2,141 research outputs found
Kaluza-Klein States versus Winding States: Can Both Be Above the String Scale?
When closed strings propagate in extra compactified dimensions, a rich
spectrum of Kaluza-Klein states and winding states emerges. Since the masses of
Kaluza-Klein states and winding states play a reciprocal role, it is often
believed that either the lightest Kaluza-Klein states or the lightest winding
states must be at or below the string scale. In this paper, we demonstrate that
this conclusion is no longer true for compactifications with non-trivial shape
moduli. Specifically, we demonstrate that toroidal compactifications exist for
which all Kaluza-Klein states as well as all winding states are heavier than
the string scale. This observation could have important phenomenological
implications for theories with reduced string scales, suggesting that it is
possible to cross the string scale without detecting any states associated with
spacetime compactification.Comment: 8 pages, LaTeX, no figure
Implicit learning of recursive context-free grammars
Context-free grammars are fundamental for the description of linguistic syntax. However, most artificial grammar learning
experiments have explored learning of simpler finite-state grammars, while studies exploring context-free grammars have
not assessed awareness and implicitness. This paper explores the implicit learning of context-free grammars employing
features of hierarchical organization, recursive embedding and long-distance dependencies. The grammars also featured
the distinction between left- and right-branching structures, as well as between centre- and tail-embedding, both
distinctions found in natural languages. People acquired unconscious knowledge of relations between grammatical classes
even for dependencies over long distances, in ways that went beyond learning simpler relations (e.g. n-grams) between
individual words. The structural distinctions drawn from linguistics also proved important as performance was greater for
tail-embedding than centre-embedding structures. The results suggest the plausibility of implicit learning of complex
context-free structures, which model some features of natural languages. They support the relevance of artificial grammar
learning for probing mechanisms of language learning and challenge existing theories and computational models of
implicit learning
A game theory model of the physician preference item supply chain
Many types of medical devices (e.g., orthopedic implants, stents, and pacemakers) have significantly higher costs and more frequent product innovations than commodity items. These physician preference items (PPI) are procured through a unique supply chain, in which physicians select which products updates to adopt, based upon clinical preference. PPI manufacturers are motivated to update products frequently to remain competitive and enhance revenues by incorporating new product features embedded in the PPI. Due to the PPI updates, physicians must progress through a learning curve after adopting a new product generation. Often, manufacturers will employ sales representatives to assist physicians with PPI learning. However, hospitals are left to address the increasing costs associated with the new product generation.
This work uses a game-theoretic approach to understand how an average physician\u27s learning curve affects the manufacturer\u27s optimal product update pace. Additionally, the impacts of sales representatives and hospital cost control efforts are studied. Results indicate that not only is the manufacturer\u27s product update pace dependent upon physician learning, but also both the manufacturer and the physician benefit when a new PPI product generation requires a shorter amount of time to be mastered. Finally, we see that a hospital\u27s PPI cost control strategy may need to vary for different devices, based upon their objectives as either a value-focused or research-focused hospital
Adventures in Thermal Duality (II): Towards a Duality-Covariant String Thermodynamics
In a recent companion paper, we observed that the rules of ordinary
thermodynamics generally fail to respect thermal duality, a symmetry of string
theory under which the physics at temperature T is related to the physics at
the inverse temperature 1/T. Even when the free energy and internal energy
exhibit the thermal duality symmetry, the entropy and specific heat are defined
in such a way that this symmetry is destroyed. In this paper, we propose a
modification of the traditional definitions of these quantities, yielding a
manifestly duality-covariant thermodynamics. At low temperatures, these
modifications produce "corrections" to the standard definitions of entropy and
specific heat which are suppressed by powers of the string scale. These
corrections may nevertheless be important for the full development of a
consistent string thermodynamics. We find, for example, that the
string-corrected entropy can be smaller than the usual entropy at high
temperatures, suggesting a possible connection with the holographic principle.
We also discuss some outstanding theoretical issues prompted by our approach.Comment: 31 pages, 6 figures, 1 conversatio
Shadows of the Planck Scale: The Changing Face of Compactification Geometry
By studying the effects of the shape moduli associated with toroidal
compactifications, we demonstrate that Planck-sized extra dimensions can cast
significant ``shadows'' over low-energy physics. These shadows can greatly
distort our perceptions of the compactification geometry associated with large
extra dimensions, and place a fundamental limit on our ability to probe the
geometry of compactification simply by measuring Kaluza-Klein states. We also
discuss the interpretation of compactification radii and hierarchies in the
context of geometries with non-trivial shape moduli. One of the main results of
this paper is that compactification geometry is effectively renormalized as a
function of energy scale, with ``renormalization group equations'' describing
the ``flow'' of geometric parameters such as compactification radii and shape
angles as functions of energy.Comment: 7 pages, LaTeX, 2 figure
Statistics in the Landscape of Intersecting Brane Models
An approach towards a statistical survey of four dimensional supersymmetric
vacua in the string theory landscape is described and illustrated with three
examples of ensembles of intersecting D-brane models. The question whether it
is conceivable to make predictions based on statistical distributions is
discussed. Especially interesting in this context are possible correlations
between low energy observables. As an example we look at correlations between
properties of the gauge sector of intersecting D-brane models and Gepner model
constructions.Comment: Submitted for the SUSY07 proceedings, 4 pages, 2 figure
Are There Oscillations in the Baryon/Meson Ratio?
All available data indicate a surplus of baryon states over meson states for
energies greater than about 1.5 GeV. Since hadron-scale string theory suggests
that their numbers should become equal with increasing energy, it has recently
been proposed that there must exist exotic mesons with masses just above 1.7
GeV in order to fill the deficit. We demonstrate that a string-like picture is
actually consistent with the present numbers of baryon and meson states, and in
fact predicts regular oscillations in their ratio. This suggests a different
role for new hadronic states.Comment: 14 pages (RevTeX), McGill/92-0
Invisible Axions and Large-Radius Compactifications
We study some of the novel effects that arise when the QCD axion is placed in
the ``bulk'' of large extra spacetime dimensions. First, we find that the mass
of the axion can become independent of the energy scale associated with the
breaking of the Peccei-Quinn symmetry. This implies that the mass of the axion
can be adjusted independently of its couplings to ordinary matter, thereby
providing a new method of rendering the axion invisible. Second, we discuss the
new phenomenon of laboratory axion oscillations (analogous to neutrino
oscillations), and show that these oscillations cause laboratory axions to
``decohere'' extremely rapidly as a result of Kaluza-Klein mixing. This
decoherence may also be a contributing factor to axion invisibility. Third, we
discuss the role of Kaluza-Klein axions in axion-mediated processes and decays,
and propose several experimental tests of the higher-dimensional nature of the
axion. Finally, we show that under certain circumstances, the presence of an
infinite tower of Kaluza-Klein axion modes can significantly accelerate the
dissipation of the energy associated with cosmological relic axion
oscillations, thereby enabling the Peccei-Quinn symmetry-breaking scale to
exceed the usual four-dimensional relic oscillation bounds. Together, these
ideas therefore provide new ways of obtaining an ``invisible'' axion within the
context of higher-dimensional theories with large-radius compactifications.Comment: 43 pages, LaTeX, 6 figure
Ultraviolet dependence of Kaluza-Klein effects on electroweak observables
In extensions of the standard model (SM) with d extra dimensions at the TeV
scale the virtual exchange of Kaluza-Klein (KK) excitations of the gauge bosons
gives contributions that change the SM relations between electroweak
observables. These corrections are finite only for d=1; for d\ge 2 the infinite
tower of KK modes gives a divergent contribution that has to be regularized
introducing a cutoff (the string scale). However, the ultraviolet dependence of
the KK effects is completely different if the running of the couplings with the
scale is taken into account. We find that for larger d the number of
excitations at each KK level increases, but their larger number is compensated
by the smaller value of the gauge coupling at that scale. As a result, for any
number of extra dimensions the exchange of the complete KK tower always gives a
finite contribution. We show that (i) for d=1 the running of the gauge coupling
decreases an 14% the effect of the KK modes on electroweak observables; (ii) in
all cases more than 90% of the total effect comes from the excitations in the
seven lowest KK levels and is then independent of ultraviolet physics.Comment: 8 pages, to appear in Phys. Rev.
- âŠ