Comments on Critical Electric and Magnetic Fields from Holography

We discuss some aspects of critical electric and magnetic fields in a field theory with holographic dual description. We extend the analysis of arxiv:1109.2920, which finds a critical electric field at which the Schwinger pair production barrier drops to zero, to the case of magnetic fields. We first find that, unlike ordinary weakly coupled theories, the magnetic field is not subject to any perturbative instability originating from the presence of a tachyonic ground state in the W-boson spectrum. This follows from the large value of the 't Hooft coupling \lambda, which prevents the Zeeman interaction term to overcome the particle mass at high B. Consequently, we study the next possible B-field instability, i.e. monopole pair production, which is the S-dual version of the Schwinger effect. Also in this case a critical magnetic field is expected when the tunneling barrier drops to zero. These Schwinger-type criticalities are the holographic duals, in the bulk, to the fields E or B reaching the tension of F1 or D1 strings respectively. We then discuss how this effect is modified when electric and magnetic fields are present simultaneously and dyonic states in the spectrum can be pair produced by a generic E - B background. Finally, we analyze finite temperature effects on Schwinger criticalities, i.e. in the AdS-Schwarzshild black hole background.Comment: 33 pages, 4 figures; v2: refs added; v3: typos corrected, to appear on JHE

Strings Inside Walls in N=1 Super Yang-Mills

We conjecture the existence of strings bounded inside walls in SU$(n)$ $\N=1$ Super Yang-Mills theory. These strings carry $\Z_{[k,n]}$ quantum number, where $[k,n]$ is the greatest common divisor between $k$, the charge of the wall, and $n$. We provide field-theoretical arguments and string-theoretical evidences, both from MQCD and from gauge-gravity correspondence. We interpret this result from the point of view of the low-energy effective action living on the $k$-wall.Comment: 25 pp. Major changes. In particular, following the recent work arXiv:0807.1908 we have been able to give a field theoretical proof of the statement. We have also corrected an important erroneous interpretation in the previous version regarding the 2+1 effective action; Typo

Weak refinement in Z

An important aspect in the specification of distributed systems is the role of the internal (or unobservable) operation. Such operations are not part of the user interface (i.e. the user cannot invoke them), however, they are essential to our understanding and correct modelling of the system. Various conventions have been employed to model internal operations when specifying distributed systems in Z. If internal operations are distinguished in the specification notation, then refinement needs to deal with internal operations in appropriate ways. However, in the presence of internal operations, standard Z refinement leads to undesirable implementations. In this paper we present a generalization of Z refinement, called weak refinement, which treats internal operations differently from observable operations when refining a system. We illustrate some of the properties of weak refinement through a specification of a telecommunications protocol

On the Possibility of a Trans-Planckian Duality

We investigate the possibility of a trans-Planckian duality, which exchanges a manifold of events (space-time), with a manifold of momenta (energy-momentum). Gravity has a dual counter-part, that is, a geometric theory defined on the manifold of momenta. We provide a mathematical framework that can possibly realize this idea, and analyze its classical behaviour.Comment: 21 pages, 4 figure

Interplay of the volume and surface plasmons in the electron energy loss spectra of C60_{60}

The results of a joint experimental and theoretical investigation of the C60 collective excitations in the process of inelastic scattering of electrons are presented. The shape of the electron energy loss spectrum is observed to vary when the scattering angle increases. This variation arising due to the electron diffraction of the fullerene shell is described by a new theoretical model which treats the fullerene as a spherical shell of a finite width and accounts for the two modes of the surface plasmon and for the volume plasmon as well. It is shown that at small angles, the inelastic scattering cross section is determined mostly by the symmetric mode of the surface plasmon, while at larger angles, the contributions of the antisymmetric surface plasmon and the volume plasmon become prominent.Comment: 11 pages, 3 figure

Perceived Psychological Restorativeness in Relation to Individual and Environmental Variables: A Study Conducted at Poetto Beach in Sardinia, Italy

This study examines how objective, social, and perceived environmental conditions in a blue space are associated with the perception of psychological restorativeness. We collected data between April 2021 and February 2022 at Poetto Beach in Sardinia, Italy. The participants (N = 255) completed a survey about perceived environmental quality, stress, weather, and restorativeness during their stay at the beach. We used linear models to evaluate the association between psychological restorativeness and social, environmental, and weather parameters. We also analyzed the nature of the association between temperature and restorativeness by viewing this relation as both linear and non-linear and by evaluating the differences in restorativeness between winter, springtime, and summer. The results suggested that the participants viewed the beach as psychologically restorative, especially during the winter season. We also found that the number of people that participants came with was negatively associated with perceived restorativeness. Finally, the results from the correlation analysis revealed that people are less stressed if they go to the beach more frequently

Magnetic Catalysis in AdS4

We study the formation of fermion condensates in Anti de Sitter space. In particular, we describe a novel version of magnetic catalysis that arises for fermions in asymptotically AdS4 geometries which cap off in the infra-red with a hard wall. We show that the presence of a magnetic field induces a fermion condensate in the bulk that spontaneously breaks CP symmetry. From the perspective of the dual boundary theory, this corresponds to a strongly coupled version of magnetic catalysis in d=2+1.Comment: 22 pages, 4 figures. v2: References added, factors of 2 corrected, extra comments added in appendix. v3: extra comments about fermion modes in a hard wall background. v4: A final factor of

Type I Non-Abelian Superconductors in Supersymmetric Gauge Theories

Non-BPS non-Abelian vortices with CP^1 internal moduli space are studied in an N=2 supersymmetric U(1) x SU(2) gauge theory with softly breaking adjoint mass terms. For generic internal orientations the classical force between two vortices can be attractive or repulsive. On the other hand, the mass of the scalars in the theory is always less than that of the vector bosons; also, the force between two vortices with the same CP^1 orientation is always attractive: for these reasons we interpret our model as a non-Abelian generalization of type I superconductors. We compute the effective potential in the limit of two well separated vortices. It is a function of the distance and of the relative colour-flavour orientation of the two vortices; in this limit we find an effective description in terms of two interacting CP^1 sigma models. In the limit of two coincident vortices we find two different solutions with the same topological winding and, for generic values of the parameters, different tensions. One of the two solutions is described by a CP^1 effective sigma model, while the other is just an Abelian vortex without internal degrees of freedom. For generic values of the parameters, one of the two solutions is metastable, while there are evidences that the other one is truly stable.Comment: 35 pages, 8 figures. v2: fixed typos and added small comments, v3 removed an unecessary figur

New evaluation of the axial nucleon form factor from electron- and neutrino-scattering data and impact on neutrino-nucleus cross sections

International audienceA joint fit to neutrino-nucleon scattering and pion electroproduction data is performed to evaluate the nucleon axial form factor in the two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. Further constraints on the model are obtained by re-evaluating the electromagnetic form factor using electron scattering data. The results of the axial form factor show sizable differences with respect to the widely used dipole model. The impact of such changes on the charged-current quasielastic neutrino-nucleus cross-section is evaluated in the SuSAv2 nuclear model, based on the relativistic mean field and including the contribution of two-body currents. How the different parametrizations of the axial form factor affect the cross-section prediction is assessed in full detail and comparisons to recent T2K and MINERvA data are presented

Ferredoxin-NADP(+) reductase from Plasmodium falciparum undergoes NADP(+)-dependent dimerization and inactivation: functional and crystallographic analysis

The completion of the Plasmodium falciparum genome sequence has recently promoted the search for new antimalarial drugs. More specifically, metabolic pathways of the apicoplast, a key organelle for survival of the parasite, have been recognized as potential targets for the development of specific new antimalarial agents. As most apicomplexan parasites, P. falciparum displays a plant-type ferredoxin-NADP(+) reductase, yielding reduced ferredoxin for essential biosynthetic pathways in the apicoplast. Here we report a molecular, kinetic and ligand binding characterization of the recombinant ferredoxin-NADP(+) reductase from P. falciparum, in the light of current data available for plant ferredoxin-NADP(+) reductases. In parallel with the functional characterization, we describe the crystal structures of P. falciparum ferredoxin-NADP(+) reductase in free form and in complex with 2'-phospho-AMP (at 2.4 and 2.7 A resolution, respectively). The enzyme displays structural properties likely to be unique to plasmodial reductases. In particular, the two crystal structures highlight a covalent dimer, which relies on the oxidation of residue Cys99 in two opposing subunits, and a helix-coil transition that occurs in the NADP-binding domain, triggered by 2'-phospho-AMP binding. Studies in solution show that NADP(+), as well as 2'-phospho-AMP, promotes the formation of the disulfide-stabilized dimer. The isolated dimer is essentially inactive, but full activity is recovered upon disulfide reduction. The occurrence of residues unique to the plasmodial enzyme, and the discovery of specific conformational properties, highlight the NADP-binding domain of P. falciparum ferredoxin-NADP(+) reductase as particularly suited for the rational development of antimalarial compounds