1,720,992 research outputs found
Wilson lines and Chern-Simons flux in explicit heterotic Calabi-Yau compactifications
Full text linkWe study to what extent Wilson lines in heterotic Calabi-Yau compactifications lead to non-trivial H-flux via Chern-Simons terms. Wilson lines are basic ingredients for Standard Model constructions but their induced H-flux may affect the consistency of the leading order background geometry and of the two-dimensional worldsheet theory. Moreover H-flux in heterotic compactifications would play an important role for moduli stabilization and could strongly constrain the supersymmetry breaking scale. We show how to compute H-flux and the corresponding superpotential, given an explicit complete intersection Calabi-Yau compactification and choice of Wilson lines. We do so by identifying large classes of special Lagrangian submanifolds in the Calabi-Yau, understanding how the Wilson lines project onto these submanifolds, and computing their Chern-Simons invariants. We illustrate our procedure with the quintic hypersurface as well as the split-bicubic, which can provide a potentially realistic three generation model
The Gravity of Warped Throats: de Sitter vacua and Gravitational Waves from Type IIB string theory
Full text linkIn this thesis we discuss challenges and opportunities arising from warping the extra dimensions
of string theory. After reviewing the required background (including the essentials of Type
IIB string theory; flux compactifications; conifolds, warping, and the Klebanov-Strassler and
GKP solutions; and the KKLT and LVS proposals) we will discuss de Sitter solutions in warped
flux compactifications. We revisit some strongly-warped solutions, present a new solution in
a weakly-warped regime and discuss the advantages of weak warping. We then consider the
robustness of the new solution in the presence of subleading corrections to the scalar potential.
We also explore the difficulties of realising alternative quintessence models as quasi-de Sitter
solutions, showing in particular that the generic behaviour of the (single field) scalar potential
arising for different types of string theory moduli does not allow for a slow-roll accelerated
expansion at the tail of a runaway.
We then take the first steps in understanding the effects of warping in gravitational wave signatures
of extra dimensions. By considering the tower of Kaluza-Klein spin-2 states arising from
a warped compactification of Type IIB string theory, we study the effects of warping on their
masses and wavefunction profiles, which we then use to compute corrections to the Newtonian
potential that one can compare with current constraints on fifth forces. This allows us to combine
theoretical consistency constraints on the parameter space with the range of parameters
experimentally excluded, thereby providing a direct connection between string theory quantities
and observations. Although a careful study of gravitational wave signals is left for future work,
we briefly outline how these results directly apply in that context and suggest which sources
might be more promising for future detection
Fundamental and Phenomenological Aspects of Anti-D-Brane Supersymmetry Breaking
Full text linkThis dissertation focusses on the investigation of both the fundamental and the phenomenological features of non-supersymmetric string theories based on supersymmetry breaking by anti-D-branes. The study of non-supersymmetric string theories is shedding light on an important corner of the string landscape that might ultimately explain the reason why, so far, supersymmetry has not been detected in the observed universe.
The first line of research aims at enriching the understanding of misaligned supersymmetry in String Theory. Misaligned supersymmetry consists in cancellations between bosonic and fermionic contributions at different energy levels in the whole string spectrum. This is interpreted as a physical mechanism that helps visualising the origin of the finiteness of string constructions, otherwise motivated based on the behaviour of the theory under modular transformations. A review is presented of how misaligned supersymmetry in closed-string theories leads to a cancellation between bosons and fermions even in non-supersymmetric scenarios. Then, it is shown that an entirely analogous cancellation can take place in non-supersymmetric open-string theories, too, by studying anti-D-branes placed on top of O-planes. These ideas are then developed via a systematic analysis of the net physical degeneracies at each energy level, studying their non-trivial cancellations and relating them to the modular properties of the partition function. Eventually, the whole concept of misaligned supersymmetry in String Theory is analysed in a mathematically rigorous way, showing the details of how the boson-fermion cancellations can take place in physical quantities, and the role of misalignment in all known 10-dimensional tachyon-free non-supersymmetric string constructions is finally discussed.
The second line of research, instead, is devoted to a phenomenological investigation and description of a class of quasi-realistic non-supersymmetric vacua including anti-D-branes. In particular, it discusses model-building scenarios featuring intersecting anti-D3- and D7-branes. Effectively, supersymmetry is broken spontaneously, despite having no scale at which sparticles appear and standard supersymmetry is restored. If the branes are placed on singularities at the tip of warped throats in Calabi-Yau orientifold flux compactifications, they may give rise to quasi-realistic particle spectra, closed- and open-string moduli stabilisation with a Minkowski/de Sitter uplift, together with a geometrical origin for the scale hierarchies. A derivation is given of the low-energy effective field theory description for such scenarios, i.e. a non-linear supergravity theory for standard and constrained supermultiplets, including soft supersymmetry-breaking matter couplings. The effect of closed-string moduli stabilisation on the open-string matter sector is worked out, incorporating non-perturbative and perturbative effects, and the mass and coupling hierarchies are computed with a view towards phenomenology
Embedding the axio-chameleon screening of fifth-forces into string theory
Full text linkLight scalar fields are ubiquitous in beyond the Standard Model and string constructions, but they naively induce unobserved fifth-forces. Axio-chameleon screening has recently been proposed as a promising mechanism to explain the absence of these fifth-forces, and the goal of this thesis is to embed it in String Theory. In particular, a good contender seems to be a D3/anti-D3-brane moving on a warped deformed conifold along both the radial and some angular directions. In the model presented here, the Standard Model is assumed to live on the brane and the radial brane-position modulus couples to it as a Quasi-Brans-Dicke scalar through the warp factor. This can be interpreted as a Quintessence field and the screening mechanism provides justification for the lack of observable fifth-forces in solar-system experiments
Misaligned supersymmetry and open strings
No full textAbstractThe study of non-supersymmetric string theories is shedding light on an important corner of the string landscape and might ultimately explain why, so far, we did not observe supersymmetry in our universe. We review how misaligned supersymmetry in closed-string theories leads to a cancellation between bosons and fermions even in non-supersymmetric string theories. We then show that the same cancellation takes place for open strings by studying an anti-Dp-brane placed on top of an Op-plane in type II string theory. Misaligned supersymmetry consists in cancellations between bosons and fermions atdifferentenergy levels, in such a way that the averaged number of states grows at a rate dominated by a factoreCeffn, withCeff< Ctot, whereCtotis the inverse Hagedorn temperature. We prove the previously conjectured complete cancellation, i.e. we prove thatCeff= 0, for a vast class of models.</jats:p
Modular invariance, misalignment and finiteness in non-supersymmetric strings
No full textAbstractIn this article we show that finite perturbative corrections in non-supersymmetric strings can be understood via an interplay between modular invariance and misaligned supersymmetry. While modular invariance is known to be crucial in closed-string models, its presence and role for open strings is more subtle. Nevertheless, we argue that it leads to cancellations in physical quantities such as the one-loop cosmological constant and prevents them from diverging. In particular, we show that if the sector-averaged number of states does not grow exponentially, as predicted by misaligned supersymmetry, all exponential divergences in the one-loop cosmological constant cancel out as well. To account for the absence of power-law divergences, instead, we need to resort to the modular structure of the partition function. We finally comment on the presence of misaligned supersymmetry in the known 10-dimensional tachyon-free non-supersymmetric string theories.</jats:p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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