Production of light pseudoscalars in external electromagnetic fields by the Schwinger mechanism

We generalize the Schwinger mechanism and calculate the probability of the decay of intense electromagnetic fields to pseudoscalar particles. We also point out that our estimate for axion emission in a previous paper was incorrect.Comment: 25 pages including 9 figures. Version that matches published versio

Weighted Approval Voting

To allow society to treat unequal alternatives distinctly we propose a natural extension of Approval Voting [7] by relaxing the assumption of neutrality. According to this extension, every alternative receives ex-ante a non-negative and finite weight. These weights may differ across alternatives. Given the voting decisions of every individual (individuals are allowed to vote for, or approve of, as many alternatives as they wish to), society elects all alternatives for which the product of total number of votes times exogenous weight is maximal. Our main result is an axiomatic characterization of this voting procedure.microeconomics ;

Incomplete Information and Small Cores in Matching Markets

We study Bayesian Nash equilibria of stable mechanisms in centralized matching markets under incomplete information. We show that truth-telling is a Bayesian Nash equilibrium of the revelation game induced by a common belief and a stable mechanism if and only if all the profiles in the support of the common belief have singleton cores. Our result matches the observations of Roth and Peranson (1999) in the National Resident Matching Program (NRMP) in the United States: (i) the cores of the profiles submitted to the clearinghouse are small and (ii) while truth-telling is not a dominant strategy most participants of the NRMP truthfully reveal their preferences.Matching Market, Incomplete Information, Small Core

Efficient and Stable Collective Choices under Gregarious Preferences

We consider collective choice problems where a set of agents have to choose an alternative from a finite set and agents may or may not become users of the chosen alternative. An allocation is a pair given by the chosen alternative and the set of its users. Agents have gregarious preferences over allocations: given an allocation, they prefer that the set of users becomes larger. We require that the final allocation be efficient and stable (no agent can be forced to be a user and no agent who wants to be a user can be excluded). We propose a two-stage sequential mechanism whose unique subgame perfect equilibrium outcome is an efficient and stable allocation which also satisfies a maximal participation property.Public Goods, Gregarious Preferences, Subgame Perfect Implementation

Limits on anomalous couplings of the Higgs boson to electroweak gauge bosons from LEP and the LHC

In this paper we assume the Higgs is an elementary scalar, and study how new physics could affect its couplings to electroweak gauge bosons. Adding LHC data to LEP data provides new, more stringent limits, particularly when the Higgs to two photon decay signal strength is taken into account. We then study the effect of anomalous angular correlations in the decay to WW*. We obtain a new limit on the rare decay to photon-Z, and use it to constrain supersymmetry, to find that staus with large mixing would be most sensitive to this channel. We also use these limits to constrain radion exchange in warped extra dimensions, finding a limit on the radion mass and interaction scale of the order of TeV. Finally, we have extrapolated the current data to obtain prospects for the full 2012 data set

Efficient and Stable Collective Choices under Crowding Preferences

We consider a set of agents who have to choose one alternative among a finite set of social alternatives. A final allocation is a pair given by the selected alternative and the group of its users. Agents have crowding preferences over allocations: between any pair of allocations with the same alternative, they prefer the allocation with the largest number of users. We require that a decision be efficient and stable (which guarantees free participation in the group of users and free exit from it). We propose a two-stage sequential mechanism whose unique subgame perfect equilibrium outcome is an efficient and stable allocation which also satisfies a maximal participation property. The social choice function implemented by the proposed mechanism is also anonymous and group stable.Public Goods, Crowding Preferences, Subgame Perfect Implementation

The Multiple-partners Assignment Game with Heterogeneous Sells and Multi-unit Demands: Competitive Equilibria

A multiple-partners assignment game with heterogeneous sells and multi-unit demands consists of a set of sellers that own a given number of indivisible units of (potentially many different) goods and a set of buyers who value those units and want to buy at most an exogenously fixed number of units. We define a competitive equilibrium for this generalized assignment game and prove its existence by only using linear programming. We show that the set of competitive equilibria (pairs of price vectors and assignments) has a Cartesian product structure: each equilibrium price vector is part of a competitive equilibrium with all equilibrium assignments, and vice versa. We also show that the set of (restricted) equilibrium price vectors has a natural lattice structure and we study how this structure is translated into the set of agents' utilities that are attainable at equilibrium.Matching, Assignment Game, Indivisible Goods, Competitive Equilibrium, Lattice

Voting by Committees under Constraints

We consider social choice problems where a society must choose a subset from a set of objects. Specifically, we characterize the families of strategy-proof voting procedures when not all possible subsets of objects are feasible, and voters' preferences are separable or additively representable.Voting, strategy-proofness, additive and separable preferences

The Accelerated universe. On the Nobel Prize in Physics 2011 awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess

Des de final de la dècada del 1920 hem sabut que les galàxies distants s'allunyen de nosaltres. Les observacions que van conduir a aquesta conclusió van ser principalment les d'Edwin Hubble. La història de l'Univers ha estat de contínua expansió i refredament, i marcada per diversos esdeveniments importants. En un univers dominat per la matèria, és bastant intuïtiu pensar que l'expansió es frenarà o, en altres paraules, que l'Univers s'hauria de desaccelerar. I no obstant això, dos equips, el Supernova Cosmology Project i el High-z Supernova Search Team, van utilitzar un subconjunt de supernoves del tipus Ia (SNIA) i van arribar al mateix resultat sorprenent: l'Univers s'està accelerant. Però llavors, què està produint l'observada acceleració cosmològica? En aquest article es discuteix el Premi Nobel de Física 2011, atorgat a Saul Perlmutter, Brian P. Schmidt i Adam G. Riess pel descobriment de l'expansió accelerada de l'Univers mitjançant observacions de supernoves distants, i revisa el context cosmològic del descobriment i l'ús de les supernoves com candeles estàndard. Algunes de les conseqüències del descobriment també es presenten.Since the end of the 1920s we have known that distant galaxies are receding from us. The observations that led to this conclusion were mainly those of Edwin Hubble. The history of the universe has been one of continuous expansion and cooling, marked by several critical events. In a matter-dominated universe, it is quite intuitive that the expansion will eventually slow down; in other words, the universe should ecelerate. And yet two teams, the Supernova Cosmology Project and the High-z Supernova Search Team, used a subset of supernova—type Ia (SNIa)—and reached the same surprising result: the universe is accelerating. But what, then, is producing the observed cosmological acceleration? This article discusses the 2011 Nobel Prize for Physics, awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for the discovery of the accelerating expansion of the universe through observations of distant supernovae, and reviews the cosmological context of the discovery and the use of supernovae as standard candles. Some of the consequences of the discovery are presented as well