Showing papers on "Electroweak interaction published in 1993"

Techniques for the Calculation of Electroweak Radiative Corrections at the One-Loop Level and Results forW-physics at LEP 200

Abstract: We review the techniques necessary for the calculation of virtual electroweak and soft photonic corrections at the one-loop level. In particular we describe renormalization, calculation of one-loop integrals and evaluation of one-loop Feynman amplitudes. We summarize many explicit results of general relevance. We give the Feynman rules and the explicit form of the counterterms of the electroweak standard model, we list analytical expressions for scalar one-loop integrals and reduction of tensor integrals, we present the decomposition of the invariant matrix element for processes with two external fermions and we give the analytic form of soft photonic corrections. These techniques are applied to physical processes with external W-bosons. We present the full set of analytical formulae and the corresponding numerical results for the decay width of the W-boson and the top quark. We discuss the cross section for the production of W-bosons in e+e− annihilation including all O(x) radiative corrections and finite width effects. Improved Born approximations for these processes are given.

Progress in electroweak baryogenesis

Abstract: Recent work on generating the excess of matter over antimatter in the early universe during the electroweak phase transition is reviewed.

The Neutralino relic density in minimal $N=1$ supergravity

Abstract: We compute the cosmic relic (dark-matter) density of the lightest supersymmetric particle (LSP) in the framework of minimal [ital N]=1 supergravity models with radiative breaking of the electroweak gauge symmetry. To this end, we recalculate the cross sections for all possible annihilation processes for a general, mixed neutralino state with arbitrary mass. Our analysis includes effects of all Yukawa couplings of third-generation fermions, and allows for a fairly general set of soft supersymmetry- (SUSY-) breaking parameters at the Planck scale. We find that a cosmologically interesting relic density emerges naturally over wide regions of parameter space. However, the requirement that relic neutralinos do not overclose the Universe does not lead to upper bounds on SUSY-breaking parameters that are strictly valid for all combinations of parameters and of interest for existing or planned collider experiments; in particular, gluino and squark masses in excess of 5 TeV cannot strictly be excluded. On the other hand, in the generic'' case of a gauginolike neutralino whose annihilation cross sections are not accidentally'' enhanced by a nearby Higgs boson or [ital Z] pole, all sparticles should lie within the reach of the proposed [ital pp] and [ital e][sup +][ital e[minus]] supercolliders. We also find thatmore » requiring the LSP to provide all dark matter predicted by inflationary models imposes a strict lower bound of 40 GeV on the common scalar mass [ital m] at the Planck scale, while the lightest sleptons would have to be heavier than 100 GeV. Fortunately, a large relic neutralino density does not exclude the possibility that charginos, neutralinos, gluinos, and squarks are all within the reach of the CERN [ital e][sup +][ital e[minus]] collider LEP 200 and the Fermilab Tevatron.« less

Low energy effects of new interactions in the electroweak boson sector.

Abstract: Novel strong interactions in the electroweak bosonic sector are expected to induce effective interactions between the Higgs doublet field and the electroweak gauge bosons which lead to anomalous $\mathrm{WWZ}$ and $\mathrm{WW}\ensuremath{\gamma}$ vertices once the Higgs field acquires a vacuum expectation value. Using a linear realization of the Goldstone bosons, we consider a complete set of dimension-six operators which are SU(2)\ifmmode\times\else\texttimes\fi{}U(1) gauge invariant and conserve $C$ and $P$. This approach allows us to study effects of new physics which originates above 1 TeV and the Higgs boson mass dependence of the results can be investigated. Four of the dimension-six operators affect low energy and present CERN LEP experiments at the tree level. Another five influence neutral and charged current experiments at the one-loop level and three of these lead to anomalous $\mathrm{WWZ}$ and $\mathrm{WW}\ensuremath{\gamma}$ vertices. Their loop contributions are at most logarithmically divergent, and these logarithmic divergences can be understood as renormalizations of the four operators which contribute at the tree level. Constraints on the remaining five operators can be obtained if one assumes the absence of cancellations between the tree level and one-loop contributions. The resulting bounds on anomalous triple gauge boson couplins are modest, which emphasizes the importance of direct measurements of the triple gauge boson vertices, e.g., in ${W}^{+}{W}^{\ensuremath{-}}$ production at LEP II.

Dynamical symmetry breaking in quantum field theories

Abstract: In this text, differential aspects of dynamical symmetry breaking (DSB) in quantum field theory are discussed in detail A deep connection between this phenomenon in condensed matter physics and particle physics is emphasized Applications of DSB to such realistic theories as chromo-dynamics and electroweak interactions are also considered Problems intimately connected with DSB (anomalies, effective lagrangian approach) are also discussed

Rapidity gaps and jets as a new-physics signature in very-high-energy hadron-hadron collisions.

Abstract: In hadron-hadron collisions, production of Higgs bosons and other color-singlet systems can occur via fusion of electroweak bosons, occasionally leaving a ``rapidity gap'' in the underlying-event structure. This observation, due to Dokshitzer, Khoze, and Troyan, is studied to see whether it serves as a signature for detection of the Higgs bosons, etc. We find it is a very strong signature at subprocess c.m. energies in excess of a few TeV. The most serious problem with this strategy is the estimation of the fraction of events containing the rapidity gap; most of the time the gap is filled by soft interactions of spectator degrees of freedom. We also study this question and estimate this ``survival probability of the rapidity gap'' to be of order 5%, with an uncertainty of a factor 3. Ways of testing this estimate and further discussion of the underlying hard-diffraction physics are presented.

Neutral currents and Glashow-Iliopoulos-Maiani mechanism in SU(3)L⊗U(1)N models for electroweak interactions

Abstract: We study the Glashow-Iliopoulos-Maiani mechanism for flavor-changing neutral-current suppression in both the gauge and Higgs sectors, for models with $\mathrm{SU}{(3)}_{L}\ensuremath{\bigotimes}\mathrm{U}{(1)}_{N}$ gauge symmetry. The models differ from one another only with respect to the representation content. The main features of these models are that in order to cancel the triangle anomalies the number of families must be divisible by three (the number of colors) and that the lepton number is violated by some lepton-gauge bosons and lepton-scalar interactions.

Top production: Sensitivity to new physics

Abstract: The production cross–section and distributions of the top quark are sensitive to new physics, e.g., the tt system can be a probe of new resonances or gauge bosons that are strongly coupled to the top quark, in analogy to Drell–Yan production. The existence of such new physics is expected in dynamical electroweak symmetry breaking schemes, and associated with the large mass of the top quark. The total top production cross–section can be more than doubled, and distributions significantly distorted with a chosen scale of new physics of ∼ 1 TeV in the vector color singlet or octet s–channel. New resonance physics is most readily discernible in the high–pT distributions of the single top quark and of the W boson.

The Chiral Approach to the Electroweak Interactions

Abstract: The effective Lagrangian approach is reviewed and applied to the case of electroweak interactions.

One-Loop Analysis of the Electroweak Breaking in Supersymmetric Models and the Fine-Tuning Problems

Abstract: We examine the electroweak breaking mechanism in the minimal supersymmetric standard model (MSSM) using the {\em complete} one-loop effective potential $V_1$. First, we study what is the region of the whole MSSM parameter space (i.e. $M_{1/2},m_o,\mu,...$) that leads to a succesful $SU(2)\times U(1)$ breaking with an acceptable top quark mass. In doing this it is observed that all the one-loop corrections to $V_1$ (even the apparently small ones) must be taken into account in order to get reliable results. We find that the allowed region of parameters is considerably enhanced with respect to former "improved" tree level results. Next, we study the fine-tuning problem associated with the high sensitivity of $M_Z$ to $h_t$ (the top Yukawa coupling). Again, we find that this fine-tuning is appreciably smaller once the one-loop effects are considered than in previous tree level calculations. Finally, we explore the ambiguities and limitations of the ordinary criterion to estimate the degree of fine-tuning. As a result of all this, the upper bounds on the MSSM parameters, and hence on the supersymmetric masses, are substantially raised, thus increasing the consistency between supersymmetry and observation.

Baryon asymmetry of the universe in the minimal Standard Model

Abstract: We calculate the baryon asymmetry of the Universe which would arise during a first-order electroweak phase transition due to minimal-standard-model processes. It agrees in sign and magnitude with the observed baryonic excess, for reasonable Kobayashi-Maskawa parameters and mt in the expected range, and plausible values of bubble velocity and other high temperature effects. © 1993 The American Physical Society.

Electroweak chiral Lagrangian and new precision measurements.

Abstract: A revised and complete list of the electroweak chiral Lagrangian operators up to dimension four is provided. The connection of these operators to the $S$, $T$, and $U$ parameters and the parameters describing the triple gauge boson vertices $\mathrm{WW}\ensuremath{\gamma}$ and $\mathrm{WWZ}$ is made, and the size of these parameters from new heavy physics is estimated using a one flavor-doublet model of heavy fermions. The coefficients of the chiral Lagrangian operators are also computed in this model.

Recent Developments in Chiral Perturbation Theory

Abstract: I review recent developments in chiral perturbation theory (CHPT) which is the effective field theory of the standard model below the chiral symmetry breaking scale. The effective chiral Lagrangian formulated in terms of the pseudoscalar Goldstone bosons ($\pi, \, K, \, \eta$) is briefly discussed. It is shown how one can gain insight into the ratios of the light quark masses and to what extent these statements are model--independent. A few selected topics concerning the dynamics and interactions of the Goldstone bosons are considered. These are $\pi \pi$ and $\pi K$ scattering, some non--leptonic kaon decays and the problem of strong pionic final state interactions. CHPT also allows to make precise statements about the temperature dependence of QCD Green functions and the finite size effects related to the propagation of the (almost) massless pseudoscalar mesons. A central topic is the inclusion of matter fields, baryon CHPT. The relativistic and the heavy fermion formulation of coupling the baryons to the Goldstone fields are discussed. As applications, photo--nucleon processes, the $\pi N$ $\Sigma$--term and non--leptonic hyperon decays are presented. Implications of the spontaneously broken chiral symmetry on the nuclear forces and meson exchange currents are also described. Finally, the use of effective field theory methods in the strongly coupled Higgs sector and in the calculation of oblique electroweak corrections is touched upon.

The Chiral Approach to the Electroweak Interactions

Abstract: The effective lagrangian approach is reviewed and applied to the case of electroweak interactions.

On the Baryon, Lepton-Flavour and Right-Handed Electron Asymmetries of the Universe

Abstract: Non-perturbative electroweak effects, in thermal equilibrium in the early universe, have the potential to erase the baryon asymmetry of the universe, unless it is encoded in a B-L asymmetry, or in some "accidentally" conserved quantity. We first consider the possibility that the BAU may be regenerated from lepton flavour asymmetries even when initially $B-L = 0$. We show that provided some, but {\it not} all the lepton flavours are violated by ${\Delta}L{ eq}0$ interactions in equilibrium, the BAU may be regenerated without lepton mass effects. We next examine the possibility of encoding the baryon asymmetry in a primordial asymmetry for the right-handed electron, which due to its weak Yukawa interaction only comes into chemical equilibrium as the sphalerons are falling out of equilibrium. This would also raise the possibility of preserving an initial baryon asymmetry when $B-L = 0$.