Models of free quantum field theories on curved backgrounds

Free quantum field theories on curved backgrounds are discussed via three explicit examples: the real scalar field, the Dirac field and the Proca field. The first step consists of outlining the main properties of globally hyperbolic spacetimes, that is the class of manifolds on which the classical dynamics of all physically relevant free fields can be written in terms of a Cauchy problem. The set of all smooth solutions of the latter encompasses the dynamically allowed configurations which are used to identify via a suitable pairing a collection of classical observables. As a last step we use such collection to construct a $*$-algebra which encodes the information on the dynamics and on the canonical commutation or anti-commutation relations depending whether the underlying field is a Fermion or a Boson.Comment: 41 page

N=1 QED in 2+1 dimensions: Dualities and enhanced symmetries

We consider three-dimensional sQED with 2 flavors and minimal supersymmetry. We discuss various models which are dual to Gross-Neveu-Yukawa theories. The $U(2)$ ultraviolet global symmetry is often enhanced in the infrared, for instance to $O(4)$ or $SU(3)$. This is analogous to the conjectured behaviour of non-supersymmetric QED with 2 flavors. A perturbative analysis of the Gross-Neveu-Yukawa models in the $D = 4 - \varepsilon$ expansion shows that the $U(2)$ preserving superpotential deformations of the sQED (modulo tuning mass terms to zero) are irrelevant, so the fixed points with enhanced symmetry are stable. We also construct an example of $\mathcal{N} = 2$ sQED with 4 flavors that exhibits enhanced $SO(6)$ symmetry.Comment: 25 pages, no figure

Europe’s Electricity Supply Security: Strengthening the Chain. CEPS Policy Brief No. 224/November 2010

Acknowledging that efficient development of electricity transmission infrastructure is crucial to achieving EU targets for a secure, competitive and sustainable electricity supply, this paper explores ways of strengthening the supply chain. Research for the paper was carried out in the context of the SECURE project (Security of Energy Considering its Uncertainties, Risks and Economic Implications), funded by the European Commission under the Seventh Framework Programme. The project develops appropriate tools for evaluating the vulnerability of the EU to the different energy supply risks, and for promoting the optimisation of EU energy insecurity mitigation strategies, including investment, demand side management and dialogue with producing countries

Variable-speed rotor helicopters: Performance comparison between continuously variable and fixed-ratio transmissions

Variable speed rotor studies represent a promising research field for rotorcraft performance improvement and fuel consumption reduction. The problems related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. There are two main variable speed concepts, depending on the type of transmission employed: Fixed Ratio Transmission (FRT) and Continuously Variable Transmission (CVT) rotors. The impact of the two types of transmission upon overall helicopter performance is estimated when both are operating at their optimal speeds. This is done by using an optimization strategy able to find the optimal rotational speeds of main rotor and turboshaft engine for each flight condition. The process makes use of two different simulation tools: a turboshaft engine performance code and a helicopter trim simulation code for steady-state level flight. The first is a gas turbine performance simulator (TSHAFT) developed and validated at the University of Padova. The second is a simple tool used to evaluate the single blade forces and integrate them over the 360 degree-revolution of the main rotor, and thus to predict an average value of the power load required by the engine. The results show that the FRT does not present significant performance differences compared to the CVT for a wide range of advancing speeds. However, close to the two conditions of maximum interest, i.e. hover and cruise forward flight, the discrepancies between the two transmission types become relevant: in fact, engine performance is found to be penalized by FRT, stating that significant fuel reductions can be obtained only by employing the CVT concept. In conclusion, FRT is a good way to reduce fuel consumption at intermediate advancing speeds; CVT advantages become relevant only near hover and high speed cruise condition

Signatures of the neutrino thermal history in the spectrum of primordial gravitational waves

In this paper we study the effect of the anisotropic stress generated by neutrinos on the propagation of primordial cosmological gravitational waves. The presence of anisotropic stress, like the one generated by free-streaming neutrinos, partially absorbs the gravitational waves (GWs) propagating across the Universe. We find that in the standard case of three neutrino families, 22% of the intensity of the wave is absorbed, in fair agreement with previous studies. We have also calculated the maximum possible amount of damping, corresponding to the case of a flat Universe completely dominated by ultrarelativistic collisionless particles. In this case 43% of the intensity of the wave is absorbed. Finally, we have taken into account the effect of collisions, using a simple form for the collision term parameterized by the mean time between interactions, that allows to go smoothly from the case of a tigthly-coupled fluid to that of a collisionless gas. The dependence of the absorption on the neutrino energy density and on the effectiveness of the interactions opens the interesting possibility of observing spectral features related to particular events in the thermal history of the Universe, like neutrino decoupling and electron-positron annihilation, both occurring at T~1 MeV. GWs entering the horizon at that time will have today a frequency \nu\sim 10^{-9} \Hz, a region that is going to be probed by Pulsar Timing Arrays.Comment: V1: 14 pages, 2 figures. To appear in Gen. Rel. Grav. V2: References Adde