474 research outputs found

    Comments on the classification of orientifolds

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    The simple current construction of orientifolds based on rational conformal field theories is reviewed. When applied to SO(16) level 1, one can describe all ten-dimensional orientifolds in a unified framework.Comment: 9 pages, Contribution to proceedings of RTN-workshop in Leuven, Belgium, September 200

    Demagnetization Analysis in Excel (DAIE). An open source workbook in Excel for viewing and analyzing demagnetization data from paleomagnetic discrete samples and u-channels

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    The Demagnetization Analysis in Excel (DAIE) program is a single Microsoft Excel file designed for viewing and analyzing stepwise demagnetization data of both discrete and u-channel samples in paleomagnetic studies. DAIE is an Excel workbook and has an open modular structure organized in 10 worksheets. It is designed for easy use and interactive operation; all the commands and choices can be entered using drop-down menus associated to single cells. The standard demagnetization diagrams and various parameters of common use are shown on the same worksheet including selectable parameters and user's choices. The characteristic remanence components may be computed by principal component analysis (PCA) on a selected interval of demagnetization steps. Saving of the PCA data can be done both sample by sample, and automatically by applying the selected choices to all the samples included in the file. The whole workbook is free both for use and editing and it is available for download on a dedicated website

    ¿CUÁNTO DURA UNA INVERSIÓN DEL CAMPO MAGNÉTICO TERRESTRE? Nuevos datos de la última inversión del campo magnético terrestre

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    CUÁNTO DURA UNA INVERSIÓN DEL CAMPO MAGNÉTICO TERRESTRE? El campo magnético terrestre (o campo geomagnético) está generado por el movimiento complejo que tiene lugar en el núcleo externo de la Tierra. En su mayor parte, este campo es asimilable al que generaría una barra de imán situada en el centro del planeta

    ENVIRONMENTAL MAGNETISM: APPLICATIONS TO PALEOCLIMATIC RECONSTRUCTIONS AND AIR POLLUTION MONITORING

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    This short course will be focused on environmental magnetism. In the first part I will present a general overview of the relevant rock magnetic techniques and parameters, briefly discussing laboratory measurements and basic data analysis. Following this general introduction, I will address the application of rock magnetic techniques to two main lines of research

    Evaluation of a long-time temperature drift in a commercial Quantum Design MPMS SQUID magnetometer using Gd2_2O3_3 as a standard

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    The long-time temperature drift in a commercial Quantum Design MPMS SQUID magnetometer was evaluated using time-dependent magnetization measurements of Gd2_2O3_3. In contrast to earlier claims, the amplitude of the drift was found not to exceed 1-1.5 K. 30 minutes after system stabilization the temperature deviation did not exceed 0.2 K and the temperature was fully stabilized in less than 3 hours

    Mass scales, supersymmetry breaking and open strings

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    We review physical motivations and possible realizations of string vacua with large internal volume and/or low string scale and discuss the issue of supersymmetry breaking. In particular, we describe the key features of Scherk-Schwarz deformations in type I models and conclude by reviewing the phenomenon of ``brane supersymmetry breaking'': the tadpole conditions of some type-I models require that supersymmetry be {\it broken at the string scale} on a collection of branes, while being exact, to lowest order, in the bulk and on other branes.Comment: Based on talks presented by the authors at STRINGS '99. Submitted to Classical and Quantum Gravity. misprints correcte

    Gauss-Bonnet gravity renders negative tension braneworlds unstable

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    We show that the Gauss-Bonnet correction to Einstein gravity induces a gravitational tachyon mode, namely an unstable spin 2 fluctuation, in the Randall-Sundrum I model. We demonstrate that this instability is generically related to the presence of a negative tension brane in the set-up, with or without Z2Z_2-symmetry across it. Indeed it is shown that the tachyon mode is a bound state localised on any negative tension brane of co-dimension one, embedded in anti-de Sitter background. We discuss the possible resolution of this instability by the inclusion of induced gravity terms on the branes or by an effective four-dimensional cosmological constant.Comment: published versio

    Paleomagnetic constraints on the Plio-Pleistocene geodynamic evolution of the external central-northern Apennines (Italy)

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    We report on new paleomagnetic results obtained from 27 sites sampled in the Plio-Pleistocene sequences at the external front of the central-northern Apennines. Previous analyses of Miocene (Messinian) sediments indicated that the present shape of the northern Apenninic arc is due to the oroclinal bending of an originally straight belt oriented around N320° and that vertical axis rotations accompanied the migration of the thrust fronts toward the Adriatic foreland [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153-3166]. We tried to provide new paleomagnetic constraints for the timing and rates of the oroclinal bending process during the Pliocene and the Pleistocene. The results suggest that CCW rotations observed in the northern part of the studied area are possibly younger than 3 Ma. No regional rotation is recorded in the Pliocene and Pleistocene sediments from the southern part of the study area, analogously to the Messinian sediments of the 'Acquasanta' domain of Speranza et al. [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153-3166]. A local significant CCW rotation (23° ± 10°) is identified in the Early Pleistocene sediments that crop out along the Adriatic coast between Ascoli and Pescara, indicating differential motion of the thrust sheets. This rotation must be younger than 1.43 Ma

    Degeneracy Between the Regge Slope of Mesons and Baryons from Supersymmetry

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    We consider the degeneracy between the Regge slope of mesons and baryons in QCD. We argue that within the "orientifold large-N approximation" asymptotically massive mesons and baryons become supersymmetric partners and hence degenerate. To this end, we generalize QCD by a SU(N) theory with a quark in the two-index antisymmetric representation. We show that in this framework the meson is represented by an oriented bosonic QCD-string and the baryon is represented by an un-oriented fermionic QCD-string. At large-N, due to an equivalence with super Yang-Mills, the tensions of the bosonic and the fermionic strings coincide. Our description of mesons and baryons as oriented and un-oriented bosonic and fermionic QCD-strings is in full agreement with the spectra of open strings in the dual type 0' string theory.Comment: v2: extended version. Appendices and references adde

    Paleomagnetic dating of non-sulfide Zn-Pb ores in SW Sardinia (Italy): a first attempt

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    A first paleomagnetic investigation aimed at constraining the age of the non-sulfide Zn-Pb ore deposits in the Iglesiente district (SW Sardinia, Italy) was carried out. In these ores, the oxidation of primary sulfides, hosted in Cambrian carbonate rocks, was related to several paleoweathering episodes spanning from the Mesozoic onward. Paleomagnetic analyses were performed on 43 cores from 4 different localities, containing: a) non-oxidized primary sulfides and host rock, b) oxidized Fe-rich hydrothermal dolomites and (c) supergene oxidation ore («Calamine»). Reliable data were obtained from 18 samples; the others show uninterpretable results due to low magnetic intensity or to scattered demagnetization trajectories. Three of them show a scattered Characteristic Remanent Magnetization (ChRM), likely carried by the original (i.e. Paleozoic) magnetic iron sulfides. The remaining 15 samples show a well defined and coherent ChRM, carried by high-coercivity minerals, acquired after the last phase of counterclockwise rotation of Sardinia (that is after 16 Myr), in a time interval long enough to span at least one reversal of the geomagnetic field. Hematite is the main magnetic carrier in the limestone, whereas weathered hydrothermal dolomite contains goethite or a mixture of both. The results suggest that paleomagnetism can be used to constrain the timing of oxidation in supergene-enriched ores
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