56,558 research outputs found
Improving PARMA Trailing
Taylor introduced a variable binding scheme for logic variables in his PARMA
system, that uses cycles of bindings rather than the linear chains of bindings
used in the standard WAM representation. Both the HAL and dProlog languages
make use of the PARMA representation in their Herbrand constraint solvers.
Unfortunately, PARMA's trailing scheme is considerably more expensive in both
time and space consumption. The aim of this paper is to present several
techniques that lower the cost.
First, we introduce a trailing analysis for HAL using the classic PARMA
trailing scheme that detects and eliminates unnecessary trailings. The
analysis, whose accuracy comes from HAL's determinism and mode declarations,
has been integrated in the HAL compiler and is shown to produce space
improvements as well as speed improvements. Second, we explain how to modify
the classic PARMA trailing scheme to halve its trailing cost. This technique is
illustrated and evaluated both in the context of dProlog and HAL. Finally, we
explain the modifications needed by the trailing analysis in order to be
combined with our modified PARMA trailing scheme. Empirical evidence shows that
the combination is more effective than any of the techniques when used in
isolation.
To appear in Theory and Practice of Logic Programming.Comment: 36 pages, 7 figures, 8 table
Observing The Hidden Sector
We study the effects of renormalization due to hidden-sector dynamics on
observable soft supersymmetry-breaking parameters in the minimal supersymmetric
extension of the Standard Model (MSSM), under various hypotheses about their
universality at a high input scale. We show that hidden-sector renormalization
effects may induce the spurious appearance of unification of the scalar masses
at some lower scale, as in mirage unification scenarios. We demonstrate in
simple two-parameter models of the hidden-sector dynamics that the parameters
may in principle be extracted from experimental measurements, rendering the
hidden sector observable. We also discuss the ingredients that would be
necessary to carry this programme out in practice.Comment: 38 pages, 13 figures, uses UApreprint.cls and subfigure.sty
(included
Nominal Unification from a Higher-Order Perspective
Nominal Logic is a version of first-order logic with equality, name-binding,
renaming via name-swapping and freshness of names. Contrarily to higher-order
logic, bindable names, called atoms, and instantiable variables are considered
as distinct entities. Moreover, atoms are capturable by instantiations,
breaking a fundamental principle of lambda-calculus. Despite these differences,
nominal unification can be seen from a higher-order perspective. From this
view, we show that nominal unification can be reduced to a particular fragment
of higher-order unification problems: Higher-Order Pattern Unification. This
reduction proves that nominal unification can be decided in quadratic
deterministic time, using the linear algorithm for Higher-Order Pattern
Unification. We also prove that the translation preserves most generality of
unifiers
Split Supersymmetry in String Theory
Type I string theory in the presence of internal magnetic fields provides a
concrete realization of split supersymmetry. To lowest order, gauginos are
massless while squarks and sleptons are superheavy. For weak magnetic fields,
the correct Standard Model spectrum guarantees gauge coupling unification with
\sin^2{\theta_W}=3/8 at the compactification scale of M_{\rm GUT}\simeq 2
\times 10^{16} GeV. I discuss mechanisms for generating gaugino and higgsino
masses at the TeV scale, as well as generalizations to models with split
extended supersymmetry in the gauge sector.Comment: 7 pages, 3 figures, prepared for the Proceedings of PASCOS-05 and of
CORFU2005 Summer Institut
Unification of Gauge, Higgs and Matter in Extra Dimensions
We consider the unification of gauge, Higgs as well as the matter fields in a
6D N=2 supersymmetric SU(8) gauge theory. The gauge symmetry SU(8) is broken
down to SU(4) x SU(2)_L x SU(2)_R x U(1)^2 in 4D through T^2/Z_6 orbifold
compactification, and the theory is reduced to 4D N=1 supersymmetric Pati-Salam
model. The electroweak Higgs fields as well as the third family of fermions are
unified in the 6D N=2 gauge multiplet. The 6D bulk gauge interaction provides
both gauge and Yukawa interactions for the third family predicting \alpha_1 =
\alpha_2 = \alpha_3 = \alpha_t = \alpha_b = \alpha_tau at the unification
scale, in good agreement with experiment. Incorporation of the first and second
family as well as other orbifolds are also briefly discussed.Comment: 13 pages and 1 figure, some minor corrections and references adde
Nominal Logic Programming
Nominal logic is an extension of first-order logic which provides a simple
foundation for formalizing and reasoning about abstract syntax modulo
consistent renaming of bound names (that is, alpha-equivalence). This article
investigates logic programming based on nominal logic. We describe some typical
nominal logic programs, and develop the model-theoretic, proof-theoretic, and
operational semantics of such programs. Besides being of interest for ensuring
the correct behavior of implementations, these results provide a rigorous
foundation for techniques for analysis and reasoning about nominal logic
programs, as we illustrate via examples.Comment: 46 pages; 19 page appendix; 13 figures. Revised journal submission as
of July 23, 200
Does the unification of BL Lac and FR I radio galaxies require jet velocity structures?
We explore the viability of the unification of BL Lacs and FR I radio
galaxies by comparing the core emission of radio galaxies with those of BL Lacs
of similar extended radio power, taking advantage of the newly measured optical
nuclear luminosity of FR I sources. The spectral properties of complete samples
are also studied in the radio-optical luminosity plane: starting from the
Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted
luminosity of FR I nuclei in the frame of a simple one--zone model, by properly
taking into account the relativistic transformations. We find that the bulk
Lorentz factors required by the spread in the observed luminosities in all
bands are significantly smaller than those implied by other, both observational
and theoretical, considerations. This discrepancy is also reflected in the fact
that FR I nuclei are over-luminous by a factor of 10-10^4, with respect to the
predictions, both in the radio and in the optical band.
In order to reconcile these results with the unification scheme, velocity
structures in the jet are suggested, where a fast spine is surrounded by a slow
(but still relativistic) layer so that the emission at different angles is
dominated by different velocity components: the fast one dominates the emission
in BL Lacs while the slow layer dominates the emission in misaligned objects.
Furthermore for the lowest luminosity BL Lacs it has to be also postulated that
their beaming factor in the radio band is lower than in the optical (and
X-ray), as would result from deceleration of the jet.
The self-consistency of the unification model therefore requires that both
intrinsic differences in the SED and different beaming properties play a
substantial role in characterizing the phenomenology of these sources.Comment: 10 pages, 11 figures, revised version, to be published in A&
- …