Nonabelian D-branes and Noncommutative Geometry

We discuss the nonabelian world-volume action which governs the dynamics of N coincident Dp-branes. In this theory, the branes' transverse displacements are described by matrix-valued scalar fields, and so this is a natural physical framework for the appearance of noncommutative geometry. One example is the dielectric effect by which Dp-branes may be polarized into a noncommutative geometry by external fields. Another example is the appearance of noncommutative geometries in the description of intersecting D-branes of differing dimensions, such as D-strings ending on a D3- or D5-brane. We also describe the related physics of giant gravitons.Comment: 21 pages, Latex, ref. adde

A Novel Long Range Spin Chain and Planar N=4 Super Yang-Mills

We probe the long-range spin chain approach to planar N=4 gauge theory at high loop order. A recently employed hyperbolic spin chain invented by Inozemtsev is suitable for the SU(2) subsector of the state space up to three loops, but ceases to exhibit the conjectured thermodynamic scaling properties at higher orders. We indicate how this may be bypassed while nevertheless preserving integrability, and suggest the corresponding all-loop asymptotic Bethe ansatz. We also propose the local part of the all-loop gauge transfer matrix, leading to conjectures for the asymptotically exact formulae for all local commuting charges. The ansatz is finally shown to be related to a standard inhomogeneous spin chain. A comparison of our ansatz to semi-classical string theory uncovers a detailed, non-perturbative agreement between the corresponding expressions for the infinite tower of local charge densities. However, the respective Bethe equations differ slightly, and we end by refining and elaborating a previously proposed possible explanation for this disagreement.Comment: 48 pages, 1 figure. v2, further results added: discussion of the relationship to an inhomogeneous spin chain, normalization in sec 3 unified, v3: minor mistakes corrected, published versio

Meson Spectroscopy in AdS/CFT with Flavour

We compute the meson spectrum of an N=2 super Yang-Mills theory with fundamental matter from its dual string theory on AdS_5 x S_5 with a D7-brane probe. For scalar and vector mesons with arbitrary R-charge the spectrum is computed in closed form by solving the equations for D7-brane fluctuations; for matter with non-zero mass m_q it is discrete, exhibits a mass gap of order m_q / sqrt(g_s N) and furnishes representations of SO(5) even though the manifest global symmetry of the theory is only SO(4). The spectrum of mesons with large spin J is obtained from semiclassical, rotating open strings attached to the D7-brane. It displays Regge-like behaviour for J << sqrt(g_s N), whereas for J >> sqrt(g_s N) it corresponds to that of two non-relativistic quarks bound by a Coulomb potential. Meson interactions, baryons and `giant gauge bosons' are briefly discussed.Comment: LaTeX, 39 pages, 4 figures, uses epsf. v2: typos corrected. references adde

Thermal Partition Functions for S-branes

We calculate the thermal partition functions of open strings on the S-brane backgrounds (the bouncing or rolling tachyon backgrounds) both in the bosonic and superstring cases. According to hep-th/0302146, we consider the discretized temperatures compatible with the pure imaginary periodicity of tachyon profiles. The ``effective Hagedorn divergence'' is shown to appear no matter how low temperature is chosen (including zero-temperature). This feature is likely to be consistent with the large rate of open string pair production discussed in hep-th/0209090 and also emission of closed string massive modes hep-th/0303139. We also discuss the possibility to remove the divergence by considering the space-like linear dilaton backgrounds as in hep-th/0306132.Comment: 33 pages, no figure; v2 typos corrected, a reference adde

Fuzzy Sphere Dynamics and Non-Abelian DBI in Curved Backgrounds

We consider the non-Abelian action for the dynamics of $N Dp'$-branes in the background of $M Dp$-branes, which parameterises a fuzzy sphere using the SU(2) algebra. We find that the curved background leads to collapsing solutions for the fuzzy sphere except when we have $D0$ branes in the $D6$ background, which is a realisation of the gravitational Myers effect. Furthermore we find the equations of motion in the Abelian and non-Abelian theories are identical in the large $N$ limit. By picking a specific ansatz we find that we can incorporate angular momentum into the action, although this imposes restriction upon the dimensionality of the background solutions. We also consider the case of non-Abelian non-BPS branes, and examine the resultant dynamics using world-volume symmetry transformations. We find that the fuzzy sphere always collapses but the solutions are sensitive to the combination of the two conserved charges and we can find expanding solutions with turning points. We go on to consider the coincident $NS$5-brane background, and again construct the non-Abelian theory for both BPS and non-BPS branes. In the latter case we must use symmetry arguments to find additional conserved charges on the world-volumes to solve the equations of motion. We find that in the Non-BPS case there is a turning solution for specific regions of the tachyon and radion fields. Finally we investigate the more general dynamics of fuzzy $\mathbb{S}^{2k}$ in the $Dp$-brane background, and find collapsing solutions in all cases.Comment: 49 pages, 3 figures, Latex; Version to appear in JHE

Comments on Half S-Branes

Following hep-th/0305177, we write the boundary state of half S-brane in bosonic string theory as a grand canonical partition function of a unitary matrix model. From this representation, it follows that the annulus amplitude can be written as a grand canonical partition function of a unitary two-matrix model. We also show that the contribution of the exponentially growing couplings to the timelike oscillators can be resummed in a certain annulus amplitude.Comment: 27 pages, lanlmac; v2: reference adde

On the usage of geomagnetic indices for data selection in internal field modelling

We present a review on geomagnetic indices describing global geomagnetic storm activity (Kp, am, Dst and dDst/dt) and on indices designed to characterize high latitude currents and substorms (PC and AE-indices and their variants). The focus in our discussion is in main field modelling, where indices are primarily used in data selection criteria for weak magnetic activity. The publicly available extensive data bases of index values are used to derive joint conditional Probability Distribution Functions (PDFs) for different pairs of indices in order to investigate their mutual consistency in describing quiet conditions. This exercise reveals that Dst and its time derivative yield a similar picture as Kp on quiet conditions as determined with the conditions typically used in internal field modelling. Magnetic quiescence at high latitudes is typically searched with the help of Merging Electric Field (MEF) as derived from solar wind observations. We use in our PDF analysis the PC-index as a proxy for MEF and estimate the magnetic activity level at auroral latitudes with the AL-index. With these boundary conditions we conclude that the quiet time conditions that are typically used in main field modelling (, and ) correspond to weak auroral electrojet activity quite well: Standard size substorms are unlikely to happen, but other types of activations (e.g. pseudo breakups ) can take place, when these criteria prevail. Although AE-indices have been designed to probe electrojet activity only in average conditions and thus their performance is not optimal during weak activity, we note that careful data selection with advanced AE-variants may appear to be the most practical way to lower the elevated RMS-values which still exist in the residuals between modeled and observed values at high latitudes. Recent initiatives to upgrade the AE-indices, either with a better coverage of observing stations and improved baseline corrections (the SuperMAG concept) or with higher accuracy in pinpointing substorm activity (the Midlatitude Positive Bay-index) will most likely be helpful in these efforts.</p

The Self-dual String Soliton in AdS_4\times S^7 spacetime

We construct self-dual string soliton solutions in $AdS_4\times S^7$ spacetime, starting from the covariant equations of motion of M5-brane. We study the properties of the solutions and find that their action are linearized, indicating the BPS nature of the solutions, and they have the same electric and magnetic charge. The straight string soliton solution represents the configuration of the membranes ending on M5-brane with a straight string intersection, and it behaves like the spiky solution in flat spacetime. The spherical string soliton solution, which could be related to the straight one by a conformal transformation, represents the membranes ending on M5-brane with a spherical intersection.Comment: 15 pages;typos corrected, references added;published versio

Non-commutative Branes from M-theory

The analysis of the worldvolume effective actions of the M-theory Kaluza-Klein monopole and 9-brane suggests that it should be possible to describe non-abelian configurations of M2-branes or M5-branes if the M2-branes are transverse to the eleventh direction and the M5-branes are wrapped on it. This is determined by the fact that the Kaluza-Klein monopole and the M9-brane are constrained to move in particular isometric spacetimes. We show that the same kind of situation is implied by the analysis of the brane descent relations in M-theory. We compute some of the non-commutative couplings of the worldvolume effective actions of these non-abelian systems of M2 and M5 branes and show that they indicate the existence of configurations corresponding to N branes expanding into a higher dimensional M-brane. The reduction to Type II brings up new descriptions of coincident D-branes at strong coupling. We show that these systems have the right non-commutative charges to describe certain expanded configurations playing a role in the framework of the AdS/CFT correspondence. Finally, we discuss the realization of non-commutative brane configurations as topological solitons in non-abelian brane-antibrane systems.Comment: 31 pages, latex file, references adde

Doenças neurológicas de ovinos na região central do Rio Grande do Sul

Neste trabalho são descritos aspectos epidemiológicos e clinico-patológicos das principais doenças neurológicas de ovinos diagnosticadas no Laboratório de Patologia Veterinária (LPV) da Universidade Federal de Santa Maria (UFSM) no período entre 1990 e 2007. A partir de uma busca nos arquivos do LPV-UFSM foram encontrados 586 exames de ovinos correspondentes a necropsias realizadas no laboratório ou a exames de amostras remetidas por veterinários de campo. Sessenta e nove casos experimentais foram excluídos do estudo. Os 517 casos restantes eram compostos de 361 casos (69,8%) com diagnóstico conclusivo e 156 casos (30,2%) com diagnóstico inconclusivo. Ovinos morreram em decorrência de doença neurológica em 58 casos (16%) do grupo com diagnóstico conclusivo. As doenças diagnosticadas mais frequentemente foram cenurose (15 casos ou 25,8%), listeriose (nove casos ou 15,5%), tétano (oito casos ou 13,7%), abscessos vertebrais (quatro casos ou 6,8%) e abscessos encefálicos (três casos ou 5,1%). Intoxicação por Erytroxylum argentinum, mielite supurativa pós-caudectomia, meningoencefalite fibrino-supurativa, polioencefalomalacia e raiva (dois casos ou 3,4% cada) foram ocasionalmente diagnosticadas. Desmielinização medular, edema da substância branca encefálica, encefalomalacia focal simétrica, hidranencefalia, hipoplasia cerebelar, intoxicação por organofosforado, intoxicação por Solanum pseudocapsicum, mielite fibrino-supurativa e provável intoxicação por closantel (um caso ou 1,7% cada) foram raramente observadas.A retrospective study of neurological diseases of sheep in southern Brazil was conducted over an 18-year period (1990-2007). A data base search was carried out in the files of the Laboratory of Veterinary Pathology (LPV) of the Universidade Federal de Santa Maria (UFSM), central Rio Grande do Sul state, Brazil. A total of 586 sheep were examined. These cases corresponded to necropsies performed in the LPV-UFSM or to mailed in samples from practitioners. Out of these, 69 experimental cases were excluded from this study. The remaining 517 cases were composed of 361 (69.8%) cases with conclusive diagnoses and 156 (30.2%) cases with inconclusive diagnoses. In 58 (16%) occasions, sheep died in consequence of neurological disease. Most important diseases included coenurosis (15 cases or 25.8%), listeriosis (9 cases or 15.5%), tetanus (8 cases or 13.7%), vertebral abscesses (4 cases or 6.8%), and cerebral abscesses (3 cases or 5.1%). Poisoning by Erytroxylum argentinum, post-caudectomy suppurative myelitis, fibrinosuppurative meningoencephalitis, polioencephalomalacia, rabies (2 cases or 3.4% each) were occasionally diagnosed. Spinal cord demyelinization, encephalic white matter edema, focal symmetrical encephalomalacia, hydranencephalia, cerebellar hypoplasia, poisoning by organophosphate, poisoning by Solanum pseudocapsicum, fibrinosuppurative myelitis, and presumptive closantel toxicity (1 case or 1.7% each) were rarely seen