Showing papers on "Electroweak interaction published in 1994"

Quantum Fields on a Lattice

Abstract: This book presents a comprehensive and coherent account of the theory of quantum fields on a lattice, an essential technique for the study of the strong and electroweak nuclear interactions. Quantum field theory describes basic physical phenomena over an extremely wide range of length or energy scales. Quantum fields exist in space and time, which can be approximated by a set of lattice points. This approximation allows the application of powerful analytical and numerical techniques, and has provided a powerful tool for the study of both the strong and the electroweak interaction. After introductory chapters on scalar fields, gauge fields and fermion fields, the book studies quarks and gluons in QCD and fermions and bosons in the electroweak theory. The last chapter is devoted to numerical simulation algorithms which have been used in recent large-scale numerical simulations. The book will be valuable for graduate students and researchers in theoretical physics, elementary particle physics, and field theory, interested in non-perturbative approximations and numerical simulations of quantum field phenomena.

Dynamics of the Standard Model

Abstract: Preface Inputs to the standard model Interactions of the standard model Symmetries and anomalies Introduction to effective Lagrangians Leptons Very low energy QCD - Pions and photons Introducing kaons and etas Kaons and the S=1 interaction Kaon mixing and CP violation The large N expansion Phenomenological models Baryon properties Hadron spectroscopy Weak interactions of heavy quarks The Higgs boson The electroweak gauge bosons Appendices References Index.

Electroweak Symmetry Breaking and Bottom-Top Yukawa Unification

Abstract: The condition of unification of gauge couplings in the minimal supersymmetric standard model provides successful predictions for the weak mixing angle as a function of the strong gauge coupling and the supersymmetric threshold scale. In addition, in some scenarios, e.g.\ in the minimal SO(10) model, the tau lepton and the bottom and top quark Yukawa couplings unify at the grand unification scale. The condition of Yukawa unification leads naturally to large values of $\tan\beta$, implying a proper top quark--bottom quark mass hierarchy. In this work, we investigate the feasibility of unification of the Yukawa couplings, in the framework of the minimal supersymmetric standard model with (assumed) universal mass parameters at the unification scale and with radiative breaking of the electroweak symmetry. We show that strong correlations between the parameters $\mu_0$ and $M_{1/2}$ appear within this scheme. These correlations have relevant implications for the sparticle spectrum, which presents several characteristic features. In addition, we show that due to large corrections to the running bottom quark mass induced through the supersymmetry breaking sector of the theory, the predicted top quark mass and $\tan\beta$ values are significantly lower than those previously estimated in the literature.

Standard Model CP-violation and Baryon asymmetry

Abstract: In CP arguments, we argue against a Standard Model explanation of the baryon asymmetry of the universe in the presence of a first order phase transition. A CP-asymmetry is found in the reflection coefficients of quarks hitting the phase boundary created during the electroweak transition. The problem is analyzed both in an academic zero temperature case and in the realistic finite temperature one. The building blocks are similar in both cases: Kobayashi-Maskawa CP-violation, CP-even phases in the reflection coefficients of quarks, and physical transitions due to fermion self-energies. In both cases an effect is present at order $\alpha_W^2 $ in rate. A regular GIM behavior is found as intuitively expected. In the finite temperature case, a crucial role is played by the damping rate of quasiparticles in a hot plasma, which is a relevant scale together with MW and the temperature. The effect is many orders of magnitude below what observation requires, and indicates that non-standard physics is indeed needed ...

Model independent constraints on leptoquarks from rare processes

Abstract: We present model independent constraints on the masses and couplings to fermions ofB andL conserving leptoquarks Such vector or scalar particles could have masses below 100 GeV and be produced at HERA; we list the generation dependent bounds that can be calculated from rare lepton and meson decays, meson-anti-meson mixing and various electroweak tests

Supersymmetric particle spectrum

Abstract: We examine the spectrum of supersymmetric particles predicted by grand unified theoretical (GUT) models where the electroweak symmetry breaking is accomplished radiatively. We evolve the soft-supersymmetry-breaking parameters according to the renormalization group equations (RGE). The minimization of the Higgs potential is conveniently described by means of tadpole diagrams. We present complete one-loop expressions for these minimization conditions, including contributions from the matter and the gauge sectors. We concentrate on the low tan $\ensuremath{\beta}$ fixed point region (that provides a natural explanation of a large top quark mass) for which we find solutions to the RGE satisfying both experimental bounds and fine-tuning criteria. We also find that the constraint from the consideration of the lightest supersymmetric particle as the dark matter of the Universe is accommodated in much of parameter space where the lightest neutralino is predominantly gaugino. The supersymmetric mass spectrum displays correlations that are model independent over much of the GUT parameter space.

Standard Model CP-violation and Baryon asymmetry Part II: Finite Temperature

Abstract: We consider the scattering of quasi-particles off the boundary created during a first order electroweak phase transition. Spatial coherence is lost due to the quasi-quark damping rate, and we show that reflection on the boundary is suppressed, even at tree-level. Simply on CP considerations, we argue against electroweak baryogenesis in the Standard Model via the charge transport mechanism. A CP asymmetry is produced in the reflection properties of quarks and antiquarks hitting the phase boundary. An effect is present at order $\alpha_W^2$ in rate and a regular GIM behaviour is found, which can be expressed in terms of two unitarity triangles. A crucial role is played by the damping rate of quasi-particles in a hot plasma, which is a relevant scale together with $M_W$ and the temperature. The effect is many orders of magnitude below what observation requires.

Beyond S, T, and U.

Abstract: The contribution to precision electroweak measurements due to TeV physics which couples primarily to the W ± and Z bosons may be parameterized in terms of the three ‘oblique correction’ parameters, S, T and U. We extend this parameterization to physics at much lower energies, > 100 GeV, and show that in this more general case neutral-current experiments are sensitive to only two additional parameters. A third new parameter enters into the W ± width. The standard electroweak theory has recently come of age, with experiments now probing its predictions with sufficient accuracy to test its r corrections in some detail. Besides providing a detailed test of the model, these precision measurements are also very useful for the constraints they impose on any potential new physics that might exist at energies higher than those that have been hitherto experimentally explored. A particularly interesting class of new physics that is constrained by these measure

Application of the Background-Field Method to the electroweak Standard Model

Abstract: Application of the background-field method yields a gauge-invariant effective action for the electroweak Standard Model, from which simple QED-like Ward identities are derived. As a consequence of these Ward identities, the background-field Green functions are shown to possess very desirable theoretical properties. The renormalization of the Standard Model in the background-field formalism is studied. A consistent on-shell renormalization procedure retaining the full gauge symmetry is presented. The structure of the counterterms is shown to greatly simplify compared to the conventional formalism. A complete list of Feynman rules for the Standard Model in the background-field method is given for arbitrary values of a quantum gauge parameter including all counterterms necessary for one-loop calculations.

Analytical and numerical methods for massive two-loop self-energy diagrams

Abstract: Motivated by the precision results in the electroweak theory studies of two-loopFeynman diagrams are performed. Specifically this paper gives a contribution to the knowledge of massive two-loop self-energy diagrams in arbitrary and especially four dimensions.This is done in three respects firstly results in terms of generalized, multivariable hypergeometric functions are presented giving explicit series for small and large momenta. Secondly the imaginary parts of these integrals are expressed as complete elliptic integrals.Finally one-dimensional integral representations with elementary functions are derived.They are very well suited for the numerical evaluations.

Electroweak gauge boson self-energies: Complete QCD corrections.

Abstract: We present the QCD corrections to the longitudinal and transverse components of the electroweak gauge boson self-energies for arbitrary momentum transfer and for different internal quark masses. Compact formulas for both the real and imaginary parts are given in the general case as well as in some physically interesting special cases. The dependence on the definition of the quark masses is discussed.

A novel approach to confront electroweak data and theory

Abstract: A novel approach to study electroweak physics at one-loop level in generic SU(2)L×U(1)Y theories is introduced. It separates the 1-loop corrections into two pieces: process specific ones from vertex and box contributions, and universal ones from contributions to the gauge boson propagators. The latter are parametrized in terms of four effective form factors $$\bar e^2 (q^2 ), \bar s^2 (q^2 ), \bar g_Z^2 (q^2 )$$ and $$\bar g_W^2 (q^2 )$$ corresponding to the γγ, γZ,ZZ andWW propagators. Under the assumption that only the Standard Model contributes to the process specific corrections, the magnitudes of the four form factors are determined atq 2=0 and atq 2=m 2 by fitting to all available precision experiments. These values are then compared systematically with predictions of SU(2)L×U(1)Y theories. In all fits α s (m Z ) and $$\bar \alpha (m_Z^2 )$$ are treated as external parameters in order to keep the interpretation as flexible as possible. The treatment of the electroweak data is presented in detail together with the relevant theoretical formulae used to interpret the data. No deviation from the Standard Model has been identified. Ranges of the top quark and Higgs boson masses are derived as functions of α s (m Z ) and $$\bar \alpha (m_Z^2 )$$ . Also discussed are consequences of the recent precision measurement of the left-right asymmetry at SLC as well as the impact of a top quark mass and an improvedW mass measurement.

Leading electroweak correction to Higgs boson production at proton colliders.

Abstract: At proton colliders, Higgs particles are dominantly produced in the gluon-gluon fusion mechanism. The Higgs-boson--gluon coupling is mediated by heavy quark loops, and the process can serve to count the number of heavy strongly interacting particles whose masses are generated by the Higgs mechanism. We present the two-loop leading electroweak radiative correction to this coupling, which is quadratically proportional to the heavy quark masses. It turns out that this correction is well under control across the physically interesting quark mass ranges.

Electroweak effective lagrangians

Abstract: In this paper I review several aspects of the use of effective Lagrangians in (mainly) electroweak physics. The conditions under which this approach is reliable and useful, as well as the limitations of the formalism, are detailed. Various applications are also presented.

Renormalization group study of the standard model and its extensions: The minimal supersymmetric standard model

Abstract: In this paper we summarize the minimal supersymmetric standard model as well as the renormalization group equations of its parameters. We proceed to examine the feasibility of the model when the breaking of supersymmetry is parametrized by the soft terms suggested by supergravity theories. In such models, the electroweak symmetry is exact at the tree level and is broken spontaneously at one-loop order. We make the additional assumption that the GUT-inspired relation ${m}_{b}={m}_{\ensuremath{\tau}}$ be valid at the scale where the gauge coupling constants unify, which constrains the value of the top quark mass. For all types of soft breaking terms expected in supergravity theories, we present the results of numerical runs which yield electroweak breaking at the required scale. These yield not only the allowed ranges for the soft supersymmetry breaking parameters, but also the value of the supersymmetric partners' masses. Taking ${m}_{b}={m}_{\ensuremath{\tau}}$ at the unification scale, ${m}_{b}=4.9$ GeV, and ${\ensuremath{\alpha}}_{3}({M}_{Z})=0.118$, we find that for models in which global supersymmetry breaking arises solely from soft gaugino masses and bilinear mixing between the two Higgs scalars the top quark mass can be no lighter than \ensuremath{\sim} 180 GeV.

Prospects of measuring the parity of Higgs particles

Abstract: We analyze the prospects of measuring the parity of Higgs particles in the Standard Model and its supersymmetric extensions. Higgs decays are discussed in this context as well as production processes including, in particular, the fusion of Higgs particles in linearly polarized photon-photon collisions.