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Tufts Institute of Cosmology			Research Interests

			*-Ultra-High Energy Cosmic Rays* The origin of the highest energy cosmic rays (Particles with energies above E>10^20 eV) remains a puzzle, more than thirty years since the first observation in 1962. This puzzle seems to have two different parts. The first one has to do with the mechanism necessary to accelerate this particles.Conventional astrophysical sites have very serious difficulties to accommodate this energies within the usual Fermi acceleration mechanisms. This fact has motivated the search for non-acceleration models, in which the high energy cosmic rays are produced by the decay of a very heavy particle. Topological defects are attractive candidates for this scenario.The second ingredient in the puzzle is the so-called GZK (Greisen-Zapsepin-Kuzmin) effect, which states that particles above 410^19 eV should not be able to propagate on the 3 K cosmic microwave background more than 50 Mpc. (The original papers treated the case of a proton, but since then it has been studied for other potential candidates, photons, heavy nuclei, and in this cases the effect is more important, imposing even stronger limits on the distance from which this particles can come from.) -Topological Defects* Topological defects are stable solitonic configurations of the fields which may have been left behind by a cosmological phase transition in the early universe. Depending on the particle physics model they could be pointlike objects (monopoles), string like (cosmic strings), or sheets (domain walls). Typically they are associated with a Grand Unified Theory, GUT, at a very high energy scale 10^16 GeV. Due to their topological stability these objects can retain their energy for very long times and release quanta of their constituents, typically with GUT scale masses, which in turn decay to produce the observed particles. As part of my research work, I studied the viability of several scenarios for the origin of the ultra-high energy cosmic rays connected with various topological defect models. This is a brief explanation. *-Monopole-Antimonopole pairs.* In this work we proposed a scenario in which monopoles and antimonopoles are connected by strings formed at a low energy phase transition (~ 100 GeV). The bound states decay by gravitational radiation, with lifetimes comparable with the age of the universe. One of the important signatures of this model is that these pairs of monopoles will behave as cold dark matter, clustering on the galactic halo, and therefore avoiding the GZK problem. On the other hand this mechanism avoids the problems of the standard monopolonium scenario, since the binding of monopoles and antimonopoles is perfectly efficient, and therefore a very low density of monopoles is sufficient to explain the UHECR by their final annihilation. *-Ordinary Cosmic Strings Cusps.* It has been recognized for quite a long time that the motion of relativistic strings can be studied within the Nambu-Goto approximation. In this approximation the equations of motion are easily solved in terms of solutions of the 2-d wave equation with a couple of constraints that have to do with our gauge conditions on the worldsheet of the string. On the other hand this motion also predicts the existence of points where the string would turn back on itself and would move at the speed of light. This points are called cusps. This is clearly a point where the infinitely thin Nambu approximation breaks down. These events have the potential to release some energy as the fields rearrange themselves around the cusp. In order to study their viability as a source of UHECR, we performed a field theory simulation of cusps in the Abelian-Higgs model. The results are in accord with the theory that the portion of the strings which overlaps near the cusp is released as radiation. Unfortunately even though the energy of the individual particles emitted by these events is much higher than the energies observed, the total energy released by a network of cosmic strings is not sufficient to explain the flux of UHECR. Massive Radiation from Cosmic Strings. Recently there has been a claim that a cosmic string network would lose energy primarily by radiation of massive particles, and that this effect could be seen even in radiation from a standing wave. This, of course, would alter the evolution of a cosmic string network so much that it could in principle rule out a set of cosmic string models by an overproduction of high energy backgrounds which are not observed in experiments. In collaboration with Ken D. Olum, I simulated such standing waves, but we found that the radiation rate from the standing waves drops exponentially with increasing wavelength. Thus we concluded that direct radiation of particles from string length cannot play a significant role in the production of high energy cosmic rays or the maintenance of a scaling network. -Superconducting Cosmic Strings. It is now well known that there is a wide class of particle physics models where cosmic strings would behave as superconducting wires. This internal degree of freedom opens up a variety of interesting effects. In particular, superconducting strings may have stable configurations, vortons and springs, which could contribute to the dark matter in the universe, or put constraints on the particle physics models that give rise to this type of strings. On the other hand these models constitute an obvious candidate for high energy particle phenomena in the universe, and they have been studied in relation to ultra-high energy cosmic rays and gamma ray bursts. In order to study their possible observational consequences, we need to understand their evolution, which unfortunately is not as simple as in ordinary cosmic string models. In this respect, and in collaboration with Ken D. Olum and Alexander Vilenkin, I have studied the dynamics of chiral cosmic string models. It turns out that with the appropiate gauge choice, the chiral neutral cosmic string evolution can be solved exactly, and important physical consequences extracted from it. We used this result to study the electromagnetic radiation emitted on a chiral cosmic string cusp, recently suggested as a possible engine for the gamma ray burst events. Also, in collaboration with Ken D. Olum and Xavier Siemens, I studied the ejection of vorton loops from the cusp-like regions. This could turn out to be an important mechanism of vorton formation, and therefore severely constrain superconducting string scenarios. *Dark Matter* In the last few of years, a new candidate for cold dark matter has emerged, the superheavy dark matter. These particles could have been created by a non-thermal mechanism in the early universe, and therefore their interactions and masses are not so severely constrained as in the traditional thermal picture. Particularly interesting is the idea that their decay in the galactic halo could explain the UHECR and avoid the GZK problem. I am interested in these models not only for their role in the high-energy cosmic ray puzzle, but for their potential signatures in other related fields of cosmology, such as the early universe physics, neutrino astronomy, etc. There are other ideas that link dark matter to the UHECR, in this case, to the hot dark matter content. The idea is that ultra-high energy neutrinos could act as messengers from a cosmological distance and interact with the relic neutrinos clustered in the halo of our galaxy or even at larger scales, but within the GZK sphere, and contribute to the high energy end of the cosmic ray spectrum. Although this is an interesting idea, it seems to be severely constrained due to the non-observation of horizontal neutrino showers in the atmosphere. There have been a number of different variations to this model. In this respect, it is important to have a reliable calculation of the density enhancement of the neutrino background at different scales, as well as a viable candidate for the neutrino source.

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