Private provision of public goods and information diffusion in social groups

We describe a model of fundraising in social groups, where private information about quality of provision is transmitted by social proximity. Individuals engage in voluntary provision of a pure collective good that is consumed by both neighbours and non-neighbours. We show that, unlike in the case of private goods, better informed individuals face positive incentives to incur a cost to share information with their neighbours. These incentives are stronger, and provision of the pure public good greater, the smaller are individuals’ social neighbourhoods

Weak Interactions in Top-Quark Pair Production at Hadron Colliders: An Update

Weak corrections for top-quark pair production at hadron colliders are revisited. Predictions for collider energies of 8 TeV, adopted to the recent LHC run, and for 13 as well as 14 TeV, presumably relevant for the next round of LHC experiments, are presented. Kinematic regions with large momentum transfer are identified, where the corrections become large and may lead to strong distortions of differential distributions, thus mimicking anomalous top quark couplings. As a complementary case we investigate the threshold region, corresponding to configurations with small relative velocity between top and antitop quark, which is particularly sensitive to the top-quark Yukawa coupling. We demonstrate, that nontrivial upper limits on this coupling, complementary to those recently derived by the CMS and the ATLAS collaorations, are well within reach of ongoing experiments. In addition we suggest a prescription that allows the implementation of these corrections in current Monte Carlo generators. Furthermore, the weak corrections have been included in the publicly available Hathor library. The numerical results presented in this article use the same setup as the recently calculated NNLO QCD corrections. The results can thus be combined to give the most precise theoretical predictions.Comment: Updated version with extended discussion on the top-quark Yukawa coupling, input parameters updated to recent value

ON THE MEASUREMENT OF A COSMOLOGICAL DIPOLE IN THE PHOTON NUMBER COUNTS OF GAMMA-RAY BURSTS

If gamma-ray bursts are cosmological or in a halo distribution their properties are expected to be isotropic (at least to 1st order). However, our motion with respect to the burst parent population (whose proper frame is expected to be that of the Cosmic Microwave Background (CMB), or that of a static halo) will cause a dipole effect in the distribution of bursts and in their photon number counts (together termed a Compton-Getting effect). We argue that the photon number count information is necessary to distinguish a genuine Compton-Getting effect from some other anisotropy and to fully test the proper frame isotropy of the bursts. Using the 2B burst catalogue and the dipole determined from the CMB, we find the surprising result that although the number weighted distribution is consistent with isotropy, the fluence weighted dipole has a correlation with the CMB dipole that has a probability of occuring only 10% of the time for an isotropic photon distribution. Furthermore, the photon and number dipoles are inconsistent under the hypothesis of isotropy, at the 2-sigma level. This could be an indication that a non-negligible fraction of gamma-ray bursts originate in the local, anisotropic universe. (shortened Abstract)Comment: Accepted by ApJ. Self-unpacking (use csh), uuencoded, compressed Postscript, 16 pages + 4 Figures (5 files

Tiebout with Politics: Capital Tax Competition and Constitutional Choices.

This paper examines how capital tax competition affects jurisdiction formation. We describe a locational model of public goods provision, where jurisdictions are represented by coalitions of consumers with similar tastes, and where the levels of taxation and local public goods provision within jurisdictions are selected by majority voting. We show that in this setting interjurisdictional tax competition results in an enlargement of jurisdictional boundaries, and can raise welfare for all members of a jurisdiction even in the absence of intrajurisdictional transfers.

Exoplanet Transit Parallax

The timing and duration of exoplanet transits has a dependency on observer position due to parallax. In the case of an Earth-bound observer with a 2 AU baseline the dependency is typically small and slightly beyond the limits of current timing precision capabilities. However, it can become an important systematic effect in high-precision repeated transit measurements for long period systems due to its relationship to secular perspective acceleration phenomena. In this short paper we evaluate the magnitude and characteristics of transit parallax in the case of exoplanets using simplified geometric examples. We also discuss further implications of the effect, including its possible exploitation to provide immediate confirmation of planetary transits and/or unique constraints on orbital parameters and orientations.Comment: 12 Pages, 3 Figures, Accepted for publication in Ap

Scale economies in nonprofit provision, technology adoption and entry

We study competition between nonprofit providers that supply a collective service through increasing-returns-to-scale technologies under conditions of free entry. When providers adopt a not-for-profit mission, the absence of a residual claimant can impede entry, protecting the position of an inefficient incumbent. Moreover, when providers supply goods that are at least partly public in nature, they may be unable to sustain the adoption of more efficient technologies that feature fixed costs, because buyers (private donors) face individual incentives to divert donations towards charities that adopt inferior, lower-fixed-cost technologies. These incentives may give rise to a technological race to the bottom, where nonprofit providers forgo opportunities to exploit scale economies. In these situations, government grants in support of core costs can have a nonneutral effect on entry, technology adoption, and industry performance

Possible constraints on exoplanet magnetic field strengths from planet-star interaction

A small percentage of normal stars harbor giant planets that orbit within a few tenths of an astronomical unit. At such distances the potential exists for significant tidal and magnetic field interaction resulting in energy dissipation that may manifest as changes within the stellar corona. We examine the X-ray emission of stars hosting planets and find a positive correlation between X-ray luminosity and the projected mass of the most closely orbiting exoplanets. We investigate possible systematics and observational biases that could mimic or confuse this correlation but find no strong evidence for any, especially for planets more massive than ~0.1 MJ. Luminosities and upper limits are consistent with the interpretation that there is a lower floor to stellar X-ray emission dependent on close-in planetary mass. Under the hypothesis that this is a consequence of planet-star magnetic field interaction, and energy dissipation, we estimate a possible field strength increase between planets of 1 and 10 MJ of a factor ~8. Intriguingly, this is consistent with recent geodynamo scaling law predictions. The high-energy photon emission of planet-star systems may therefore provide unique access to the detailed magnetic, and hence geodynamic, properties of exoplanets.Comment: Submitted to ApJ, 35 pages, 6 figure

A Spherical Harmonic Approach to Redshift Distortion and a Measurement of Omega from the 1.2 Jy IRAS Redshift Survey

We present a formalism for analysing redshift distortions based on a spherical harmonic expansion of the density field. We use a maximum likelihood estimator for the combination of density and bias parameters, $\Omega^0.6/b$. We test the method with $N$-body simulations and apply it to the 1.2 Jy IRAS redshift survey.Comment: 8 pages--uuencoded postscript fil

Infrared limit in external field scattering

Scattering of electrons/positrons by external classical electromagnetic wave packet is considered in infrared limit. In this limit the scattering operator exists and produces physical effects, although the scattering cross-section is trivial.Comment: 12 pages; published version; minor corrections; comments adde