Algebras and non-geometric flux vacua

In this work we classify the subalgebras satisfied by non-geometric Q-fluxes in type IIB orientifolds on T^6/(Z_2 x Z_2) with three moduli (S,T,U). We find that there are five subalgebras compatible with the symmetries, each one leading to a characteristic flux-induced superpotential. Working in the 4-dimensional effective supergravity we obtain families of supersymmetric AdS_4 vacua with all moduli stabilized at small string coupling g_s. Our results are mostly analytic thanks to a judicious parametrization of the non-geometric, RR and NSNS fluxes. We are also able to leave the flux-induced C_4 and C_8 RR tadpoles as free variables, thereby enabling us to study which values are allowed for each Q-subalgebra. Another novel outcome is the appearance of multiple vacua for special sets of fluxes. However, they generically have g_s > 1 unless the net number of O3/D3 or O7/D7 sources needed to cancel the tadpoles is large. We also discuss briefly the issues of axionic shift symmetries and cancellation of Freed-Witten anomalies.Comment: 61 pages, LaTex, v2: added reference

Cash flow or income? : the choice of base for company taxation

Considerable interest has been expressed in recent years by tax theorists as well as practitioners, for the taxation of companies based on their cash flow. Unlike the equity-income tax base, which requires the deductibility of economic depreciation and debt financing costs, the cash-flow base expenses capital at the point of purchase, eliminating the need for the subsequent costing of this capital. This paper raises some of the issues that would arise in trying to implement a company tax either in the form of an indexed equity-income or a cash-flow tax. Issues raised include: (i) administrative complexity; (ii) international tax coordination and competition; and (iii) transition problems. In a closed economy the cash-flow tax seems a simple, efficient form of company taxation, administratively straightforward and neutral with regard to investment decisions. The more complicated equity-income tax is harder to defend in a closed economy.Economic Theory&Research,Public Sector Economics&Finance,Environmental Economics&Policies,Banks&Banking Reform,International Terrorism&Counterterrorism

Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement

We calculate how the relic density of dark matter particles is altered when their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa interaction between the annihilating particles. Maintaining a dark matter abundance consistent with current observational bounds requires the normalization of the s-wave annihilation cross section to be decreased compared to a model without enhancement. The level of suppression depends on the specific parameters of the particle model, with the kinetic decoupling temperature having the most effect. We find that the cross section can be reduced by as much as an order of magnitude for extreme cases. We also compute the mu-type distortion of the CMB energy spectrum caused by energy injection from such Sommerfeld-enhanced annihilation. Our results indicate that in the vicinity of resonances, associated with bound states, distortions can be large enough to be excluded by the upper limit |mu|<9.0x10^(-5) found by the COBE/FIRAS experiment.Comment: 10 pages, 6 figures, accepted for publication in Physical Review D. Corrections to eqs. 9,10,14 and 16. Figures updated accordingly. No major changes to previous results. Website with online tools for Sommerfeld-related calculations can be found at http://www.mpa-garching.mpg.de/~vogelsma/sommerfeld

Numerical simulations of the internal shock model in magnetized relativistic jets of blazars

The internal shocks scenario in relativistic jets is used to explain the variability of the blazar emission. Recent studies have shown that the magnetic field significantly alters the shell collision dynamics, producing a variety of spectral energy distributions and light-curves patterns. However, the role played by magnetization in such emission processes is still not entirely understood. In this work we numerically solve the magnetohydodynamic evolution of the magnetized shells collision, and determine the influence of the magnetization on the observed radiation. Our procedure consists in systematically varying the shell Lorentz factor, relative velocity, and viewing angle. The calculations needed to produce the whole broadband spectral energy distributions and light-curves are computationally expensive, and are achieved using a high-performance parallel code.Comment: 7 pages, 5 figures, proceeding of the "Swift: 10 Years of Discovery" conference (December 2014, Rome, Italy

Commentary on session III: U.S.-Mexico remittances: recent trends and measurement issues

Summary and discussion of the three papers in this session: "Leveraging remittances for development" by Dilip Ratha; "Remittances and their microeconomic impacts: evidence from Latin America" by Catalina Amuedo-Dorantes; and "The relationship between international migration, trade, and development: some paradoxes and findings" by J. Edward Taylor. ; The rest of this commentary explores recent trends in U.S.?Mexico remittances, explaining how they are measured and comparing them with forecasts of remittances based on an econometric model and with trends in other developing countries.Emigration and immigration ; International trade ; Economic development ; Developing countries ; Emigrant remittances ; Econometric models ; Mexico