Quantum theory of SASE FEL

We describe a free-electron laser (FEL) in the Self-Amplified Spontaneous Emission (SASE) regime quantizing the electron motion and taking into account propagation effects. We demonstrate quantum purification of the SASE spectrum, i.e. in a properly defined quantum regime the spiking behavior disappears and the SASE power spectrum becomes very narrow

Quantum effects in the collective light scattering by coherent atomic recoil in a Bose-Einstein condensate

We extend the semiclassical model of the collective atomic recoil laser (CARL) to include the quantum mechanical description of the center-of-mass motion of the atoms in a Bose-Einstein condensate (BEC). We show that when the average atomic momentum is less than the recoil momentum $\hbar\vec q$, the CARL equations reduce to the Maxwell-Bloch equations for two momentum levels. In the conservative regime (no radiation losses), the quantum model depends on a single collective parameter, $\rho$, that can be interpreted as the average number of photons scattered per atom in the classical limit. When $\rho\gg 1$, the semiclassical CARL regime is recovered, with many momentum levels populated at saturation. On the contrary, when $\rho\le 1$, the average momentum oscillates between zero and $\hbar\vec q$, and a periodic train of $2\pi$ hyperbolic secant pulses is emitted. In the dissipative regime (large radiation losses) and in a suitable quantum limit, a sequential superfluorescence scattering occurs, in which after each process atoms emit a $\pi$ hyperbolic secant pulse and populate a lower momentum state. These results describe the regular arrangement of the momentum pattern observed in recent experiments of superradiant Rayleigh scattering from a BEC.Comment: submitted for publication on Phys. Rev.

Superradiant light scattering and grating formation in cold atomic vapours

A semi-classical theory of coherent light scattering from an elongated sample of cold atoms exposed to an off-resonant laser beam is presented. The model, which is a direct extension of that of the collective atomic recoil laser (CARL), describes the emission of two superradiant pulses along the sample's major axis simultaneous with the formation of a bidimensional atomic grating inside the sample. It provides a simple physical picture of the recent observation of collective light scattering from a Bose-Einstein condensate [S. Inouye et al., Science N.285, p. 571 (1999)]. In addition, the model provides an analytical description of the temporal evolution of the scattered light intensity which shows good quantitative agreement with the experimental results of Inouye et al.Comment: submitted to Optics Communications, LaTex version, 2 postscript figure

Low metallicity stars in our Galaxy

The advent of m class telescopes has allowed the detailed spectroscopic study of sizeable numbers of extremely metal-poor Galactic stars which are the witnesses of the formation of the early Galaxy. Their chemical composition displays some distinctive trends which should provide a strong constraint on the physical nature of the first generation(s) of stars and on their nucleosynthetic output. I will review recent results in the field following the periodic table, from lithium to uranium and shortly comment on the intriguing classes of Carbon Enhanced Metal Poor (CEMP) stars, for many of which there is no analogue among solar metallicity stars. In spite of these exciting results, the number of known stars of metallicity below [Fe/H]=-3.3 remains quite small and it would be desirable to discover more, both to clearly understand the metal-weak tail of Halo metallicity distribution and to clarify the abundance trends at the lowest metallicities. Most of these extremely rare objects have been discovered by the wide field objective prism surveys, HK survey and Hamburg-ESO survey. In the near future the Sloan Digital Sky Survey and its continuation SEGUE are expected to boost significantly the numbers of known extremely metal poor stars. We are living exciting times but an even more exciting future lies ahead !Comment: To appear in the proceedings of the CRAL-Conference Series I "Chemodynamics: from first stars to local galaxies", Lyon 10-14 July 2006, France, Eds. Emsellem, Wozniak, Massacrier, Gonzalez, Devriendt, Champavert, EAS Publications Serie

Abundances in Sagittarius: present state and perspectives for the use of VLT

Sagittarius, the nearest external galaxy, will be amenable to detailed abundance studies with VLT and other 8m class telescopes. Such data, in conjuction with the similar data of our own galaxy, will allow a deeper understanding of chemical evolution. Our study conducted with NTT shows the presence of metal-rich stars with radial velocities compatible with Sagittarius membership and of stars as metal-poor as [Fe/H]=-1.5. In this talk I shall address the way in which VLT instruments will allow to clarify this intricate situation.Comment: Invited talk at the ESO VLT Opening Symposium, Antofagasta, March 1-4 199

Time as a statistical variable and intrinsic decoherence

We propose a novel approach to intrinsic decoherence without adding new assumptions to standard quantum mechanics. We generalize the Liouville equation just by requiring the dynamical semigroup property of time evolution and dropping the unitarity requirement. With no approximations and statistical assumptions we find a generalized Liouville equation which depends on two characteristic time t1 and t2 and reduces to the usual equation in the limit t1 = t2 -> 0. However, for t1 and t2 arbitrarily small but finite, our equation can be written as a finite difference equation which predicts state reduction to the diagonal form in the energy representation. The rate of decoherence becomes faster at the macroscopic limit as the energy scale of the system increases. In our approach the evolution time appears, a posteriori, as a statistical variable with a Poisson-gamma function probability distribution as if time evolution would take place randomly at average intervals t2 each evolution having a time width t1. This view point is supported by the derivation of a generalized Tam Mandelstam inequality. The relation with previous work by Milburn, with laser and micromaser theory and many experimental testable examples are described.Comment: 24 pages, e-mail: [email protected]. revised versio

On the lithium content of the globular cluster M92

I use literature data and a new temperature calibration to determine the Li abundances in the globular cluster M 92. Based on the same data, Boesgaard et al. have claimed that there is a dispersion in Li abundances in excess of observational errors. This result has been brought as evidence for Li depletion in metal-poor dwarfs. In the present note I argue that there is no strong evidence for intrinsic dispersion in Li abundances, although a dispersion as large as 0.18 dex is possible. The mean Li abundance, A(Li)=2.36, is in good agreement with recent results for field stars and TO stars in the metal-poor globular cluster NGC 6397. The simplest interpretation is that this constant value represents the primordial Li abundance.Comment: A&A accepte

Using interval weights in MADM problems

The choice of weights vectors in multiple attribute decision making (MADM) problems has generated an important literature, and a large number of methods have been proposed for this task. In some situations the decision maker (DM) may not be willing or able to provide exact values of the weights, but this difficulty can be avoided by allowing the DM to give some variability in the weights. In this paper we propose a model where the weights are not fixed, but can take any value from certain intervals, so the score of each alternative is the maximum value that the weighted mean can reach when the weights belong to those intervals. We provide a closed-form expression for the scores achieved by the alternatives so that they can be ranked them without solving the proposed model, and apply this new method to an MADM problem taken from the literature.Este trabajo forma parte del proyecto de investigación: MEC-FEDER Grant ECO2016-77900-P

On the origin of HE0107-5240, the most iron deficient star presently known

We show that the "puzzling" chemical composition observed in the extremely metal poor star HE0107-5240 may be naturally explained by the concurrent pollution of at least two supernovae. In the simplest possible model a supernova of quite low mass (~15 Msun), underwent a "normal" explosion and ejected ~0.06 Msun of 56Ni while a second one was massive enough (~35 Msun) to experience a strong fall back that locked in a compact remnant all the carbon-oxygen core. In a more general scenario, the pristine gas clouds were polluted by one or more supernovae of relatively low mass (less than ~25 Msun). The successive explosion of a quite massive star experiencing an extended fall back would have largely raised the abundances of the light elements in its close neighborhood.Comment: 10 pages; 3 figures; accepted for publication in the The Astrophysical Journal Letter

Quantum SASE FEL with laser wiggler

In this letter we specify the physical parameters necessary to operate a SASE FEL in the quantum regime with a laser wiggler. We also show that this is more feasible in the quantum regime than in the classical one. Specific examples are given.Comment: to appear on NIM-