137,115 research outputs found

    Transverse effects in multifrequency Raman generation

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    The theory of ultrabroadband multifrequency Raman generation is extended, for the first time, to allow for beam-propagation effects in one and two transverse dimensions. We show that a complex transverse structure develops even when diffraction is neglected. In the general case, we examine how the ultrabroadband multifrequency Raman generation process is affected by the intensity, phase quality, and width of the input beams, and by the length of the Raman medium. The evolution of power spectra, intensity profiles, and global characteristics of the multifrequency beams are investigated and explained. In the two-dimensional transverse case, bandwidths comparable to the optical carrier frequency, spanning the whole visible spectrum and beyond, are still achievable

    Anthropically Selected Baryon Number and Isocurvature Constraints

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    The similarity of the observed baryon and dark matter densities suggests that they are physically related, either via a particle physics mechanism or anthropic selection. A pre-requisite for anthropic selection is the generation of superhorizon-sized domains of different Omega_{B}/Omega_{DM}. Here we consider generation of domains of different baryon density via random variations of the phase or magnitude of a complex field Phi during inflation. Baryon isocurvature perturbations are a natural consequence of any such mechanism. We derive baryon isocurvature bounds on the expansion rate during inflation H_{I} and on the mass parameter mu which breaks the global U(1) symmetry of the Phi potential. We show that when mu < H_{I} (as expected in SUSY models) the baryon isocurvature constraints can be satisfied only if H_{I} is unusually small, H_{I} < 10^{7} GeV, or if non-renormalizable Planck-suppressed corrections to the Phi potential are excluded to a high order. Alternatively, an unsuppressed Phi potential is possible if mu is sufficiently large, mu > 10^{16} GeV. We show that the baryon isocurvature constraints can be naturally satisfied in Affleck-Dine baryogenesis, as a result of the high-order suppression of non-renormalizable terms along MSSM flat directions.Comment: 8 pages, 1 eps figure, LaTeX. Minor typo correcte

    A Minimal Sub-Planckian Axion Inflation Model with Large Tensor-to-Scalar Ratio

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    We present a minimal axion inflation model which can generate a large tensor-to-scalar ratio while remaining sub-Planckian. The modulus of a complex scalar field Ξ¦\Phi with a λ∣Φ∣4\lambda |\Phi|^4 potential couples directly to the gauge field of a strongly-coupled sector via a term of the form (∣Φ∣/MPl)mFF~(|\Phi|/M_{Pl})^{m} F \tilde{F}. This generates a minimum of the potential which is aperiodic in the phase. The resulting inflation model is equivalent to a Ο•4/(m+1)\phi^{4/(m+1)} chaotic inflation model. For the natural case of a leading-order portal-like interaction Φ†ΦFF~\Phi^{\dagger}\Phi F \tilde{F}, the model is equivalent to a Ο•4/3\phi^{4/3} chaotic inflation model and predicts a tensor-to-scalar ratio r=16/3N=0.097r = 16/3N = 0.097 and a scalar spectral index ns=1βˆ’5/3N=0.970n_{s} = 1-5/3N = 0.970. The value of ∣Φ∣|\Phi| remains sub-Planckian throughout the observable era of inflation, with βˆ£Ξ¦βˆ£β‰²0.01MPl|\Phi| \lesssim 0.01 M_{Pl} for N≲60N \lesssim 60 when λ∼1\lambda \sim 1.Comment: One minor alteration. Version to be published in JCA

    Enhanced Dark Matter Annihilation Rate for Positron and Electron Excesses from Q-ball Decay

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    We show that Q-ball decay in Affleck-Dine baryogenesis models can account for dark matter when the annihilation cross-section is sufficiently enhanced to explain the positron and electron excesses observed by PAMELA, ATIC and PPB-BETS. For Affleck-Dine baryogenesis along a d = 6 flat direction, the reheating temperature is approximately 30 GeV and the Q-ball decay temperature is in the range 10-100 MeV. The LSPs produced by Q-ball decay annihilate down to the observed dark matter density if the cross-section is enhanced by a factor ~ 10^3 relative to the thermal relic cross-section.Comment: 4 pages, version to be published in Physical Review Letter

    Complex learning communities

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    A new breed of learning community which is driven by the need to generate learning, creativity and economic capacity is emerging as a result of the demands of the Information Society. Radical heterogeneity and multiple drivers make these learning communities significantly different from previously identified learning communities such as corporate Communities of Practice or Virtual Learning Communities. If full benefit is to be realised from such Complex Learning Communities (CLCs), then better understanding of their complex behaviour and methods of maximising their effectiveness are required. This short paper presents an overview of CLCs and reports on the development of a research agenda designed to address the identified gaps in knowledge

    Foreword from the Principal [June 2009]

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    As Principal and Vice-Chancellor of the University of Strathclyde I am very pleased to introduce this special issue of the Fraser Commentary which, in addition to the regular forecast and review of the Scottish economy, is focussed on the role of higher education and training in Scotland
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