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Topics in Theoretical Particle Physics and Cosmology Thesis by Michael P. Salem In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 2007 (Defended May 21, 2007) ii c 2007 Michael P. Salem All Rights Reserved iii Acknowledgements Much of this research would not have occurred without the hard work and insight of my collaborators. For this I thank Christian Bauer, Matthew Dorsten, Michael Graesser, Lawrence Hall, and Taizan Watari. I have been fortunate to have collaborators who were all willing to patiently explain ideas within and beyond the scope of our research. For this I am especially grateful to Christian, Michael, and Taizan, who have greatly increased my understanding of theoretical physics. I am pleased to acknowledge my graduate adviser Mark Wise, who has provided invaluable guidance and support throughout my graduate studies. I have benefited much from his suggestions and his pedagogy, while his humor and congeniality helped make graduate school a very enjoyable experience. For his guidance and support I am also grateful to my undergraduate adviser Tanmay Vachaspati. His clever ideas initiated a successful collaboration on my undergraduate thesis, which set the stage for my subsequent progress. I am grateful that both of my advisers have always received me warmly and respectfully. I have learned much from the other members of Mark's research group. For this I thank Lotty Ackerman, Moira Gresham, Alejandro Jenkins, Jennifer Kile, Sonny Mantry, Donal O'Connell, Sean Tulin, and Peng Wang. Lotty, Moira, and Donal have been especially patient and helpful in answering my questions. I have also benefited from interactions with Michael Ramsey-Musolf, Marc Kamionkowski, and members of Marc's research group, in particular Jonathan Pritchard and Kris Sigurdson. All of these people helped to create a pleasant and productive working environment. For this I am also grateful to my previous and present officemates, in particular Paul Cook and Jie Wang and especially Moira Gresham. Through most of my graduate studies I have been financially supported primarily by a Caltech teaching assistantship. I am thankful to David Goodstein and especially David Politzer and my Ph1 students for making this responsibility an enjoyable learning experience. I have also received financial support via the Caltech department of physics, the John A. McCone Chair, and the U.S. Department of Energy under contract number DE-FG03-92ER40701. I am grateful to Carol Silberstein and Charlene Cartier for their friendliness and their extensive administrative support. My loving wife Sara has provided encouragement, emotional support, and patient understanding of the demands of my graduate work. Meanwhile her thoughtful and careful planning for our time iv together has made my life and my work all the more rewarding. For these, and everything else Sara gives, I am endlessly grateful. I am also thankful for my siblings, Jeff, Joe, Karen, and Sherry, their spouses Jason and Mark, and my parents Frank and Marie. Their constant praise has encouraged me throughout my studies. More importantly, they contributed most to make me who I am. Finally, I am thankful for the countless researchers who have preceded me. Their elucidation has increased the beauty of the world for me, and without their progress none of this work would have been possible. I appreciate that the knowledge, opportunity, and abundance that makes my life so wonderful was built on the cumulative work of generations of these and other people, who persisted through experiences far more difficult than mine, to create a better world. I can only hope to express my gratitude through my own contributions toward a more knowledgeable, just, and prosperous future for humanity. v Abstract We first delve into particle phenomenology with a study of soft-collinear effective theory (SCET), an effective theory for Quantum Chromodynamics for when all particles are approximately on their light-cones. In particular, we study the matching of SCETI involving ultrasoft and collinear particles onto SCETII involving soft and collinear particles. We show that the modes in SCETII are sufficient to reproduce all of the infrared divergences of SCETI , a result that was previously in contention. Next we move into early universe cosmology and study alternative mechanisms for generating primordial density perturbations. We study the inhomogeneous reheating mechanism and extend it to describe the scenario where the freeze-out process for a heavy particle is modulated by sub- dominant fields that received fluctuations during inflation. This scenario results in perturbations that are comparable to those generated by the original inhomogeneous reheating scenarios. In addition, we study yet another alternative to single field inflation whereby the curvature perturbation is generated by interactions at the end of inflation, as opposed to when inflaton modes exit the horizon. We clarify the circumstances under which this process can dominate over the standard one and we show that it may result in a spectrum with an observable level of non-Gaussianities. We then turn to studies of the landscape paradigm, which hypothesizes that the observed universe is just one among a multitude of possibilities that are realized in separate causal regions. Such a landscape has been used to explain the smallness of the cosmological constant, at least when only it scans across the landscape. We study the scenario where both the cosmological constant and the strength of gravity, parameterized by the effective Planck mass, scan across the landscape. We find that selection effects acting on the cosmological constant are significantly weaker in this scenario and we find the measured value of the Planck mass to be exponentially unlikely under certain plausible assumptions about the landscape. Finally, we study some other models of the landscape as part of a possible explanation for quark-sector flavor parameters in the Standard Model. In this picture quark Yukawa couplings result from overlap integrals involving quark and Higgs wavefunctions in compactified extra dimensions, and the values we measure result from random selection from a landscape of possibilities. We find that many of the salient features of the measured flavor parameters are typical of the landscape distribution. vi Contents Acknowledgements iii Abstract v 1 Introduction 1 1.1 Soft-Collinear Effective Theory . 1 1.2 Alternative Mechanisms to Generate Primordial Density Perturbations . 5 1.3 Low-Energy Consequences of a Landscape . 12 2 Infrared Regulators and SCETII 17 2.1 Introduction . 17 2.2 Matching from SCETI onto SCETII . 19 2.3 Infrared Regulators in SCET . 23 2.3.1 Problems with Known IR Regulators . 23 2.3.2 A New Regulator for SCET . 25 2.4 Conclusions . 28 3 Fluctuating Annihilation Cross Sections and the Generation of Density Pertur- bations 30 3.1 Introduction . 30 3.2 Analytical Determination of the Perturbations . 32 3.3 Explicit Models for Coupling S to Radiation . 34 3.4 Models for Producing the Fluctuations . 36 3.5 Evolution of Density Perturbations . 39 3.6 Numerical Results . 41 3.7 Conclusions . 42 4 On the Generation of Density Perturbations at the End of Inflation 44 4.1 Introduction . 44 4.2 Background . 46 vii 4.3 The Specific Model . 47 4.4 A More Detailed Analysis . 48 4.5 Generalizing the Model . 52 4.5.1 Varying the Potential for φ . 52 4.5.2 Relaxing the Constraint on λσ . 53 4.6 Discussion and Conclusions . 55 5 The Scale of Gravity and the Cosmological Constant within a Landscape 57 5.1 Introduction . 57 5.2 Anthropic Constraints on the Scale of Gravity . 61 5.2.1 Inflation . 63 5.2.1.1 Satisfying Slow-Roll for N e-folds of Inflation . 63 5.2.1.2 The Curvature Perturbation ζ . 63 5.2.1.3 Reheating . 65 5.2.2 Baryogenesis . 65 5.2.3 Big Bang Nucleosynthesis . 67 5.2.4 Matter Domination . 68 5.2.5 Structure Formation . 70 5.2.5.1 Halo Virialization . 72 5.2.5.2 Galaxy Formation . 73 5.2.5.3 Star Formation . 75 5.2.6 Stellar Dynamics . 77 5.2.6.1 Stellar Lifetimes and Spectra . 78 5.2.6.2 Heavy Element Production . 81 5.2.7 Stability of Stellar Systems . 82 5.2.8 Summary . 85 5.3 The Probability Distribution for the Scale of Gravity . 88 5.4 Anthropic Constraints on Λ and the Scale of Gravity . 93 5.5 Nonstandard Paths toward Structure Formation . 99 5.6 Analysis of a Structure Formation Constraint . 101 5.7 Conclusions . 104 6 Quark Masses and Mixings from the Landscape 106 6.1 Introduction . 106 6.2 Prelude: Hierarchy without Flavor Symmetry . 109 6.2.1 The Distribution of Mass Eigenvalues . 110 6.2.2 Pairing Structure in Electroweak Interactions . 113 viii 6.2.3 Problems . 115 6.3 A Toy Landscape: Quarks in One Extra Dimension . 115 6.3.1 Emergence of Scale-Invariant Distributions . 116 6.3.2 Quark-Sector Phenomenology of the Gaussian Landscape . 120 6.3.3 Environmental Selection Effects . 124 6.3.4 Summary . 125 6.4 Testing Landscape Model Predictions . 127 6.4.1 The Chi-Square Statistic . 128 6.4.2 The P-Value Statistic . 129 6.5 Geometry Dependence . 131 6.5.1 A Gaussian Landscape on T 2 . 131 6.5.2 Changing the Number of Dimensions . 133 6.5.3 Information Not Captured by the Number of Dimensions . 137 6.6 Approximate Probability Distribution Functions . 140 6.6.1 Gaussian Landscape with One Extra Dimension . 140 6.6.2 Gaussian Landscapes on D = 2 and D = 3 using fD(y) in Eq. (6.61) . 141 6.7 Discussion and Conclusions . 143 Bibliography 146 ix List of Figures 2.1 Diagrams in SCETI that contribute to the matching .
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