Experimental & Theoretical Aspects of the Electroweak Sector

The electroweak sector of the Standard Model (SM) has been extremely successful in predicting and matching observations. The basic form of it was sketched out some fifty years ago with the elucidation of the Higgs mechanism in a non-Abelian Yang-Mills gauge theory, yet the existence of a central player in the story, the (or a) Higgs boson, was confirmed only in 2012. In the intervening years, a great deal of experimental research was done to measure parameters of the model and confirm other predictions. In this sense, it has been an extremely fruitful theory in addition to being robust. But questions regarding the origin of the values of certain parameters in the theory, and especially regarding obvious but unexplained hierarchies between them, beg to be answered. The question of the technical naturalness of the Higgs mass has been one of the most significant motivating factors behind theories of beyond-the-Standard-Model (BSM) physics, though other striking features (for instance, the large discrepancy between quark masses) have also motivated theories (for instance, 2-Higgs-doublet models and models with Yukawa unification). Thus the electroweak sector has also proven fruitful for BSM theorists. The present paper may be divided into two parts: a description and characterization of the electroweak sector as it exists in the Standard Model on the one hand (a SM part), and an exploration of what may lie beyond it on the other (a BSM part). In the SM part, we first review the conceptual development of the electroweak model of Glashow, Weinberg, and Salam (touching on Yang-Mills theory and the Higgs mechanism), then present the key phenomenology of the electroweak theory. This leads into a presentation of this author's work in studying nal-state radiation (FSR) uncertainties in a measurement of sin2 W, with W being the weak mixing angle, done by the Compact Muon Solenoid (CMS) group at the Large Hadron Collider (LHC) in 2011. The framework necessary to understand the analysis is laid out in the text but this author played only the small role described in the section on FSR. The full analysis was presented in the papers by N. Tran and the CMS Collaboration, referenced in the text. The BSM part begins with an interlude that includes a review one of the most discussed puzzles of the SM and a discussion of "naturalness." We then present some of the basics of supersymmetry, including its history, the SUSY algebra, and the MSSM. SUSY is probably the leading contender for an explanation of seemingly "unnatural" parameters. In the next chapter, we present a supersymmetric model in which a new generation of \vector-like" quarks (as opposed to chiral) mixes with the third generation. Such a mixing raises the value of the top Yukawa yt necessary to give a top quark of the observed mass, mt = 173 GeV. Since the one-loop quantum corrections to the Higgs mass scale as yt to the fourth power , even a minor increase in yt can have a large effect. With current experimental bounds, yt may increase by as much as 6%, which implies the top's contribution to the Higgs mass increases by up to 26%. The model preserves gauge unification and gives a Higgs mass mh ~ 125.5 GeV without requiring soft supersymmetry-breaking masses above 1 TeV while satisfying all experimental constraints and predicting new quarks around the TeV scale, discoverable at the LHC. We conclude with a summary of the model and remarks on future prospects

Wind signatures in the X-ray emission line profiles of the late O supergiant ζ\zeta Orionis

X-ray line profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. In this paper we apply several analysis techniques to the emission lines in the Chandra HETGS spectrum of the late-O supergiant zeta Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blue shifts and profile asymmetry, as well as broadening in the X-ray emission lines of zeta Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for zeta Ori are very similar to those for the earlier O star, zeta Pup, which we have previously shown to be well-fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in zeta Ori, as compared to zeta Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both zeta Ori and zeta Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from zeta Ori can be understood within the framework of the standard wind-shock scenario for hot stars.Comment: MNRAS, accepted; 16 pages, 5 figure

Experimental & Theoretical Aspects of the Electroweak Sector

The electroweak sector of the Standard Model (SM) has been extremely successful in predicting and matching observations. The basic form of it was sketched out some fifty years ago with the elucidation of the Higgs mechanism in a non-Abelian Yang-Mills gauge theory, yet the existence of a central player in the story, the (or a) Higgs boson, was confirmed only in 2012. In the intervening years, a great deal of experimental research was done to measure parameters of the model and confirm other predictions. In this sense, it has been an extremely fruitful theory in addition to being robust. But questions regarding the origin of the values of certain parameters in the theory, and especially regarding obvious but unexplained hierarchies between them, beg to be answered. The question of the technical naturalness of the Higgs mass has been one of the most significant motivating factors behind theories of beyond-the-Standard-Model (BSM) physics, though other striking features (for instance, the large discrepancy between quark masses) have also motivated theories (for instance, 2-Higgs-doublet models and models with Yukawa unification). Thus the electroweak sector has also proven fruitful for BSM theorists. The present paper may be divided into two parts: a description and characterization of the electroweak sector as it exists in the Standard Model on the one hand (a SM part), and an exploration of what may lie beyond it on the other (a BSM part). In the SM part, we first review the conceptual development of the electroweak model of Glashow, Weinberg, and Salam (touching on Yang-Mills theory and the Higgs mechanism), then present the key phenomenology of the electroweak theory. This leads into a presentation of this author's work in studying nal-state radiation (FSR) uncertainties in a measurement of sin2 W, with W being the weak mixing angle, done by the Compact Muon Solenoid (CMS) group at the Large Hadron Collider (LHC) in 2011. The framework necessary to understand the analysis is laid out in the text but this author played only the small role described in the section on FSR. The full analysis was presented in the papers by N. Tran and the CMS Collaboration, referenced in the text. The BSM part begins with an interlude that includes a review one of the most discussed puzzles of the SM and a discussion of "naturalness." We then present some of the basics of supersymmetry, including its history, the SUSY algebra, and the MSSM. SUSY is probably the leading contender for an explanation of seemingly "unnatural" parameters. In the next chapter, we present a supersymmetric model in which a new generation of \vector-like" quarks (as opposed to chiral) mixes with the third generation. Such a mixing raises the value of the top Yukawa yt necessary to give a top quark of the observed mass, mt = 173 GeV. Since the one-loop quantum corrections to the Higgs mass scale as yt to the fourth power , even a minor increase in yt can have a large effect. With current experimental bounds, yt may increase by as much as 6%, which implies the top's contribution to the Higgs mass increases by up to 26%. The model preserves gauge unification and gives a Higgs mass mh ~ 125.5 GeV without requiring soft supersymmetry-breaking masses above 1 TeV while satisfying all experimental constraints and predicting new quarks around the TeV scale, discoverable at the LHC. We conclude with a summary of the model and remarks on future prospects