215 research outputs found
Production of Massive Fermions at Preheating and Leptogenesis
No full textWe present a complete computation of the inflaton decay into very massive fermions during preheating. We show that heavy fermions are produced very efficiently up to masses of order 10^{17}-10^{18} GeV; the accessible mass range is thus even broader than the one for heavy bosons. We apply our findings to the leptogenesis scenario, proposing a new version of it, in which the massive right-handed neutrinos, responsible for the generation of the baryon asymmetry, are produced during preheating. We also discuss other production mechanisms of right-handed neutrinos in the early Universe, identifying the neutrino mass parameters for which the observed baryon asymmetry is reproduced
Ultra High Energy Cosmic Rays and Inflation
No full textTwo processes of matter creation after inflation: 1) gravitational creation of superheavy (quasi)stable particles, and 2) non-thermal phase transitions leading to formation of topological defects, may be relevant to the resolution of the puzzle of cosmic rays observed with energies beyond GZK cut-off. Both possibilities are reviewed in this talk.Two processes of matter creation after inflation: 1) gravitational creation of superheavy (quasi)stable particles, and 2) non-thermal phase transitions leading to formation of topological defects, may be relevant to the resolution of the puzzle of cosmic rays observed with energies beyond GZK cut-off. Both possibilities are reviewed in this talk
Phenomenology of Axion Miniclusters
No full textI review possible observational phenomena appearing in models leading to dense small-scale substructures in the axionic dark matter. Also, I discuss their imaginable implications for the direct dark matter searches in the laboratory
Cosmology and Dark Matter
No full textThis lecture course covers cosmology from the particle physicist perspective. Therefore, the emphasis will be on the evidence for the new physics in cosmological and astrophysical data together with minimal theoretical frameworks needed to understand and appreciate the evidence. I review the case for non-baryonic dark matter and describe popular models which incorporate it. In parallel, the story of dark energy will be developed, which includes accelerated expansion of the Universe today, the Universe origin in the Big Bang, and support for the Inflationary theory in CMBR data.This lecture course covers cosmology from the particle physicist perspective. Therefore, the emphasis will be on the evidence for the new physics in cosmological and astrophysical data together with minimal theoretical frameworks needed to understand and appreciate the evidence. I review the case for non-baryonic dark matter and describe popular models which incorporate it. In parallel, the story of dark energy will be developed, which includes accelerated expansion of the Universe today, the Universe origin in the Big Bang, and support for the Inflationary theory in CMBR data
Bisimilar stochastic systems
No full textStochastic systems have been widely investigated and employed in numerousapplications in different areas such as finance, biology and engineering asthey allow accounting for imprecisions so often faced in every practical tasks. Often that task would require to find the best action sequence in order to optimize the outcome. When the model is small, one can efficiently employ algorithmic techniques to synthesize such a control policy. Hence, in case of more complex models, instead of solving control tasks there directly, one may want to approximate them with simpler ones and then use those algorithms. This method is called abstraction for it abstracts the original “physical” model to an “abstract” one, only needed to ease the computations. Ideally, this abstract model is somewhat similar to the original one, as we want to extrapolate results achieved over the former to the setting of the latter. One way this similarity can be ensured is by means of the (bi)simulation methods, that give sufficient conditions to the closeness of behaviors of the two systems being compared. Such techniques became popular in discrete non-stochastic models, then advanced to continuous ones and started making steps to discrete stochastic systems. Yet, definite results were not achieved for abstractions of continuous stochastic models. There were trials to extend ideas from continuous non-stochastic framework, or discrete stochastic one, but they were mostly fragmentary. This thesis brings those methods together to build a unified framework and shows immediate benefits of doing this.To define the closeness between the systems we look at their path-wise properties, which cover most of the tasks whose relevance was praised in the literature. That comprises both additive cost-like criteria and formal specifications, e.g. encoded by LTL formulae of the kind “reach the goal set through the safe set while avoiding dangerous states”. We derive guarantees on the approximation error and suggest how to build an abstraction for a given tolerance level. These guarantees work mostly for the finite time horizon properties, hence for the rest we develop task-dependent solution methods, further connecting with the existing literature. Besides those concrete results, we also put some effort in developing the conceptual side of the bisimulation framework for stochastic systems. For example, we know how important it is to choose a definition of behavior here, since bisimiliarity is useful as long as it guarantees closeness of behaviors one is interested in.We hence stress the importance of keeping in mind the final goal while extrapolating abstract solution methods, and show which issues may arise when this goal is forgotten. We also extend the framework we deal with beyond bisimulation of stochastic systems only, providing a formalization of approximate relations and their connections with pseudo-metrics, proving several theorems in probabilistic approximation, whose generality is greater than the scope of this thesis, and also provide a category-theoretical basis for bisimulations of stochastic systems, hence opening one more door from which this problem can be approached.Team Bart De Schutte
Is astronomy possible with neutral ultra-high energy cosmic ray particles existing in the Standard Model?
No full textThe recently observed correlation between HiRes stereo cosmic ray events with energies E ~ 10 EeV and BL Lacs occurs at an angle which strongly suggests that the primary particles are neutral. We analyze whether this correlation, if not a statistical fluctuation, can be explained within the Standard Model, i.e., assuming only known particles and interactions. We have not found a plausible process which can account for these correlations. The mechanism which comes closest -- the conversion of protons into neutrons in the IR background of our Galaxy -- still under-produces the required flux of neutral particles by about 2 orders of magnitude. The situation is different at E ~ 100 EeV where the flux of cosmic rays at Earth may contain up to a few percent of neutrons pointing back to the extragalactic sources.The recently observed correlation between HiRes stereo cosmic ray events with energies E ~ 10 EeV and BL Lacs occurs at an angle which strongly suggests that the primary particles are neutral. We analyze whether this correlation, if not a statistical fluctuation, can be explained within the Standard Model, i.e., assuming only known particles and interactions. We have not found a plausible process which can account for these correlations. The mechanism which comes closest -- the conversion of protons into neutrons in the IR background of our Galaxy -- still under-produces the required flux of neutral particles by about 2 orders of magnitude. The situation is different at E ~ 100 EeV where the flux of cosmic rays at Earth may contain up to a few percent of neutrons pointing back to the extragalactic sources
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