We calculate the oblique electroweak corrections and confront with the experiments in an extension of the Standard Model. The new fields added are a vector-like weak doublet and a singlet fermion. After electroweak symmetry breaking there is a mixing between the components of the new fields, but no mixing allowed with the standard fermions. Four electroweak parameters, $\hat{S}$, $\hat{T}$, W, Y are presented in the formalism of Barbieri et al., these are the generalization of the Peskin-Takeuchi S, T, U's. The vector-like extension is slightly constrained, $\hat{T}$ requires the new neutral fermion masses not to be very far from each other, allowing higher mass difference for higher masses and smaller mixing. $\hat{S}, W, Y$ gives practically no constraints on the masses. This extension can give a positive contribution to $\hat{T} $, allowing a heavy Higgs boson in electroweak precision tests of the Standard Model.
In this proceeding, we highlight the computation of leading fermionic three-loop corrections to electroweak precision observables (EWPOs) accomplished recently. We summarize the numerical analysis and provide an outlook.
The origin of the fermion generations is discussed. A strong interactions spin $I_{S}$ is introduced which unifies the quarks and leptons as two multiplets of this spin. The electroweak vector bosons and gluons emerge as the fused states of these fermion multiplets.
In models with supersymmetry breaking by boundary conditions on an extra dimension it is possible to relate in a quantitative natural way the Fermi scale to the size of the extra dimension. We analyze in detail the compatibility of such models with ElectroWeak Precision Tests.
We present the minimal model of electroweak baryogenesis induced by fermions. The model consists of an extension of the Standard Model with one electroweak singlet fermion and one pair of vector like doublet fermions with renormalizable couplings to the Higgs. A strong first order phase transition is radiatively induced by the singlet-doublet fermions, while the origin of the baryon asymmetry is due to asymmetric reflection of the same set of fermions on the expanding electroweak bubble wall. The singlet-doublet fermions are stabilized at the electroweak scale by chiral symmetries and the Higg…
We consider instanton-induced high energy processes in an electroweak-type model. Multi-fermion final states are taken into account in Gauss approximation. Unlike previous results we calculate nonzero fermion modes contribution to cross-section by straightforward procedure. This contribution can be important at energies of sphaleron mass order.
A solution of the standard electroweak theory with a single lepton family is constructed, consisting of a cosmic string and a fermion condensate within its core. The stability of this system to small perturbations is examined, and it is found that stability is not enhanced relative to the bare electroweak string. The presence of quark zero modes is shown to violate the existence criteria for embedded defects.
We establish constraints on a general four-fermion contact interaction from precise measurements of electroweak parameters. We compute the one-loop contribution for the leptonic $Z$ width, anomalous magnetic, weak-magnetic, electric and weak dipole moments of leptons in order to extract bounds on the energy scale of these effective interactions.
We discuss recent results for the asymptotic behavior of fermion scattering amplitudes in the Sudakov limit. The results include next-to-next-to-leading logarithmic electroweak corrections and are used for the analysis of fermion-antifermion pair production in e+e- annihilation at high energy. The importance of the subleading logarithmic contributions is emphasized and the fermionic contributions to the Abelian form factor at two loops are discussed.
The calculation of electroweak precision observables in the MSSM is reviewed. The description of the 1997 data and the results of updated global fits are discussed in comparison with the Standard Model.
Recent developments in the field of complete electroweak tree-level calculations for six-fermion final states in e+ e- collisions are briefly reviewed. Particular attention is given to top-quark and Higgs boson physics, which are items of primary importance at the Next Linear Collider. The relevance of electroweak backgrounds and finite-width effects is discussed, showing the importance of complete calculations for precision studies at the colliders operating in the TeV energy range.
We present the analytic calculation of the planar master integrals which contribute to compute the two-loop light-fermion electroweak corrections to the production of a Higgs boson in association with a jet in gluon-gluon fusion. The complete dependence on the electroweak-boson mass is retained. The master integrals are evaluated by means of the differential equations method and the analytic results are expressed in terms of multiple polylogarithms up to weight four.
We reemphasize the importance of discriminating fermion-loop and bosonic electroweak corrections in the analysis of electroweak precision data. Most recent data are indeed precise enough to require corrections beyond (trivial) fermion loops. An analysis of these data in terms of the observables $Δx \equiv ε_{N1} - ε_{N2}$, $Δy \equiv - ε_{N2}$ and $ε\equiv - ε_{N3}$ identifies the required additional corrections as vertex corrections at the $W^\pm f\bar f^\prime$ and $Z^0 f\bar f$ vertices. Standard-model values for these corrections are consistent with the experimental data.
We consider the generation of fermion masses in an emergent model of electroweak symmetry breaking with composite $W,Z$ gauge bosons. A universal bulk fermion profile in a warped extra dimension is used for all fermion flavors. Electroweak symmetry is broken at the UV (or Planck) scale where boundary mass terms are added to generate the fermion flavor structure. This leads to flavor-dependent nonuniversality in the gauge couplings. The effects are suppressed for the light fermion generations but are enhanced for the top quark where the $Zt{\bar t}$ and $Wt{\bar b}$ couplings can deviate at the…
Problems related to fermions, unitarity and chiral anomaly in high energy electroweak interactions, are investigated. Particular attention is paid to the correct functional integration over fermion fields in the background of instanton-anti\-instanton type configurations. This leads to an expansion of correlation functions in terms of a small parameter, $ρ/R$, when the instanton-antiinstanton separation ($R$) is large compared to their sizes ($ρ$). Applying such a method to widely discussed cases of fermion-number violation in the electroweak theory, we conclude that there are no theoretical b…
The two-loop electroweak corrections to the anomalous magnetic moment of the muon, generated by fermionic loops, are calculated. An interesting role of the top quark in the anomaly cancellation is observed. New corrections, including terms of order $G_μαm_t^2$, are computed and a class of diagrams previously thought to vanish are found to be important. The total fermionic correction is $-(23\pm 3) \times 10^{-11}$ which decreases the electroweak effects on $g-2$, predicted from one-loop calculations, by 12\%. We give an updated theoretical prediction for $g-2$ of the muon.
The S,T, and U formalism for studying electroweak precision data has recently been updated to include the effects of new light physics and within this analysis the latest LEP and SLC data tightly constrain models of physics beyond the Standard Model. We re-examine the constraints on additional strong and weakly interacting fermion families in the light of these developments. We conclude that the precision data favour models with at least one fermion with a mass below 150GeV.
Recently the two-loop next-to-leading logarithmic (NLL) virtual corrections to arbitrary processes with massless external fermions have been calculated. Within the spontaneously broken electroweak theory the one- and two-loop mass singularities have been derived to NLL accuracy and expressed as universal correction factors depending only on the quantum numbers of the external particles. This talk summarizes the results for massless fermionic processes and presents new aspects arising in the extension of the corresponding loop calculations to massive external fermions. As a preliminary result, …
We update our analysis of technicolour theories with techniquarks in higher dimensional representations of the technicolour gauge group in the light of the new electroweak precision data on the Z resonance.
A new dynamical symmetry breaking model of electroweak interactions is proposed based on interacting fermions. Two fermions of different SU_{L}(2) representations form a symmetry breaking condensate and generate the lepton and quark masses. The weak gauge bosons get their usual standard model masses from a gauge invariant Lagrangian of a doublet scalar field composed of the new fermion fields. The new fermion fields become massive by condensation. It is shown that the new charged fermions are produced at the next linear colliders in large number. The model is a low energy one which cannot be r…
We study the impact of LEP2 constraints on the dimensionless coefficients of the electroweak chiral Lagrangian on the precision observables using the improved renormalization group equations. We find that the current uncertainty in the triple and quartic gauge boson couplings can accommodate electroweak symmetry breaking models with $S(Λ=1 \textrm{TeV}) > 0$.
A critical assessment is given of the comparison between the new versions of the programs TOPAZ0 40i and ZFITTER 510. The relevance for precision calculations around the $\zb$ resonance is briefly discussed. << more references added >>
The role of the top quark in the Standard Model predictions for electroweak precision observables is reviewed and the implications of the experimental data for the indirect determination of the top mass range is discussed.
Precision measurements of electroweak observables provide stringent tests of the Standard Model structure and an accurate determination of its parameters. An overview of the present experimental status is presented.
We compute the net electric current generated during a first order electroweak phase transition when fermions transit from the false to the true vacuum. This current is generated by the CP-violating fermion interaction with the Higgs during the phase transition and is quantified in terms of a CP-violating phase in the bubble wall separating the symmetric from the symmetry-broken phases. We comment on the seed magnetic field that this current is able to generate and it is possible implications for magnetogenesis in the early universe during the electroweak phase transition.
Invited Talk presented at the First Arctic Workshop on Particle Physics, Saariselkae, August 1994 and at the Workshop on Elementary Particles and Quantum Field Theory, Moscow, September 1994
This is a pedagogical and self-contained review on obtaining electroweak precision constraints on TeV scale new physics using the effective theory method. We identify a set of relevant effective operators in the standard model and calculate from them corrections to all major electroweak precision observables. The corrections are compared with data to put constraints on the effective operators. Various approaches and applications in the literature are reviewed.
The little Higgs idea is an alternative to supersymmetry as a solution to the gauge hierarchy problem. In this note, I review various little Higgs models and their phenomenology with emphases on the precision electroweak constraints in these models.
Precision measurements of electroweak observables provide stringent tests of the Standard Model structure and an accurate determination of its parameters. A brief overview of the present experimental status is presented. A more extensive discussion can be found in Ref. hep-ph/9711279
The status of the electroweak Standard Model is reviewed in the light of recent precision data and new theoretical results which have contributed to improve the predictions for precision observables, together with the remaining inherent theoretical uncertainties. Consequences for possible new physics are also discussed.
1. Introduction 2. Status of the Data 3. Precision Electroweak Data and the Standard Model 4. A More General Analysis of Electroweak Data 4.1 Basic Definitions and Results 4.2 Experimental Determination of the Epsilon Variables 4.3 Comparing the Data with the Minimal Supersymmetric Standard Model 5. Theoretical Limits on the Higgs Mass 6. Conclusion
I give a basic introduction to precision electroweak analysis, beginning with calculation at tree-level which most simply illustrates the procedure. I then work out the formalism for one-loop corrections to the vector boson self energies (oblique corrections). This is a tractable subset of the complete electroweak program. Not only does this exercise provide an analytically accessible demonstration of the theory involved in electroweak precision analyses, it also teaches students a useful technique to analyze a large class of theories beyond the Standard Model.
If the electroweak symmetry is broken by some unspecified strong dynamics, composite fermions may exist with definite transformation properties under SU(2)_L x SU(2)_R/SU(2)_{L+R} and may play a role in giving masses by mixing to all the standard quarks and leptons. Assuming this to be the case, we analyze the role of Singlets, Doublets and Triplets in the ElectroWeak Precision Tests and in Flavour Physics. Doublets and Triplets are generically disfavoured. In the Singlet case, we specify the breaking patterns of the flavour group that allow to keep the CKM picture of flavour physics and we di…
The first part of these lectures provides a brief introduction to the concepts and techniques of effective field theory. The second part reviews precision electroweak constraints using effective theory methods. Several simple extensions of the Standard Model are considered as illustrations. The appendix contains some new results on the one-loop contributions of electroweak triplet scalars to the T parameter and contains a discussion of decoupling in that case.
In this talk we discuss the phenomenology of models with replicated electroweak gauge symmetries, based on a framework with the gauge structure [SU(2) or U(1)] x U(1) x SU(2) x SU(2).
The model of radiative neutrino mass with dark matter proposed by one of us is extended to include a real singlet scalar field. There are then two important new consequences. One is the realistic possibility of having the lightest neutral singlet fermion (instead of the lightest neutral component of the dark scalar doublet) as the dark matter of the Universe. The other is a modification of the effective Higgs potential of the Standard Model, consistent with electroweak baryogenesis.
Adding radion perturbations (up to second order) to the static (RS) metric allows us to calculate the general first and second order interactions of the radion field with the electroweak vector bosons. We use these interactions to compute precision electroweak observables in the case of Higgs-radion mixing and compare with experiment.
Review of the electroweak sector of the minimal standard model, Basic LEP processes at the tree level, 1-loop radiative corrections in the on-shell renormalization scheme, Star scheme and oblique corrections, Introduction to oblique parameters.
We calculate the critical temperature $(T_c$) of the electroweak phase transition in the minimal standard model considering simultaneously temperature ($T$) and fermion chemical potential ($μ_f$) effects over the effective potential. The calculation is performed in the one-loop approximation to the effective potential at non-zero temperature using the real time formalism of the thermal field theory. We show that it exists a fermion chemical potential critical value ($μ_f^c$) for which the Higgs boson condensate vanishes at T=0. If $T$ and $μ_f$ effects are considered simultaneously, it is show…
Measurements of the cross section for $\bar ν_e e^-$ elastic scattering with unprecedented precision have recently been proposed. The impact of these experiments for detecting possible deviations from the standard electroweak theory is analyzed and compared with that of several other measurements.
The status of several precisely measured electroweak parameters is reviewed. Natural relations among them are shown to constrain the Higgs mass, m_H, as well as various ``New Physics'' effects. Indications of an anomalous Zb anti-b coupling are discussed. Constraints on excited W^* bosons are given.
This is the first in a series of two papers. We use the Schwinger-Keldysh formalism to derive semiclassical Boltzmann transport equations for massive chiral fermions and scalar particles. Our considerations include complex mass terms and mixing fermion and scalar fields, such that CP-violation is naturally included, rendering the equations particularly suitable for studies of baryogenesis at a first order electroweak phase transition. In part II we discuss the collision terms.
New heavy particles in an SU(2)_L multiplet, sometimes introduced in extensions of the standard model, have highly degenerate tree-level mass M if their couplings to the Higgs bosons are very small or forbidden. However, loop corrections may generate the gauge-symmetry-breaking mass splitting within the multiplet, which does not vanish in the large M limit due to the threshold singularity. We calculate the electroweak contribution to the mass splitting for a heavy fermion multiplet, to the two-loop order. Numerically, two-loop electroweak contributions are typically O(MeV).
We study the impact of additional beyond-the-Standard Model (BSM) fermions, charged under the Standard Model (SM) $SU(2)_L \otimes U(1)_Y$ gauge group, on the electroweak phase transition (EWPT) in a 2-Higgs-Doublet-Model (2HDM) of type II. We find that the strength of the EWPT can be enhanced by about 40% compared to the default 2HDM. Therefore, additional light fermions are a useful tool to weaken the tension between increasing mass constraints on BSM scalars and the requirement of additional light scalar degrees of freedom to accommodate a strong first order EWPT. The findings are of partic…
I consider the low energy limit of Little Higgs models. The method consists in eliminating the heavy fields using their classical equations of motion in the infinite mass limit. After the elimination of the heavy degrees of freedom one can directly read off deviations from the precision electroweak data. I also examine the effects on the low energy precision experiments.
We present the results from a calculation of the full electroweak two-loop fermionic contributions to the effective leptonic mixing angle of the $Z$ boson, $\sin^2θ_{eff}$, in the Standard Model. On-shell renormalization and analytic calculations are performed for the three-point vertex functions at zero external momenta, whereas irreducible three-point integrals for non-vanishing external momenta are evaluated semi-analytically applying two different methods. Comparisons with a previous calculation show complete agreement.
The effect of perturbations of electroweak strings on quark zero modes is studied in 2+1 and 3+1 dimensions. As first discovered by Naculich, it is found that the bosonic perturbations that destabilize the string give a mass to the zero modes and also lift their degeneracy. The effect of the zero modes on the stability of the string is discussed qualitatively and we argue that the fermionic vacuum instability found by Naculich should lead to a distortion of the bosonic string but not be responsible for decay.
A model of electroweak-scale right-handed neutrino (\ewnur) model was constructed five years ago in which the right-handed neutrinos are members of mirror fermion weak doublets and where the Majorana masses of the right-handed neutrinos are found to be {\em naturally} of the order of the electroweak scale. These features facilitate their searches at the LHC through signals such as like-sign dilepton events. This model contains, in addition to the mirror quarks and leptons, extra scalars transforming as weak triplets. In this paper, we study the constraints imposed on these additional particles…
In 5 dimensions the electroweak symmetry can be broken by boundary conditions, leading to a new type of Higgsless theories. These could in principle improve on the 4D case by extending the perturbative domain to energies higher than $4 πv$ and by allowing a better fit to the electroweak precision tests. Nevertheless, it is unlikely that both these improvements can be achieved, as we show by discussing these problems in an explicit model.
I summarise the virtual effects of the new particles predicted by supersymmetric extensions of the Standard Model on the high-precision electroweak observables measured at LEP/SLC, the Tevatron and CLEO. I will then discuss in some details the two-loop SUSY-QCD corrections to the $ρ$ parameter.
Extra-dimensional Higgsless models with electroweak symmetry breaking through boundary conditions generically have difficulties with electroweak precision constraints, when the fermions are localized to the ``branes'' in the fifth dimension. In this paper we show that these constraints can be relaxed by allowing the light fermions to have a finite extent into the bulk of the fifth dimension. The $T$ and $U$ electroweak parameters can be naturally suppressed by a custodial symmetry, while the $S$ parameter can be made to vanish through a cancellation, if the leakage into the bulk of the light g…
We give some details of the recently completed calculation of the full two-loop fermionic corrections to the effective leptonic weak mixing angle. Among others, we describe the C++ library DiaGen/IdSolver, which was used to reduce the two-loop light fermion vertex diagrams to linear combinations of master integrals with rational function coefficients.
We study the low energy limit of Little Higgs models. The method consists in eliminating the heavy fields using their classical equations of motion in the infinite mass limit. After the elimination of the heavy degrees of freedom we can directly read off deviations from the precision electroweak data. We also examine the effects on the low energy precision experiments.
