WIMP Dark Matter and Unitarity-Conserving Inflation via a Gauge Singlet Scalar

A gauge singlet scalar with non-minimal coupling to gravity can drive inflation and later freeze out to become cold dark matter. We explore this idea by revisiting inflation in the singlet direction (S-inflation) and Higgs Portal Dark Matter in light of the Higgs discovery, limits from LUX and observations by Planck. We show that large regions of parameter space remain viable, so that successful inflation is possible and the dark matter relic abundance can be reproduced. Moreover, the scalar singlet can stabilise the electroweak vacuum and at the same time overcome the problem of unitarity-violation during inflation encountered by Higgs Inflation, provided the singlet is a real scalar. The 2-$\sigma$ Planck upper bound on $n_s$ imposes that the singlet mass is below 2 TeV, so that almost the entire allowed parameter range can be probed by XENON1T.Comment: 22 pages, 7 figures; v2: minor changes, references added, matches published versio

Dollar Denominated Debt and Optimal Security Design

During a crisis, developing countries regret having issued dollar denominated debt because they have to pay more when they have less. Ex ante, however, they may be worse off issuing local currency debt because the equilibrium interest rate might rise, making it more expensive for them to borrow. Many authors have assumed that lenders and borrowers have contrary goals, and that local currency (peso) debt is better for the borrower (Bolivia), and dollar debt is better for the lender (America). We show that if each country is represented by a single consumer with quadratic utilities, in perfect competition, then both will agree ex ante on whether dollar debt or peso debt is better. (In fact all assets can be Pareto ranked). But we show that it might well be dollar debt that Pareto dominates. In particular, if the lender is sufficiently risk averse and the borrower sufficiently impatient, and the lender's endowment is sufficiently riskless, then dollar debt Pareto dominates peso debt. However, if there are persistent gains to risk sharing between the countries, then peso debt Pareto dominates dollar debt. In the special case where utilities are linear in the first period and quadratic in the second period, we can completely characterize the Pareto ranking of any asset by a formula depending only on marginal utilities at autarky. In the more general case where utilities are linear in the first period and have positive third derivative in the second period, we show that when persistent gains to risk sharing hold, America must gain from Peso debt but Bolivia might lose. Thus the presumption that peso debt is more favorable to Bolivia than to America is false. Our framework of optimal security design can be used to demonstrate one rationale for credit controls. If the purchasing power of a dollar overseas varies with the quantity of debt issued, then both America and Bolivia can gain from capital controls, because a tax that reduces the quantity of Bolivian debt might make the real dollar payoffs in Bolivia more `peso-like', and therefore under persistent gains to risk pooling, better for America and Bolivia.Dollar debt, Currency, Indexed bonds, Security design, Capital controls

Signaling Concerns about Fairness: Cooperation under Uncertain Social Preferences

This paper investigates incomplete information and signaling about players?inequity aversion in the simultaneous and sequential-move prisoner?s dilemma game. We first evaluate the role of incomplete information according to: (1) whether uncertainty helps select the effcient equilibrium outcome, and (2) whether more cooperation can be sustained under incomplete than under complete information. We then examine the possibility of information transmission among individuals in a signaling game. A separating equilibrium can be supported in which players with high concerns about fairness bear the cost of cooperating in order to reveal their type to opponents, thus promoting cooperation in subsequent periods. We also fi?nd a pooling equilibrium in which a player unconcerned about inequity aversion initially cooperates in order to mislead the uninformed player. This misleading strategy induces cooperation from the uninformed player in the subsequent stage of the game, moment at which the unconcerned player takes the opportunity to defect. This "backstabbing" equilibrium helps explain frequently observed behavior in ?finitely-repeated experiments.Prisoner?s Dilemma; Inequity aversion; Incomplete Information; Signaling

Quantacell: Powerful charging of quantum batteries

We study the problem of charging a quantum battery in finite time. We demonstrate an analytical optimal protocol for the case of a single qubit. Extending this analysis to an array of N qubits, we demonstrate that an N-fold advantage in power per qubit can be achieved when global operations are permitted. The exemplary analytic argument for this quantum advantage in the charging power is backed up by numerical analysis using optimal control techniques. It is demonstrated that the quantum advantage for power holds when, with cyclic operation in mind, initial and final states are required to be separable.Comment: 11 pages, 3 figures, comments welcom