Showing papers on "Electroweak interaction published in 2001"

Bounds on Universal Extra Dimensions

Abstract: We show that the bound from the electroweak data on the size of extra dimensions accessible to all the standard model elds is rather loose. These \universal" extra dimensions could have a compactication scale as low as 300 GeV for one extra dimension. This is because the Kaluza-Klein number is conserved and thus the contributions to the electroweak observables arise only from loops. The main constraint comes from weak-isospin violation eects. We also compute the contributions to the S parameter and the Zb b vertex. The direct bound on the compactication scale is set by CDF and D0 in the few hundred GeV range, and the Run II of the Tevatron will either discover extra dimensions or else it could signicantly raise the bound on the compactication scale. In the case of two universal extra dimensions, the current lower bound on the compactication scale depends logarithmically on the ultra-violet cuto of the higher dimensional theory, but can be estimated to lie between 400 and 800 GeV. With three or more extra dimensions, the cuto dependence may be too strong to allow an estimate.

SOFTSUSY: a program for calculating supersymmetric spectra

Abstract: SOFTSUSY is a program which accurately calculates the spectrum of superparticles in the CP-conserving Minimal Supersymmetric Standard Model (MSSM), with a full flavour mixing structure The program solves the renormalisation group equations with theoretical constraints on soft supersymmetry breaking terms provided by the user Weak-scale gauge coupling and fermion mass data (including one-loop finite MSSM corrections) are used as a boundary condition, as well as successful radiative electroweak symmetry breaking The program can also calculate a measure of fine-tuning The program structure has been designed to easily generalise to extensions of the MSSM This article serves as a self-contained guide to prospective users, and indicates the conventions and approximations used

Solitons in Field Theory and Nonlinear Analysis

Abstract: Preface * Primer of Field Theory * Sigma Models * Multiple Instantons and Characteristic Classes * Generalized Abelian Higgs Equations * Chern-Simons Systems: Abelian Case * Chern-Simons Systems: Non-Abelian Case * Electroweak Vortices * Dyons * Ordinary Differential Equations * Strings in Cosmology * Vortices and Antivortices * Born-Infeld Solutions * References * Bibliography * Index

Intersecting brane worlds

Abstract: Chiral fermions naturally appear at intersections of branes at angles. We propose a string scenario in which different SM gauge interactions propagate on different (intersecting) brane worlds, partially wrapped in the extra dimensions. Quarks and leptons live at brane intersections, located at different positions in the extra dimensions. Replication of families follows because branes generically intersect at several points. Gauge and Yukawa couplings can be computed in terms of the compactification radii. Yukawa couplings are hierarchical, proportional to exp −A, where A is the area of a string world-sheet extending among three intersections. The models are non-supersymmetric but the string scale may be lowered down to 1-10 TeV. The proton is however stable due to discrete symmetries, exact in perturbation theory. The scenario has KK, winding and other new excited states (`gonions'), with masses below the string scale and accessible to accelerators. The models contain scalar tachyons with the quantum numbers of SM Higgs doublets, and we propose that they induce electroweak symmetry breaking in a novel way. We present specific string models with the above properties, built from D4-branes wrapping on T2 × (T2)2/N, and leading to 3-family semirealistic spectra.

Radion dynamics and electroweak physics

Abstract: The dynamics of a stabilized radion in the Randall-Sundrum model with two branes is investigated, and the effects of the radion on electroweak precision observables are evaluated The radius is assumed to be stabilized using a bulk scalar field as suggested by Goldberger and Wise First the mass and the wave function of the radion is determined including the back reaction of the bulk stabilization field on the metric, giving a typical radion mass of the order of the weak scale This is demonstrated by a perturbative computation of the radion wave function A consequence of the background configuration for the scalar field is that after including the back reaction the Kaluza-Klein states of the bulk scalars couple directly to the standard model fields on the TeV brane Some cosmological implications are discussed, and in particular it is found that the shift in the radion at late times is in agreement with the four-dimensional effective theory result The effect of the radion on the oblique parameters is evaluated using an effective theory approach In the absence of a curvature-scalar Higgs mixing operator, these corrections are small and give a negative contribution to S In the presence of such a mixingmore » operator, however, the corrections can be sizable due to the modified Higgs and radion couplings« less

Experimental probes of localized gravity: On and off the wall

Abstract: The phenomenology of the Randall-Sundrum model of localized gravity is analyzed in detail for the two scenarios where the Standard Model (SM) gauge and matte fields are either confined to a TeV scale 3-brane or may propagate in a slice of five dimensional anti-deSitter space. In the latter instance, the authors derive the interactions of the graviton, gauge, and fermion Kaluza-Klein (KK) states. The resulting phenomenological signatures are shown to be highly dependent on the value of the 5-dimensional fermion mass and differ substantially from the case where the SM fields lie on the TeV-brane. In both scenarios, they examine the collider signatures for direct production of the graviton and gauge KK towers as well as their induced contributions to precision electroweak observables. These direct and indirect signatures are found to play a complementary role in the exploration of the model parameter space. In the case where the SM field content resides on the TeV-brane, they show that the LHC can probe the full parameter space and hence will either discover or exclude this model if the scale of electroweak physics on the 3-brane is less than 10 TeV. They also show that spontaneous electroweak symmetry breaking of the SMmore » must take place on the TeV-brane.« less

Gauge Theories of the Strong and Electroweak Interaction

Abstract: Basic concepts: The fundamental constituents of matter and their interactions - Elements of relativistic quantum mechanics - The quark model of the hadrons - Phenomenology of the electroweak interaction - The quark parton model - Quantum theory of gauge fields - Quantum chromodynamics - Theory of the electroweak interaction - Outlook to grand unified theories and supersymmetry

Constraints on the Minimal Supergravity Model and Prospects for SUSY Particle Production at Future Linear e^+ e^- Colliders

Abstract: We perform a complete analysis of the supersymmetric particle spectrum in the Minimal Supergravity (mSUGRA) model where the soft SUSY breaking scalar masses, gaugino masses and trilinear couplings are unified at the GUT scale, so that the electroweak symmetry is broken radiatively. We show that the present constraints on the Higgs boson and superparticle masses from collider searches and precision measurements still allow for large regions of the mSUGRA parameter space where charginos, neutralinos, sleptons and top squarks as well as the heavier Higgs particles, are light enough to be produced at the next generation of $\ee$ linear colliders with center of mass energy around $\sqrt{s} \sim 800$ GeV, with sizeable cross sections. An important part of this parameter space remains even when we require that the density of the lightest neutralinos left over from the Big Bang, which we calculate using standard assumptions, falls in the range favored by current determinations of the Dark Matter density in the Universe. Already at a c.m. energy of 500 GeV, SUSY particles can be accessible in some parameter range, and if the energy is increased to $\sqrt{s} \simeq 1.2$ TeV, the $e^+e^-$ collider will have a reach for high precision studies of SUSY particles in a range that is comparable to the discovery range of the LHC.

Constraints on the minimal supergravity model and prospects for SUSY particle production at future linear e+e− colliders

Abstract: We perform a complete analysis of the supersymmetric particle spectrum in the Minimal Supergravity (mSUGRA) model where the soft SUSY breaking scalar masses, gaugino masses and trilinear couplings are unified at the GUT scale, so that the electroweak symmetry is broken radiatively. We show that the present constraints on the Higgs boson and superparticle masses from collider searches and precision measurements still allow for large regions of the mSUGRA parameter space where charginos, neutralinos, sleptons and top squarks as well as the heavier Higgs particles, are light enough to be produced at the next generation of e+e− linear colliders with center of mass energy around √s ~ 800 GeV, with sizeable cross sections. An important part of this parameter space remains even when we require that the density of the lightest neutralinos left over from the Big Bang, which we calculate using standard assumptions, falls in the range favored by current determinations of the Dark Matter density in the Universe. Already at a c.m. energy of 500 GeV, SUSY particles can be accessible in some parameter range, and if the energy is increased to √s 1.2 TeV, the e+e− collider will have a reach for high precision studies of SUSY particles in a range that is comparable to the discovery range of the LHC.

One-loop leading logarithms in electroweak radiative corrections . I. Results

Abstract: We discuss the evaluation of the collinear single-logarithmic contributions to virtual electroweak corrections at high energies. More precisely, we prove the factorization of the mass singularities originating from loop diagrams involving collinear virtual gauge bosons coupled to external legs. We discuss, in particular, processes involving external longitudinal gauge bosons, which are treated using the Goldstone-boson equivalence theorem. The proof of factorization is performed within the 't Hooft–Feynman gauge at one-loop order and applies to arbitrary electroweak processes that are not mass-suppressed at high energies. As basic ingredients we use Ward identities for Green functions with arbitrary external particles involving a gauge boson collinear to one of these. The Ward identities are derived from the BRS invariance of the spontaneously broken electroweak gauge theory.

Electroweak radiative corrections to W boson production in hadronic collisions

Abstract: Some results of a calculation of electroweak radiative corrections to {ital W} and {ital Z} boson production in hadronic collisions are presented.

Higgs mechanism and bulk gauge boson masses in the Randall-Sundrum model

Abstract: Assuming the breaking of gauge symmetries by the Higgs mechanism, we consider the associated bulk gauge boson masses in the Randall-Sundrum background. With the Higgs field confined on the TeV-brane, the W and Z boson masses are naturally an order of magnitude smaller than their Kaluza-Klein excitation masses. The electroweak precision data require the lowest excited state to lie above about 30 TeV, with fermions on the TeV-brane. This bound is reduced to about 10 TeV if the fermions reside sufficiently close to the Planck-brane. Thus, some tuning of parameters is needed. We also discuss the bulk Higgs case, where the bounds are an order of magnitude smaller.

How can a heavy Higgs boson be consistent with the precision electroweak measurements

Abstract: The fit of precision electroweak data to the Minimal Standard Model currently gives an upper limit on the Higgs boson mass of 170 GeV at 95% confidence. Nevertheless, it is often said that the Higgs boson could be much heavier in more general models. In this paper, we critically review models that have been proposed in the literature that allow a heavy Higgs boson consistent with the precision electroweak constraints. All have unusual features, and all can be distinguished from the Minimal Standard Model either by improved precision measurements or by other signatures accessible to next-generation colliders.

Gauge-Higgs unification in higher dimensions

Abstract: The electroweak Higgs doublets are identified as components of a vector multiplet in a higher-dimensional supersymmetric field theory. We construct a minimal model in 6D where the electroweak SU(2)⊗U(1) gauge group is extended to SU(3), and unified 6D models with the unified SU(5) gauge symmetry extended to SU(6). In these realistic theories the extended gauge group is broken by orbifold boundary conditions, leaving Higgs doublet zero modes which have Yukawa couplings to quarks and leptons on the orbifold fixed points. In one SU(6) model the weak mixing angle receives power law corrections, while in another the fixed point structure forbids such corrections. A 5D model is also constructed in which the Higgs doublet contains the fifth component of the gauge field. In this case Yukawa couplings are introduced as nonlocal operators involving the Wilson line of this gauge field.

Large Two-Loop Contributions to g-2 from a Generic Pseudoscalar Boson

Abstract: We calculate the dominant contributions to the muon g - 2 at the two-loop level due to a pseudoscalar boson that may exist in any exotic Higgs sector in most extensions of the standard model. The leading effect comes from diagrams of the Barr-Zee type. A sufficiently light pseudoscalar Higgs boson can give rise to contribution as large as the electroweak contribution which is measurable in the next round of g - 2 experiment. The coming improved data on muon g - 2 can put the best limit on the possible existence of a light pseudoscalar boson in physics beyond the standard model.

First principle derivation of semiclassical force for electroweak baryogenesis

Abstract: We perform a systematic gradient expansion on kinetic equations and derive the CP-violating semiclassical force for fermions propagating in presence of a CP-violating wall at a first order electroweak phase transition. The force appears at order in the flow term of the kinetic equation and agrees with the semiclassical force used for baryogenesis computations. In particular we consider the force for charginos in both the MSSM and NMSSM. We then study the continuity equations for the vector and axial vector currents and stress the role of the latter as the one containing baryogenesis sources. We also show that there is no CP-violating force for bosons to order in gradient expansion.

One-loop leading logarithms in electroweak radiative corrections . II. Factorization of collinear singularities

Abstract: � Springer-Verlag / Societa Italiana di Fisica 2001 Abstract. We discuss the evaluation of the collinear single-logarithmic contributions to virtual electroweak corrections at high energies. More precisely, we prove the factorization of the mass singularities originating from loop diagrams involving collinear virtual gauge bosons coupled to external legs. We discuss, in partic- ular, processes involving external longitudinal gauge bosons, which are treated using the Goldstone-boson equivalence theorem. The proof of factorization is performed within the 't Hooft-Feynman gauge at one- loop order and applies to arbitrary electroweak processes that are not mass-suppressed at high energies. As basic ingredients we use Ward identities for Green functions with arbitrary external particles involving a gauge boson collinear to one of these. The Ward identities are derived from the BRS invariance of the spontaneously broken electroweak gauge theory.

Neutrino masses and mixing in brane world theories

Abstract: We present a comprehensive study of five-dimensional brane-world models for neutrino physics based on flat compactifications. Particular emphasis is put on the inclusion of bulk mass terms. We derive a number of general results for such brane-world models with bulk mass terms. In particular, in the limit of small brane-bulk couplings, the electroweak eigenstates are predominantly given as a superposition of three light states with non-trivial small admixtures of bulk states. As a consequence, neutrinos can undergo standard oscillations as well as oscillation into bulk Kaluza-Klein states. We use this structure to construct a specific model based on orbifolding and bulk Majorana masses which is compatible with all observed oscillation phenomena. The solar neutrino deficit is explained by oscillations into sterile bulk states while the atmospheric neutrino deficit is due to ?? - ?? oscillations with naturally maximal mixing. In addition, the model can accommodate the LSND result and a significant neutrino dark matter component. We also analyze the constraints from supernova energy loss on neutrino brane-world theories and show that our specific model is consistent with these constraints.

Electroweak symmetry breaking from dimensional deconstruction

Abstract: We propose a new class of four-dimensional theories for natural electroweak symmetry breaking, relying neither on supersymmetry nor on strong dynamics at the TeV scale. The new TeV physics is perturbative, and radiative corrections to the Higgs mass are finite. The softening of this mass occurs because the Higgs is an extended object in theory space, resulting in an accidental symmetry. A novel Higgs potential emerges naturally, requiring a second light SU(2) doublet scalar.

Hybrid inflation and baryogenesis at the TeV scale

Abstract: We consider the construction of inverted hybrid inflation models in which the vacuum energy during inflation is on the TeV scale, and the inflaton couples to the Higgs field. Such models are of interest in the context at some recently proposed models of electroweak baryogenesis. We demonstrate how constraints on these models arise from quantum corrections, and how self-consistent examples may be constructed, albeit at the expense of fine tuning. We discuss two possible ways in which the baryon asymmetry of the universe may be produced in these models. One of them is based on preheating and a consequent nonthermal electroweak symmetry restoration, and the other on the formation of Higgs winding configurations by the Kibble mechanism at the end of inflation.