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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 19:55:54 +0000Applications of Effective Field Theory to Electron Scattering
https://resolver.caltech.edu/CaltechETD:etd-03082005-201156
Authors: Diaconescu, Luca Radu
Year: 2005
DOI: 10.7907/QJBW-X946
<p>In this work two calculations are presented. In the first, we compute the vector analyzing power (VAP) for the elastic scattering of transversely polarized electrons from protons at low energies, using an effective theory of electrons, protons, and photons. We study all contributions through second order in E/M, where E and M are the electron energy and nucleon mass, respectively. The leading order VAP arises from the imaginary part of the interference of one- and two-photon exchange amplitudes. Sub-leading contributions are generated by the nucleon magnetic moment and charge radius, as well as recoil corrections to the leading-order amplitude. Working to second order in E/M), we obtain a prediction for Aₙ that is free of unknown parameters and that agrees with the recent measurement of the VAP in backward angle electron proton scattering.</p>
<p>In the second part of this thesis the longitudinal asymmetry due to Z exchange is calculated in quasi-elastic electron-deuteron scattering at momentum transfers |Q²| of about 0.1 GeV² relevant for the SAMPLE experiment. The deuteron and $pn$ scattering-state wave functions are obtained from solutions of a Schrodinger equation with the Argonne v18 potential. Electromagnetic and weak neutral one- and two-nucleon currents are included in the calculation. The two-nucleon currents of pion range are shown to be identical to those derived in Effective Field Theory. The results indicate that two-body contributions to the asymmetry are small (about 0.2%) around the quasi-elastic peak, but become relatively more significant (about 3%) in the high-energy wing of the quasi-elastic peak.</p>https://thesis.library.caltech.edu/id/eprint/897Probing Physics and the Standard Model and Beyond with Electroweak Baryogenesis and Effective Theories of the Strong Interactions
https://resolver.caltech.edu/CaltechETD:etd-05272005-162229
Authors: Lee, Christopher
Year: 2005
DOI: 10.7907/GJPE-HN05
<p>We address in this thesis two primary questions aimed at improving our ability to calculate reliably in the Standard Model of particle physics and probing possible new particles which may exist beyond it.</p>
<p>First, we embark on an attempt to account for the abundance of matter in the present Universe if earlier in its history matter and antimatter were equally abundant. We explore whether baryogenesis at the electroweak phase transition could successfully account for the observed density of baryons in the Universe, using the closed-time-path (CTP) formalism of quantum field theory to calculate the buildup and relaxation of particle densities during the phase transition. For our model of the new particles and sources of CP violation necessary to account for the baryon asymmetry of the Universe, we adopt the Minimal Supersymmetric Extension of the Standard Model (MSSM). We look for regions of the parameter space in the MSSM that could give rise to sufficiently large baryon asymmetry without violating constraints on these parameters from existing experiments, in particular, constraints on masses of Higgs and supersymmetric particles from accelerator searches and precision electroweak tests, and on CP-violating parameters of the MSSM from searches for electric dipole moments of elementary particles.</p>
<p>Next, we explore how to get around our ignorance of the dynamics of strongly interacting particles in the nonperturbative regime of Quantum Chromodynamics (QCD) by the clever use of effective field theories. Two applications are explored: the decay of Z bosons to hadronic jets using soft-collinear effective theory (SCET) and the radiative decays of quarkonia to light hadrons using SCET and non-relativistic QCD (NRQCD). These tools facilitate the proof of factorization of decay rates into perturbatively-calculable and nonperturbative parts. Universality of the latter among different observables provides predictive power even in our ignorance of the details of the nonperturbative physics.</p>https://thesis.library.caltech.edu/id/eprint/2160Constraints on Physics Beyond the Standard Model and its Observable Effects
https://resolver.caltech.edu/CaltechETD:etd-05282007-034337
Authors: Kile, Jennifer E.
Year: 2007
DOI: 10.7907/KSBC-RD46
<p>In this work, we describe three analyses, all of which involve physics beyond the Standard Model. The first two discussed here are closely related; they use effective operator analyses to constrain the contributions of physics beyond the SM to observable processes. The third project involves the investigations of a particular extra-dimensions model which addresses the cosmological constant problem.</p>
<p>The first project which we will discuss uses the scale of neutrino mass to place model-independent constraints on the coefficients of the chirality-changing terms in the muon decay Lagrangian. We list all of the dimension-six effective operators which contribute to muon decay and Dirac mass for the neutrino. We then calculate the one-loop contributions that each of these operators makes to neutrino mass. Taking a generic element of the neutrino mass matrix to be of order ~ 1 eV, we derive limits on the contributions of these operators to the muon Michel parameters which are approximately four orders of magnitude more stringent than the current experimental results, and well below near-future experimental sensitivity. We also find two chirality-changing operators, which, due to their flavor structure, are unconstrained by neutrino mass yet contribute to muon decay. However, as these two operators differ from those constrained by neutrino mass only by their flavor indices, we naively expect their contributions to also be small; if their effects instead turn out to be observable, this may be an indication of beyond-the-Standard-Model physics with an interesting flavor structure.</p>
<p>In the second analysis, we again perform an effective operator analysis, this time applied to Higgs production at a linear collider. Here we include all dimension-six operators containing fermions which contribute to Higgs production. We again include operators that contain right-handed Dirac neutrinos. We obtain limits on these operators from electroweak precision observables, the scale of neutrino mass, and limits on neutrino magnetic moments, and use these limits to constrain the contributions of these operators to the Higgs production cross-section. Although we find that all operators containing right-handed neutrinos contribute negligibly to Higgs production, we do find three operators containing only SM fields which could have observable contributions at an e⁺e⁻ linear collider.</p>
<p>Lastly, we discuss the characteristics of a particular extra-dimensions model originally proposed by Carroll and Guica [54]. This model has two extra dimensions compactified into a sphere, a bulk magnetic field, and a bulk cosmological constant. In this model, the cosmological constant seen by a four-dimensional observer can be set to zero by fine-tuning the bulk magnetic field against the bulk cosmological constant. If branes with a tension are added at each of the poles of the two-sphere, solutions with zero four-dimensional cosmological constant are still possible, but the compactified dimensions must acquire a deficit angle which depends on the brane tension. However, the brane tension does not affect the fine-tuning relationship between the bulk cosmological constant and the bulk magnetic field. This feature led to the hope that, after this fine-tuning, the model might self-tune, keeping the four-dimensional cosmological constant zero regardless of what happens to the brane tension by adjusting the deficit angle. We speculated that this self-tuning property would imply a massless scalar mode in the perturbed Einstein's equations; as there exist very stringent limits on scalar-tensor theories of gravity, a massless scalar mode would make this model incompatible with observation. We conducted a search for such modes, and found none which satisfied the boundary conditions. This finding led us to speculate that this model does not, in fact, have a self-tuning property.</p>https://thesis.library.caltech.edu/id/eprint/2201Neutrina Mass Implications for Physics Beyond the Standard Model
https://resolver.caltech.edu/CaltechETD:etd-05312007-144931
Authors: Wang, Peng
Year: 2007
DOI: 10.7907/4AGY-DF85
We begin by working out an effective field theory valid below some new physics scale for Dirac neutrinos and Majorana neutrinos, respectively. For Dirac neutrinos, we obtain a complete basis of effective dimension four and dimension six operators that are invariant under the gauge symmetry of the Standard Model. As for Majorana neutrinos, we come up with a complete basis of effective dimension five and dimension seven operators that are invariant under the gauge symmetry of the Standard Model. Using the effective theory, we derive model-independent, "naturalness" upper bounds on the magnetic moments of Dirac neutrinos and Majorana neutrinos generated by physics above the scale of electroweak symmetry breaking. In the absence of fine-tuning of effective operator coefficients, for Dirac neutrinos, we find that current information on neutrino mass implies that the bound on neutrino magnetic moments is several orders of magnitude stronger than those obtained from analyses of solar and reactor neutrino data and astrophysical observations. As for Majorana neutrinos, the magnetic moment contribution to the mass is Yukawa suppressed. The bounds we derive for magnetic moments of Majorana neutrinos are weaker than present experimental limits if neutrino magnetic moments are generated by new physics at around 1 TeV, and surpass current experimental sensitivity only for new physics scales >10-100 TeV. The discovery of a neutrino magnetic moment near present limits would thus signify that neutrinos are Majorana particles. Then, we use the scale of neutrino mass to derive model-independent naturalness constraints on possible contributions to muon decay Michel parameters. We show that -- in the absence of fine-tuning -- the most stringent bounds on chirality-changing operators relevant to muon decay arise from one-loop contributions to neutrino mass. The bounds we obtain on their contributions to the Michel parameters are four or more orders of magnitude stronger than bounds previously obtained in the literature. We also show that, if neutrinos are Dirac fermions, there exist chirality-changing operators that contribute to muon decay but whose flavor structure allows them to evade neutrino mass naturalness bounds. We discuss the implications of our analysis for the interpretation of muon decay experiments. Finally, we use the upper limit on the neutrino mass to derive model-independent naturalness constraints on some non-Standard-Model interactions of beta decays. In the absence of fine-tuning of effective operator coefficients, our results yield constraints on scalar and tensor weak interactions one or more orders of magnitude stronger than a recent global fit after combined with the current experimental limits. We also show that, if neutrinos are Majorana fermions, there exist four-fermion operators that contribute to beta decay but whose flavor structure allows them to evade neutrino mass naturalness bounds. Constraints on the beta decay parameters by CKM Unitarity, ratio of positive pion decays, and pion beta decays are discussed as well.https://thesis.library.caltech.edu/id/eprint/2342Neutrino Mass Constraints on Electroweak Parameters
https://resolver.caltech.edu/CaltechETD:etd-05252007-084814
Authors: Erwin, Rebecca Joan
Year: 2007
DOI: 10.7907/E3FG-2H43
<p>We use the scale of neutrino mass and naturalness considerations to obtain model-independent expectations for the magnitude of possible contributions to muon decay Michel parameters from new physics above the electroweak symmetry-breaking scale. Focusing on Dirac neutrinos, we obtain a complete basis of effective dimension four and dimension six operators that are invariant under the gauge symmetry of the Standard Model and that contribute to both muon decay and neutrino mass. We show that - in the absence of fine tuning - the most stringent neutrino mass naturalness bounds on chirality-changing vector operators relevant to muon decay arise from one-loop operator mixing. The bounds we obtain on their contributions to the Michel parameters are two orders of magnitude stronger than bounds previously obtained in the literature. In addition, we analyze the implications of one-loop matching considerations and find that the expectations for the size of various scalar and tensor contributions to the Michel parameters are considerably smaller than those derived from previous estimates of two-loop operator mixing. We also show, however, that there exist gauge-invariant operators that generate scalar and tensor contributions to muon decay but whose flavor structure allows them to evade neutrino mass naturalness bounds. We discuss the implications of our analysis for the interpretation of muon decay experiments.</p>
<p>We then repeat the analysis with Majorana neutrinos. Since the lowest dimension mass operator in this case is a five-dimensional operator, we start with a new basis of effective dimension five and dimension seven operators that contribute to muon decay and neutrino mass through radiative corrections. In contrast to similar studies of magnetic moments and masses using Dirac and Majorana neutrinos, which found substantially weaker bounds for Majorana magnetic moments, we find that the limits on muon decay Michel parameters from Majorana neutrinos are similar in magnitude to the limits from Dirac neutrinos. We also find, similar to the Dirac case, that there are operators in our basis whose coefficients are not bound by neutrino mass.</p>
<p>Finally, we calculate one-loop renormalization factors of twist-two operators in massless QCD with domain-wall fermions. The Shamir type domain-wall fermion, with an infinitely large extra dimension to describe the massless fermion, is used.</p>https://thesis.library.caltech.edu/id/eprint/2068