no need to assume a neutrino mass. The ultraviolet
cut-offs, which correspond in some sense to the matching of the effective
theory with the exact one, are discussed. Finally the energy due to long
distance neutrino exchange is of the order of , i.e. negligible with respect to the neutron mass density.Comment: Latex file (Revtex), 34 pages, 8 postscripted figure
Neutrino ground state in a dense star
It has recently been argued that long range forces due to the exchange of
massless neutrinos give rise to a very large self-energy in a dense,
finite-ranged, weakly-charged medium. Such an effect, if real, would
destabilize a neutron star. To address this issue we have studied the related
problem of a massless neutrino field in the presence of an external, static
electroweak potential of finite range. To be precise, we have computed to one
loop the exact vacuum energy for the case of a spherical square well potential
of depth alpha and radius R. For small wells, the vacuum energy is reliably
determined by a perturbative expansion in the external potential. For large
wells, however, the perturbative expansion breaks down. A manifestation of this
breakdown is that the vacuum carries a non-zero neutrino charge. The energy and
neutrino charge of the ground state are, to a good approximation for large
wells, those of a neutrino condensate with chemical potential mu=alpha. Our
results demonstrate explicitly that long-range forces due to the exchange of
massless neutrinos do not threaten the stability of neutron stars.Comment: 27 pages, 7 figures, revtex; several typos corrected; a few
references added; title changed slightly; includes a "note added in proof";
version to appear in Phys. Rev.
Scattering From a Two Dimensional Array of Flux Tubes: A Study of The Validity of Mean Field Theory
Mean Field Theory has been extensively used in the study of systems of anyons
in two spatial dimensions. In this paper we study the physical grounds for the
validity of this approximation by considering the Quantum Mechanical scattering
of a charged particle from a two dimensional array of magnetic flux tubes. The
flux tubes are arranged on a regular lattice which is infinitely long in the
``'' direction but which has a (small) finite number of columns in the
``'' direction. Their physical size is assumed to be infinitesimally small.
We develop a method for computing the scattering angle as well as the
reflection and transmission coefficients to lowest order in the Aharonov--Bohm
interaction. The results of our calculation are compared to the scattering of
the same particle from a region of constant magnetic field whose magnitude is
equal to the mean field of all the flux tubes. For an incident plane wave, the
Mean Field approximation is shown to be valid provided the flux in each tube is
much less than a single flux quantum. This is precisely the regime in which
Mean Field Theory for anyons is expected to be valid. When the flux per tube
becomes of order 1, Mean Field Theory is no longer valid.Comment: 23 pages, University of British Columbia Preprint UBCTP93-01
Generalized messengers of supersymmetry breaking and the sparticle mass spectrum
We investigate the sparticle spectrum in models of gauge-mediated
supersymmetry breaking. In these models, supersymmetry is spontaneously broken
at an energy scale only a few orders of magnitude above the electroweak scale.
The breakdown of supersymmetry is communicated to the standard model particles
and their superpartners by "messenger" fields through their ordinary gauge
interactions. We study the effects of a messenger sector in which the
supersymmetry-violating F-term contributions to messenger scalar masses are
comparable to the supersymmetry-preserving ones. We also argue that it is not
particularly natural to restrict attention to models in which the messenger
fields lie in complete SU(5) GUT multiplets, and we identify a much larger
class of viable models. Remarkably, however, we find that the superpartner mass
parameters in these models are still subject to many significant contraints.Comment: 24 pages, LaTeX, uses epsf.sty, 4 figures. Assumptions clarified,
numerical bounds tweaked, typos correcte
Strong CP and Mu Problems in Theories with Gauge Mediated Supersymmetry Breaking
We provide a simple solution to the and strong CP problems in the
context of gauge mediated supersymmetry breaking. The generic appearance of R
symmetry in dynamical supersymmetry breaking is used to implement Peccei-Quinn
symmetry. Acceptable and terms as well as the large symmetry breaking
scale are induced in the presence of nonrenormalizable interactions.
Cosmological consequences of this scheme turn out to yield constraints on the
PQ symmetry breaking scale and the number of the messenger/heavy quarks.
Complexity in introducing non-R Peccei-Quinn symmetry is contrasted with the
case of R symmetry.Comment: 10 pages, Revtex. Significantly modified version to apear in Phys.
Rev.
A One-Scale Model of Dynamical Supersymmetry Breaking
A model of gauge-mediated supersymmetry breaking is constructed in which the
low-energy physics depends on a single dynamical scale. Strong coupling
dynamics of gauge theories plays an important role, in particular through its
effects on beta functions and through confinement. The model does not have
distinct messenger and supersymmetry-breaking sectors. The scale of
supersymmetry breaking is of order 10-100 \TeV, implying that the decay of the
next-to-lightest superpartner into the gravitino is prompt. Superoblique
corrections are enhanced. A Dirac fermion and one complex scalar, in a 10 or
\bar{10} of (global) SU(5), are predicted to be relatively light and to satisfy
certain mass relations with the standard model squarks and sleptons.Comment: 28 pages, uses revtex, h-physrev.bs
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