120919640
Gravitational Behaviour of an Effective Topological Field Theory
Effective topological field theories describe the properties of Dirac fermions in the low-energy regime. In this work, we introduce a new emergent gravity model by considering Dirac fermions invariant under local de Sitter transformations in four-dimensional open manifolds. In the context of Cartan geometry, fermions couple to spacetime through a $Spin(5)$ Cartan connection that can be decomposed in spin connection and tetrads. In presence of a gravitational instanton background, we show that the corresponding effective topological field theory becomes equivalent to a dynamical gravitational theory with positive cosmological constant and Barbero-Immirzi parameter. At classical level and in absence of matter, this theory is compatible with general relativity.
96459470
(3+1) -dimensional topological quantum field theory from a tight-binding model of interacting spinless fermions
Currently, there is much interest in discovering analytically tractable (3+1)-dimensional models that describe interacting fermions with emerging topological properties. Towards that end we present a three-dimensional tight-binding model of spinless interacting fermions that reproduces, in the low-energy limit, a (3+1)-dimensional Abelian topological quantum field theory called the BF model. By employing a mechanism equivalent to Haldane's Chern insulator, we can turn the noninteracting model into a three-dimensional chiral topological insulator. We then isolate energetically one of the two Fermi points of the lattice model. In the presence of suitable fermionic interactions, the system, in the continuum limit, is equivalent to a generalized (3+1)-dimensional Thirring model. The low-energy limit of this model is faithfully described by the BF theory. Our approach directly establishes the presence of (2+1)-dimensional BF theory at the boundary of the lattice and it provides a way to detect the topological order of the model through fermionic density measurements.
73703329
Non-Abelian Chern-Simons Theory from a Hubbard-like Model
Here, we provide a simple Hubbard-like model of spin-1/2 fermions that gives rise to the SU(2)-symmetric Thirring model that is equivalent, in the low-energy limit, to the Yang-Mills-Chern-Simons model. First, we identify the regime that simulates the SU(2) Yang-Mills theory. Then, we suitably extend this model so that it gives rise to the SU(2) Chern-Simons theory with level k≥2 that can support non-Abelian anyons. This is achieved by introducing multiple fermionic species and modifying the Thirring interactions, while preserving the SU(2) symmetry. Our proposal provides the means to theoretically and experimentally probe non-Abelian SU(2) level k topological phases.
2580056
Skyrmion Superfluidity in Two-Dimensional Interacting Fermionic Systems
In this article we describe a multi-layered honeycomb lattice model of interacting fermions which supports a new kind of parity-preserving skyrmion superfluidity. We derive the low-energy field theory describing a non-BCS fermionic superfluid phase by means of functional fermionization. Such effective theory is a new kind of non-linear sigma model, which we call double skyrmion model. In the bi-layer case, the quasiparticles of the system (skyrmions) have bosonic statistics and replace the Cooper-pairs role. Moreover, we show that the model is also equivalent to a Maxwell-BF theory, which naturally establishes an effective Meissner effect without requiring a breaking of the gauge symmetry. Finally, we map effective superfluidity effects to identities among fermionic observables for the lattice model. This provides a signature of our theoretical skyrmion superfluidy that can be detected in a possible implementation of the lattice model in a real quantum system.
17349103
Chandra observations of X-ray source overdensities in distant galaxy cluster fields
The first systematic study of the serendipitous X-ray source density around 10 high z (0.24 2sigma per field, and >5sigma overall) only on scale of 8'X8' and peculiar to cluster fields. The blank field fluctuations, i.e. the most sensitive measurements to date of the X-ray cosmic variance on this scale, indicate (1 sigma) variations within ~15-25% around the average value. The first marginal evidence that these overdensities increase with cluster redshift is also presented. We speculate that the most likely explanation is that the sources in excess are AGNs tracing filaments connected to the clusters. If the association of the overdensities with large-scale structures and their positive relation with cluster redshift is confirmed, these studies could represent a new, direct, way of mapping the cosmic web and its cosmological parameters at high z.
18676788
Simbol-X Background Minimization: Mirror Spacecraft Passive Shielding Trade-Off Study
The present work shows a quantitative trade‐off analysis of the Simbol‐X Mirror Spacecraft (MSC) passive shielding, in the phase space of the various parameters: mass budget, dimension, geometry and composition. A simplified physical (and geometrical) model of the sky screen, implemented by means of a GEANT4 simulation, has been developed to perform a performance‐driven mass optimization and evaluate the residual background level on Simbol‐X focal plane.
140606740
From (under)ground to space: The birth of Space Science in Italy and the growth of astrophysics in its National Institutions
The first attempt by Italian scientists to carry on Astrophysical observations from Space can be traced back to the early 60ʼs just after the launch of the first satellites. Dominated by Physicists for decades, slowly space astrophysics has become one of the major fields of modern Astronomy. From the first experiments flown on balloons and rockets to the construction of todayʼs sophisticated satellites a historical scenario will be presented in the framework of Italian Science and, more generally, of European and World growth of the field. The first experiments in Universities, the setting of the CNR (National Research Council) labs, later transformed in institutes, to the recent merging of all Astronomical Observatories, will be discussed with mention to historical episodes. Main results and achievements will be outlined. How politics has affected the development of this branch of astronomical research will also be briefly considered within the limits assigned to the present talk. Some of the information contained in this paper comes from original documents, some from interviewing people who played a major role in the events, some from the literature but most of it is from personal reminiscences.
3198083
The low luminosity AGN in the LINER galaxy M 81: BeppoSAX discovery of highly ionized gas
The LINER nucleus of the nearby spiral galaxy M 81 was pointed by BeppoSAX, which caught it at the highest (2- 10) keV flux level observed so far. The LECS, MECS and PDS data, extending over (0.1-100) keV, are used to investi- gate the physical similarities and differences between LINERs and AGNs. The continuum is well fitted by a power law of pho- ton index 1:84, modified by little absorption due to cold material; this extends to (0.1-100) keV the validity of a similar ASCA result. Superimposed on the continuum BeppoSAX de- tects a 6.7 keV emission line (confirming another ASCA result) and an absorption edge at 8:6 keV. Both spectral features are consistent with being produced by iron at the same high ion- ization level, and probably also with the same column density. So, we suggest that they originate from transmission through highly ionized thin material. Concerning the origin of the con- tinuum emission, we do not observe signs of reflection from the optically thick material of an accretion disk, as usually found in Seyfert 1's (a 6.4 keV emission line and a broad bump peaking at 10-20 keV). The low bolometric luminosity of the nucleus of M 81 is consistent with being produced by advection dominated accretion; in this case the X-ray emission should be dominated by Comptonization, rather than by bremsstrahlung, in order to reproduce the steep spectrum observed over the (0.1-100) keV band.
10219162
BeppoSAX observations of Mkn 3: Piercing through the torus of a Seyfert 2 galaxy
An ewBeppoSAX broad-band (0.6-150 keV) spec- trum of the Seyfert 2 galaxy Mkn 3 is presented. The spectrum provides a direct measurement of a large, neutral column of gas with NH 10 24 cm 2 in the source direction. The source, as bright as 3C 273 above 10 keV, has a steep ( 1.8) spec- trum without any evidence of a high-energy cutoff up to at least 150 keV. At lower energies, the data are best modeled with the addition of an unabsorbed reflection component. Combining these data with previous Ginga and ASCA observations, the Fe K and reflection continuum indicate that the reprocessed emission is responding slower than the intrinsic continuum vari- ations suggesting a size of the reprocessor > 2 pc. Identifying such a reprocessor with a (close to edge-on) obscuring torus, the overall result fits well into unified models since, presum- ably, one can interpret the strong absorption as due to transmis- sion through the rim of the torus and the unabsorbed (directly viewed) reflection component as due to reprocessing from the torus inner surface.