Showing papers on "Electroweak interaction published in 1999"

The Masses of the Neutral CP-even Higgs Bosons in the MSSM: Accurate Analysis at the Two-Loop Level

Abstract: We present detailed results of a diagrammatic calculation of the leading two-loop QCD corrections to the masses of the neutral CP-even Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM). The two-loop corrections are incorporated into the full diagrammatic one-loop result and supplemented with refinement terms that take into account leading electroweak two-loop and higher-order QCD contributions. The dependence of the results for the Higgs-boson masses on the various MSSM parameters is analyzed in detail, with a particular focus on the part of the parameter space accessible at LEP2 and the upgraded Tevatron. For the mass of the lightest Higgs boson, mh, a parameter scan has been performed, yielding an upper limit on mh which depends only on tan�. The results for the Higgs-boson masses are compared with results obtained by renormalization group methods. Good agreement is found in the case of vanishing mixing in the scalar quark sector, while sizable deviations occur if squark mixing is taken into account.

Predictions for all processes e^+e^- -> 4 fermions + gamma

Abstract: The complete matrix elements for e^+ e^- -> 4f and e^+ e^- -> 4f + gamma are calculated in the Electroweak Standard Model for polarized massless fermions. The matrix elements for all final states are reduced to a few compact generic functions. Monte Carlo generators for e^+ e^- -> 4f and e^+ e^- -> 4f + gamma are constructed. We compare different treatments of the finite widths of the electroweak gauge bosons; in particular, we include a scheme with a complex gauge-boson mass that obeys all Ward identities. The detailed discussion of numerical results comprises integrated cross sections as well as photon-energy distributions for all different final states.

End point of the hot electroweak phase transition

Abstract: We study the hot electroweak phase transition by four-dimensional lattice simulations and give the phase diagram. A continuum extrapolation is done. We find that the phase transition is first order for Higgs-boson masses ${m}_{H}l66.5\ifmmode\pm\else\textpm\fi{}1.4\mathrm{GeV}$. Above this end point a rapid crossover occurs. Our result agrees with that of the dimensional reduction approach. It also indicates that the fermionic sector of the standard model (SM) may be included perturbatively. We obtain that the end point in the SM is $72.4\ifmmode\pm\else\textpm\fi{}1.7\mathrm{GeV}$. Thus, the LEP Higgs-boson mass lower bound excludes any electroweak phase transition in the SM.

Supersymmetry and electroweak breaking from extra dimensions at the TeV scale

Abstract: We analyze some features of the role that extra dimensions, of radius R in the ${\mathrm{TeV}}^{\ensuremath{-}1}$ range, can play in the soft breaking of supersymmetry and the spontaneous breaking of electroweak symmetry. We use a minimal model where the gauge and Higgs boson sector of the MSSM are living in the bulk of five dimensions and the chiral multiplets in a four-dimensional boundary. Supersymmetry is broken in the bulk by the Scherk-Schwarz mechanism and transmitted to the boundary by radiative corrections. The particle spectrum is completely predicted as a function of a unique R charge. The massless sector corresponds to the pure standard model and electroweak symmetry is radiatively broken with a light Higgs boson weighing $\ensuremath{\lesssim}110 \mathrm{GeV}.$ The $\ensuremath{\mu}$ problem is solved and Higgsinos, gauginos, and heavy Higgs bosons acquire masses $\ensuremath{\sim}1/R.$ Chiral sfermions acquire radiative squared-masses $\ensuremath{\sim}{\ensuremath{\alpha}}_{i}{/R}^{2}.$ The effective potential is explicitly computed in the bulk of extra dimensions and some cosmological consequences can be immediately drawn from it. Gauge coupling running and unification are studied in the presence of Scherk-Schwarz supersymmetry breaking. The unification is similar to that in the supersymmetric theory.

Top Quark Seesaw Theory of Electroweak Symmetry Breaking

Abstract: We study electroweak symmetry breaking involving the seesaw mechanism of quark condensation. These models produce a composite Higgs boson involving the left-handed top quark, yet the top quark mass arises naturally at the observed scale. We describe a schematic model which illustrates the general dynamical ideas. We also consider a generic low-energy effective theory which includes several composite scalars, and we use the effective potential formalism to compute their spectrum. We develop a more detailed model in which certain features of the schematic model are replaced by additional dynamics.

Production of neutral Higgs-boson pairs at LHC

Abstract: The reconstruction of the Higgs potential in the Standard Model or supersymmetric theories demands the measurement of the trilinear Higgs couplings. These couplings affect the multiple production of Higgs bosons at high energy colliders. We present a systematic overview of the cross sections for the production of pairs of (light) neutral Higgs bosons at the LHC. The analysis is carried out for the Standard Model and its minimal supersymmetric extension.

Electroweak precision measurements and collider probes of the standard model with large extra dimensions

Abstract: The elementary particles of the standard model may reside in more than 3+1 dimensions. We study the consequences of large compactified dimensions on scattering and decay observables at high-energy colliders. Our analysis includes global fits to electroweak precision data, indirect tests at high-energy electron-positron colliders (CERN LEP2 and NLC), and direct probes of the Kaluza-Klein resonances at hadron colliders (Fermilab Tevatron and CERN LHC). The present limits depend sensitively on the Higgs sector, both the mass of the Higgs boson and how many dimensions it feels. If the Higgs boson is trapped on a (3+1)-dimensional wall with the fermions, large Higgs boson masses (up to 500 GeV) and relatively light Kaluza-Klein mass scales (less than 4 Tev) can provide a good fit to precision data. That is, a light Higgs boson is not necessary to fit the electroweak precision data, as it is in the standard model. If the Higgs boson propagates in higher dimensions, precision data prefer a light Higgs boson (less than 260 GeV) and a higher compactification scale (greater than 3.8 TeV). Future colliders can probe much larger scales. For example, a 1.5 TeV electron-positron linear collider can indirectly discover Kaluza-Klein excitations up to 31 TeV ifmore » 500 fb{sup -1} integrated luminosity is obtained. (c) 1999 The American Physical Society.« less

Nonequilibrium electroweak baryogenesis at preheating after inflation

Abstract: We present a novel scenario for baryogenesis in a hybrid inflation model at the electroweak scale, in which the standard model Higgs field triggers the end of inflation. One of the conditions for successful baryogenesis, the departure from thermal equilibrium, is naturally achieved at the stage of preheating after inflation. The inflaton oscillations induce large occupation numbers for long-wavelength configurations of the Higgs and gauge fields, which leads to a large rate of sphaleron transitions. We estimate this rate during the first stages of reheating and evaluate the amount of baryons produced due to a particular type of higher-dimensional CP violating operator. The universe thermalizes through fermion interactions, at a temperature below critical, T{sub rh}(less-or-similar sign)100 GeV, preventing the wash-out of the produced baryon asymmetry. Numerical simulations in 1+1 dimensions support our theoretical analyses. (c) 1999 The American Physical Society.

Phenomenology of low quantum gravity scale models

Abstract: We study some phenomenological implications of models where the scale of quantum gravity effect lies much below the four-dimensional Planck scale. These models arise from M-theory vacua where either the internal space volume is large or the string coupling is very small. We provide a critical analysis of ways to unify electroweak, strong, and gravitational interactions in M theory. We discuss the relations between different scales in two M vacua: type I strings and Ho\ifmmode \check{r}\else \v{r}\fi{}ava-Witten supergravity models. The latter allows possibilities for an 11-dimensional scale at TeV energies with one large dimension below separating our four-dimensional world from a hidden one. Different mechanisms for breaking supersymmetry (gravity mediated, gauge mediated, and Scherk-Schwarz mechanisms) are discussed in this framework. Some phenomenological issues such as dark matter (with masses that may vary in time), origin of neutrino masses, and axion scale are discussed. We suggest that these are indications that the string scale may be lying in the ${10}^{10}--{10}^{14 }$GeV region.

Effects of standard model Kaluza-Klein excitations on electroweak observables

Abstract: The presence of an extra dimension of size $R\ensuremath{\approx}{\mathrm{TeV}}^{\ensuremath{-}1}$ introduces a tower of Kaluza-Klein gauge boson excitations that affects the standard model relations between electroweak observables. The mixing of the W and Z bosons with their excitations changes their masses and couplings to fermions. This effect depends on the Higgs field, which may live in the bulk of the extra dimension, on its boundary, or may be a combination of both types of fields. We use high-precision electroweak data to constrain $1/R$. We find limits from 1 to 3 TeV from different observables, with a model-independent lower bound of 2.5 TeV.

Electroweak and Flavor Physics in Extensions of the Standard Model with Large Extra Dimensions

Abstract: We study the implications of extra dimensions of size $R\sim 1/TeV$ on electroweak and flavor physics due to the presence of Kaluza-Klein excitations of the SM gauge-bosons. We consider several scenarios with the SM fermions either living in the bulk or being localized at different points of an extra dimension. Global fits to electroweak observables provide lower bounds on 1/R, which are generically in the 2-5 TeV range. We find, however, certain models where the fit to electroweak observables is better than in the SM, because of an improvement in the prediction to the weak charge Q_W. We also consider the case of softly-broken supersymmetric theories and we find new non-decoupling effects that put new constraints on 1/R. If quarks of different families live in different points of the extra dimension, we find that the Kaluza-Klein modes of the SM gluons generate (at tree level) dangerous flavor and CP-violating interactions. The lower bounds on 1/R can increase in this case up to 5000 TeV, disfavoring these scenarios in the context of TeV-strings.

Measurement of Neutral and Charged Current Cross-Sections in Positron-Proton Collisions at Large Momentum Transfer

Abstract: The inclusive single and double differential cross-sections for neutral and charged current processes with four-momentum transfer squared Q^2 between 150 and 30,000 GeV2 and with Bjorken x between 0.0032 and 0.65 are measured in e^+ p collisions. The data were taken with the H1 detector at HERA between 1994 and 1997, and they correspond to an integrated luminosity of 35.6 pb^-1. The Q^2 evolution of the parton densities of the proton is tested, yielding no significant deviation from the prediction of perturbative QCD. The proton structure function F_2(x,Q^2) is determined. An extraction of the u and d quark distributions at high x is presented. At high Q^2 electroweak effects of the heavy bosons Z0 and W are observed and found to be consistent with Standard Model expectation.

Electroweak constraints on extended models with extra dimensions

Abstract: Electroweak measurements place significant bounds on higher-dimensional versions of the standard model in which the gauge and Higgs fields have Kaluza-Klein excitations. These bounds may be altered quantitatively if chiral matter is also allowed to propagate in the higher-dimensional ``bulk.'' We determine the electroweak constraints on a number of models of this type, including scenarios in which only the leptons or only the first two generations of matter fields propagate in the bulk. We also consider the possibility that different factors of the electroweak gauge group may be distinguished by their bulk or three-brane assignment, and study a minimal extra-dimensional ${Z}^{\ensuremath{'}}$ model. We find typical bounds on the compactification scale between 1.5 and $4 \mathrm{TeV},$ and comment on models in which these bounds might be significantly relaxed.

Baryogenesis below the electroweak scale

Abstract: We propose a new alternative for baryogenesis which resolves a number of the problems associated with grand unified theory (GUT) and electroweak scenarios, and which may allow baryogenesis even in modest extensions of the standard model. If the Universe never reheats above the electroweak scale following inflation, GUT baryon production does not occur, and at the same time thermal sphalerons, gravitinos, and monopoles are not produced in abundance. Nevertheless, nonthermal production of sphaleron configurations via preheating could generate the observed baryon asymmetry of the Universe.

Covariant analysis of the light front quark model

Abstract: A manifestly covariant formalism is used as a guide to construct a covariant extension of the light-front quark model. Our analysis demonstrates in detail that covariance necessarily requires the inclusion of zero-mode contributions. The main goal of this paper is to use this technique in order to extend the standard light-front formalism such that all form factors that are necessary to represent the Lorentz structure of a hadronic matrix element can be calculated on the same footing. The form factors that have been calculated in the standard approach are reproduced, except for those that describe transitions that involve vector mesons. The covariant approach permits also the calculation of the scalar form factor for transitions between pseudoscalar mesons, and the form factor a−(q2) for transitions between pseudoscalar and vector mesons, which is not possible in the standard light-front formalism. The practical application of the covariant extension of the light-front quark model is successful only if the formulas for form factors are evaluated with standard light-front vertex functions. The latter violate the conditions for strict Lorentz covariance of the formalism. In order to explore the predictive power of this approach, we calculate various properties of pseudoscalar and vector mesons in the u-, d-, s-quark sector. We find good agreement with all available data for electroweak transitions.

Model independent electroweak penguins in B decays to two pseudoscalars

Abstract: We study the effects of electroweak penguin (EWP) amplitudes in B meson decays into two charmless pseudoscalars in the approximation of retaining only the dominant EWP operators Q9 and Q10. Using flavor SU(3) symmetry, we derive a set of model-independent relations between EWP contributions and tree-level decay amplitudes one of which was noted recently by Neubert and Rosner. Two new applications of these relations are demonstrated in which uncertainties due to EWP corrections are eliminated in order to determine a

Magnetohydrodynamics of the early Universe and the evolution of primordial magnetic fields

Abstract: We show that the decaying magnetohydrodynamic turbulence leads to a more rapid growth of the correlation length of a primordial magnetic field than that caused by the expansion of the Universe. As an example, we consider the magnetic fields created during the electroweak phase transition. The expansion of the Universe alone would yield a correlation length at the present epoch of 1 AU, whereas we find that the correlation length is likely of order 100 AU, and cannot possibly be longer than ${10}^{4}\mathrm{AU}$ for non-helical fields. If the primordial field is strongly helical, the correlation length can be much larger, but we show that even in this case it cannot exceed 100 pc. All these estimates make it hard to believe that the observed galactic magnetic fields can result from the amplification of seed fields generated at the electroweak phase transition by the standard galactic dynamo.

FCNC top quark decay in the MSSM: a door to SUSY physics in high luminosity colliders?

Abstract: We study the FCNC top quark decays t -> c h in the framework of the MSSM, where h= h^0,H^0,A^0 is any of the supersymmetric neutral Higgs bosons. We include the leading set of SUSY-QCD and SUSY electroweak contributions. While the FCNC top quark decay into the SM Higgs boson has such a negligible rate that will not be accessible to any presently conceivable accelerator, we find that there is a chance that the potential rates in the MSSM can be measured at the high luminosity colliders round the corner, especially at the LHC and possibly at a future LC, but we deem it difficult at the upgraded Tevatron. In view of the large SUSY-QCD effects that we find in the Higgs channels, and due to some discrepancies in the literature, we have revisited the FCNC top quark decay into gluon, t -> c g, in our framework. We confirm that the possibility of sizeable rates does not necessarily require a general pattern of gluino-mediated FCNC interactions affecting both the LH and the RH sfermion sectors -- the LH one being sufficient. However, given the present bounds on sparticle masses, the gluon channel turns out to lie just below the expected experimental sensibility, so our general conclusion is that the Higgs channels t -> c h (especially the one for the light CP-even Higgs) have the largest potential top quark FCNC rates in the MSSM, namely of order 10^-4.

Physics implications of flat directions in free fermionic superstring models. I. Mass spectrum and couplings

Abstract: From the {open_quotes}top-down{close_quotes} approach we investigate physics implications of the class of {ital D}- and {ital F}-flat directions formed from non-Abelian singlets which are proved flat to all orders in the nonrenormalizable superpotential, for a prototype quasi-realistic free fermionic string model with the standard model gauge group and three families (CHL5). These flat directions have at least an additional U(1){sup {prime}} unbroken at the string scale. For each flat direction, the complete set of effective mass terms and effective trilinear superpotential terms in the observable sector are computed to all orders in the VEV{close_quote}s of the fields in the flat direction. The {open_quotes}string selection rules{close_quotes} disallow a large number of couplings allowed by gauge invariance, resulting in a massless spectrum with a large number of exotics, in most cases excluded by experiment, thus signifying a generic flaw of these models. Nevertheless, the resulting trilinear couplings of the massless spectrum possess a number of interesting features which we analyze for two representative flat directions: for the fermion texture, baryon- and lepton-number violating couplings, {ital R}-parity breaking, non-canonical {mu} terms, and the possibility of electroweak and intermediate scale symmetry breaking scenarios for U(1){sup {prime}}. The gauge coupling predictions are obtained in themore » electroweak scale case. Fermion masses possess t-b and {tau}-{mu} universality, with the string scale Yukawa couplings {ital g} and g/{radical} (2) , respectively. Fermion textures are present for certain flat directions, but only in the down-quark sector. Baryon- and lepton-number violating couplings can trigger proton decay, N-{bar N} oscillations, leptoquark interactions and {ital R}-parity violation, leading to the absence of a stable LSP. {copyright} {ital 1999} {ital The American Physical Society}« less

The end point of the first-order phase transition of the SU(2) gauge-Higgs model on a four-dimensional isotropic lattice

Abstract: We report results of a study of the end point of the electroweak phase transition of the SU(2) gauge-Higgs model defined on a four-dimensional isotropic lattice with Nt = 2. Finite-size scaling study of Lee-Yang zeros yields λc = 0.00116(16) for the end point. Combined with a zero-temperature measurement of Higgs and W boson masses, this leads to MH,c = 68.2 ± 6.6 GeV for the critical Higgs boson mass. An independent analysis of Binder cumulant gives a consistent value λc = 0.00102(3) for the end point.