204,643 research outputs found
Gauge coupling unification in E 6 F-theory GUTs with matter and bulk exotics from flux breaking
No full textWe consider gauge coupling unification in E 6 F-Theory Grand Unified Theories (GUTs) where E 6 is broken to the Standard Model (SM) gauge group using fluxes. In such models there are two types of exotics that can affect gauge coupling unification, namely matter exotics from the matter curves in the 27 dimensional representation of E 6 and the bulk exotics from the adjoint 78 dimensional representation of E 6. We explore the conditions required for either the complete or partial removal of bulk exotics from the low energy spectrum. In the latter case we shall show that (miraculously) gauge coupling unification may be possible even if there are bulk exotics at the TeV scale. Indeed in some cases it is necessary for bulk exotics to survive to the TeV scale in order to cancel the effects coming from other TeV scale matter exotics which would by themselves spoil gauge coupling unification. The combination of matter and bulk exotics in these cases can lead to precise gauge coupling unification which would not be possible with either type of exotics considered by themselves. The combination of matter and bulk exotics at the TeV scale represents a unique and striking signature of E 6 F-theory GUTs that can be tested at the LHC
Constraining Dark Matter through CMB
No full textSelf-Annihilating Dark Matter (DM) candidates can modify the recombination history of the Universe, injecting additional energy into the thermal gas, and modifying its ionization state. The electron fraction history affects the CMB temperature and polarization power spectra; observations of the modifications (or of the absence) of the latter can then be used to constrain the power injected by annihilating DM, and ultimately the DM parameters themselves. DM annihilations able to modify the CMB spectra are active at redshifts 100 ×z× 1000, thus involving only a smooth density field, and permitting to ignore structure formation. Current WMAP7 data on the TT, TE and EE angular power spectra already permit to rule out interesting regions in the 〈σv〉 -mx space; the forthcoming PLANCK ones will permit to explore the "thermal WIMP" region. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution- NonCommercial-ShareAlike Licence
Unified Dark Matter models with fast transition
No full textWe investigate the general properties of Unified Dark Matter (UDM) fluid models where the pressure and the energy density are linked by a barotropic equation of state (EoS) p = p(ρ) and the perturbations are adiabatic. The EoS is assumed to admit a future attractor that acts as an effective cosmological constant, while asymptotically in the past the pressure is negligible. UDM models of the dark sector are appealing because they evade the so-called ``coincidence problem'' and ``predict'' what can be interpreted as wDE ≈ −1, but in general suffer the effects of a non-negligible Jeans scale that wreak havoc in the evolution of perturbations, causing a large Integrated Sachs-Wolfe effect and/or changing structure formation at small scales. Typically, observational constraints are violated, unless the parameters of the UDM model are tuned to make it indistinguishable from ΛCDM. Here we show how this problem can be avoided, studying in detail the functional form of the Jeans scale in adiabatic UDM perturbations and introducing a class of models with a fast transition between an early Einstein-de Sitter CDM-like era and a later ΛCDM-like phase. If the transition is fast enough, these models may exhibit satisfactory structure formation and CMB fluctuations. To consider a concrete case, we introduce a toy UDM model and show that it can predict CMB and matter power spectra that are in agreement with observations for a wide range of parameter values
The d-f luminescence of Eu2+, Ce3+ and Yb2+ ions in Cs2MP2O7 (M = Ca2+, Sr2+)
No full textThe efficient narrow band emission of Eu2+ in Cs2MP2O7 (M = Ca2+, Sr2+) is characterized by a large Stokes shift and a high quenching temperature which makes the material promising for application in warm white LEDs. The unusual Eu2+ luminescence properties were reported recently but an explanation for the peculiar behavior is lacking. In this paper we aim at providing new insights in the luminescence of the Eu2+ emission in Cs2MP2O7 through measurements at cryogenic temperatures (down to 4 K) and by comparison with the d-f luminescence of Ce3+ and Yb2+ in the same host. The results reveal a sharp onset of the Eu2+ emission and excitation bands at 4 K. Usually the sharp onset for narrow excitation and emission bands coincide at an energy corresponding to the zero-phonon (purely electronic) transition, but for Eu2+ in Cs2MP2O7 there is a large shift of 3500 cm-1 between the onsets, consistent with the large Stokes shift observed. The onset shift can be explained by emission from a lower energy distorted excited 4f65d1 state. For Ce3+, the f-d absorption bands are at energies expected based on the relation between the absorption energies for Eu2+ and Ce3+ reported by Dorenbos. Contrary to Eu2+, the emission for Ce3+ shows a normal Stokes shift and therefore the emission bands are at much higher energies than predicted from the energy of the Eu2+ emission and the Dorenbos relations. Based on the present results the unusually large Stokes shift for the Eu2+ emission in Cs2MP2O7 is assigned to a Jahn-Teller like deformation in the excited 4f65d1 state of Eu2+ that is not present in the 5d state of Ce3+
Fast d–f emission in Ce3+, Pr3+ and Nd3+ activated RbCl
No full textIn the search for new scintillator materials, Ce3+ doped chlorides are a promising class of materials, combining a high efficiency and fast response time. Even shorter response times may be achieved by replacing Ce3+ by Pr3+ or Nd3+ as the lifetime of the d–f emission is substantially shorter for these ions. Here we report on the luminescence properties of Ce3+, Pr3+ and Nd3+ in RbCl and investigate the potential as a scintillator material. Under UV excitation Ce3+ shows d–f emission between 325 and 425 nm. The emission originates from multiple (differently charge compensated) Ce3+ sites. The luminescence lifetime varies with wavelength and is ∼40 ns for the longer wavelength emission. For RbCl:Pr3+ three d–f emission band are observed between 250 and 350 nm which can be assigned to transitions from the lowest energy fd state to different 3HJ (J = 4–6) states within the 4f2 configuration of Pr3+. The decay time is ∼17 ns. For the Nd3+ activated sample a weak emission band around 220 nm is observed only at 8 K which may be due to d–f emission. The very short lifetime (4 ns) is faster than the radiative lifetime, indicating that the d–f emission is quenched by relaxation to lower lying 4f3 states or by the process of photoionization. Under VUV excitation at wavelengths below 175 nm (the bandgap of RbCl) the d–f emission is very weak for Ce3+, Pr3+ and Nd3+ doped RbCl and the emission spectra are dominated by defect related emission. This indicates that energy transfer from the host lattice to the fd states is inefficient which prevents application as a scintillator material
Bianchi type-I universe in f(R, T) modified gravity with quark matter and Λ
No full text32nd International Physics Congress of Turkish-Physical-Society (TPS) -- SEP 06-09, 2016 -- Bodrum, TURKEYIn this study, we investigate homogeneous and anisotropic Bianchi type I universe in the presence of quark matter source in f (R, T) gravity (Harko et al. in Phys. Rev. D 84:024020, 2011) with cosmological constant A (where R is the Ricci scalar and T is the trace of the energy momentum tensor). For this aim we have used the anisotropy feature of Bianchi type I universe and equation of states (EoS) of quark matter. We explore the exact solution f(R, T)=R + 2f(T) model for Bianchi type I universe model. When t -> infinity, we get very small cosmological constant value, this result agrees with recent observations.Turkish Phys So
“F. Suárez a J. Senftleben: Reification of Prime Matter“
No full textThe author compares the doctrine according to which the prime matter includes already entitative act at F. Suarez and J. Senftleben. By that he analyzes the basic arguments against the thomistic doctrine, according to which the first matter is pure potentiality
Analysis of the radiative lifetime of d-f emission
No full textThe radiative lifetime of excited states is governed by Fermi’s Golden Rule. For many applications, the radiative decay rate is an important parameter. For example, for scintillators materials in PET scanners, a short response time is crucial and it has been realized that the d-f emission of Pr3+ is faster than for the widely applied d-f emission from Ce3+. In this paper, the radiative decay rate of d-f emission from Pr3+ is systematically investigated in a wide variety of host lattices, including scintillators materials. The variation in the decay rate is analyzed based on Fermi’s Golden Rule. The trend observed is best described using a full cavity model to correct for local-field effects and a λ3 factor to account for the energy of the transition. Still, there is a considerable scatter of the experimental data around the best fit to these data. The variation is explained by uncertainties in the refractive indices and a variation in the transition dipole moment of the d-f transition for Pr3+. Based on the results, the shortest radiative lifetime that can be achieved for Pr3+ d-f emission is predicted to be ∼6 n
Variations on the Author
No full text“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
Why We Have Day and Night
No full textby Peter F. Neumeyer.https://creativematter.skidmore.edu/curious_cover/1039/thumbnail.jp
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