Topological defects and ultra-high energy cosmic rays

Laburpena

The observation of ultra-high-energy cosmic rays (UHECR) with energies above 1011 GeV poses a serious challenge to the particle acceleration mechanisms so far proposed. 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. 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 UHECR. In the present thesis we study in detail several scenarios involving different topological defects. The first part of the thesis is devoted to ordinary cosmic string scenarios, in particular, cusp evaporation and direct particle emission. We analyze these models by using numerical simulations and conclude that they are not able to account for the flux of ultra-high energy cosmic ray observed. We then consider superconducting string models, especially in connection with cusp-like regions with chiral (null) currents. We calculate the electromagnetic radiation emitted from these regions and the formation of stable loops by self-intersection of the string near the cusp. This process could have interesting consequences for cosmic ray production. Finally, we present a different scenario in which very massive monopoles (m & sim; 1014 GeV) are bound by a light string formed at approximately 300 GeV. These monopoles do not have the usual magnetic charge, or in fact any unconfined flux. Gravitational radiation is the only significant energy-loss mechanism for the bound systems. Their lifetimes can then be comparable with the age of the universe, and their final annihilation will then contribute to the high energy end of the cosmic ray spectrum. The observation of ultra-high-energy cosmic rays (UHECR) with energies above 1011 GeV poses a serious challenge to the particle acceleration mechanisms so far proposed. 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. 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 UHECR. In the present thesis we study in detail several scenarios involving different topological defects. The first part of the thesis is devoted to ordinary cosmic string scenarios, in particular, cusp evaporation and direct particle emission. We analyze these models by using numerical simulations and conclude that they are not able to account for the flux of ultra-high energy cosmic ray observed. We then consider superconducting string models, especially in connection with cusp-like regions with chiral (null) currents. We calculate the electromagnetic radiation emitted from these regions and the formation of stable loops by self-intersection of the string near the cusp. This process could have interesting consequences for cosmic ray production. Finally, we present a different scenario in which very massive monopoles (m ∼ 1014 GeV) are bound by a light string formed at approximately 300 GeV. These monopoles do not have the usual magnetic charge, or in fact any unconfined flux. Gravitational radiation is the only significant energy-loss mechanism for the bound systems. Their lifetimes can then be comparable with the age of the universe, and their final annihilation will then contribute to the high energy end of the cosmic ray spectrum.