1,721,009 research outputs found
Dynamic AdS/QCD: A holographic approach to asymptotically free gauge theories
No full textDynamic AdS/QCD is a bottom-up model inspired from top-down, probe-brane extensions to the renowned AdS/CFT correspondence. The AdS/CFT correspondence states that there is an equivalence in the physics describing an N = 4 super-Yang-Mills gauge theory and that pertaining to a type IIB superstring theory on an AdS5S5 background. Moreover, the duality between the two theories in a particular limit requires that the gauge theory be strongly coupled when the superstring theory is weakly coupled. From this, we have a tool for studying strongly coupled gauge theories in a physically equivalent gravitational description. Using the AdS/CFT correspondence as a springboard, relationships between more complex superstring constructions on varying backgrounds have been studied to push the dual gauge theory to be more like those of QCD. In this thesis, we use one such bottom-up model, Dynamic AdS/QCD, to probe the behaviours of strongly coupled, asymptotically free gauge theories, including QCD.We show that the model correctly describes the hyperscaling relations of quark masses and the condensate, the behaviours expected of the scalar, vector and axial meson spectra in theories with infrared fixed points and how the condensate is affected by temperature and magnetic field in such theories. We finally go on to show that it is possible to describe QCD-like theories with a Lorentz-symmetry breaking vacuum, e.g. a striped condensate, and follow up with the cosmological ramifications of such a proposal and conclude that outside of a totally disconnected dark sector, this behaviour is highly constrained.<br/
Hyper-scaling relations in the conformal window from dynamic AdS/QCD
No full textDynamic AdS/QCD is a holographic model of strongly coupled gauge theories with the dynamics included through the running anomalous dimension of the quark bilinear, gamma. We apply it to describe the physics of massive quarks in the conformal window of SU(N_c) gauge theories with N_f fundamental flavours, assuming the perturbative two loop running for gamma. We show that to find regular, holographic, renormalization group flows in the infra-red the decoupling of the quark flavours at the scale of the mass is important and enact it through suitable boundary conditions when the flavours become on shell. We can then compute the quark condensate and the mesonic spectrum (M_rho, M_pi, M_sigma) and decay constants. We compute their scaling dependence on the quark mass for a number of examples. The model matches perturbative expectations for large quark mass and naive dimensional analysis (including the anomalous dimensions) for small quark mass. The model allows study of the intermediate regime where there is an additional scale from the running of the coupling and we present results for the deviation of scalings from assuming only the single scale of the mass
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
Translational symmetry breaking in field theories and the cosmological constant
No full textWe argue, at a very basic effective field theory level, that higher dimension operators in scalar theories that break symmetries at scales close to their ultraviolet completion cutoff include terms that favor the breaking of translation (Lorentz) invariance, potentially resulting in striped, checkerboard or general crystal-like phases. Such descriptions can be thought of as the effective low energy description of QCD-like gauge theories near their strong coupling scale where terms involving higher dimension operators are generated. Our low energy theory consists of scalar fields describing operators such as q¯q and q¯F(2n)q. Such scalars can have kinetic mixing terms that generate effective momentum dependent contributions to the mass matrix. We show that these can destabilize the translationally invariant vacuum. It is possible that in some real gauge theory such operators could become sufficiently dominant to realize such phases, and it would be interesting to look for them in lattice simulations. We present a holographic model of the same phenomena which includes renormalization group running. A key phenomenological motive to look at such states is recent work that shows that the nonlinear response in R2 gravity to such short-range fluctuations can mimic a cosmological constant. Intriguingly in a cosmology with such a Starobinsky inflation term, to generate the observed value of the present day acceleration would require stripes at the electroweak scale. Unfortunately, low energy phenomenological constraints on Lorentz violation in the electron-photon system appear to strongly rule out any such possibility outside of a disconnected dark sector
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Inverse magnetic catalysis in bottom-up holographic QCD
No full textWe explore the effect of magnetic field on chiral condensation in QCD via a simple bottom-up holographic model which inputs QCD dynamics through the running of the anomalous dimension of the quark bilinear. Bottom-up holography is a form of effective field theory and we use it to explore the dependence on the coefficients of the two lowest order terms linking the magnetic field and the quark condensate. In the massless theory, we identify a region of parameter space where magnetic catalysis occurs at zero temperature but inverse magnetic catalysis at temperatures of order the thermal phase transition. The model shows similar nonmonotonic behavior in the condensate with B at intermediate T as the lattice data. This behavior is due to the separation of the transition at which a thermal width develops for the mesons and the chiral transition in the holographic framework. The introduction of quark mass raises the scale of B where inverse catalysis takes over from catalysis until the inverse catalysis lies outside the regime of validity of the effective description leaving just catalysis
Meson spectra of asymptotically free gauge theories from holography
No full textUsing holography, we study the low-lying mesonic spectrum of a range of asymptotically free gauge theories. First, we revisit a simple top-down holographic model of QCD-like dynamics with predictions in the M?-M? plane. The meson masses in this model are in very good agreement with lattice gauge theory calculations in the quenched approximation. We show that the key ingredient for the meson mass predictions is the running of the anomalous dimension of the quark condensate ?. This provides an explanation for the agreement of holographic and quenched lattice gauge theory calculations. We then study the “dynamic AdS/QCD model,” in which the gauge theory dynamics is included by a choice for the running of ?. We use the naive two-loop perturbative running of the gauge coupling extrapolated to the nonperturbative regime to estimate the running of ? across a number of theories. We consider models with quarks in the fundamental, adjoint, two-index symmetric and two-index antisymmetric representations. We display predictions for M?, M?, M? and the lightest glueball mass. Many of these theories, where the contribution to the running of ? is dominated by the gluons, give very similar spectra, which also match with lattice expectations for QCD. On the other hand, a significant difference between spectra in different holographic models is seen for theories where the quark content changes the gradient of the running of ? around the scale at which chiral symmetry breaking is triggered at ??1. For these walking theories, we see an enhancement of the ? mass and a suppression of the ? mass. Both phenomena are characteristic for walking behavior in the physical meson masses
Holograms of a dynamical top quark
No full textWe present holographic descriptions of dynamical electroweak symmetry breaking models that incorporate the top mass generation mechanism. The models allow computation of the spectrum in the presence of large anomalous dimensions due to walking and strong Nambu–Jona-Lasinio interactions. Technicolor and QCD dynamics are described by the bottom-up Dynamic AdS/QCD model for arbitrary gauge groups and numbers of quark flavors. An assumption about the running of the anomalous dimension of the quark bilinear operator is input, and the model then predicts the spectrum and decay constants for the mesons. We add Nambu–Jona-Lasinio interactions responsible for flavor physics from extended technicolor, top-color, etc., using Witten’s multitrace prescription. We show the key behaviors of a top condensation model can be reproduced. We study generation of the top mass in (walking) one doublet and one family technicolor models and with strong extended technicolor interactions. The models clearly reveal the tensions between the large top mass and precision data for δρ. The necessary tunings needed to generate a model compatible with precision constraints are simply demonstrated
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