Fermion-antifermion Pair Research Articles

Recent results on Sudakov logarithms in electroweak processes

Recent results for the asymptotic behavior of fermion scattering amplitudes in the Sudakov limit are discussed including next-to-leading logarithmic (NLL) and NNLL corrections. These are used for the analysis of the dominant electroweak corrections to the fermion-antifermion pair production in e + e − annihilation at high energy. The importance of angular dependent as well as subleading terms in emphasized.

Sudakov logarithms in four-fermion electroweak processes at high energy

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.

Phase diagram of compact QED coupled to a four-Fermi interaction

Compact lattice quantum electrodynamics with four species of fermions is simulated with massless quarks by adding a four-Fermi interaction to the action. Simulations in the chiral limit are done on ${8}^{4}$ lattices for exploratory purposes, and ${16}^{4}$ lattices for quantitative purposes, and the phase diagram, parametrized by the gauge and the four-Fermi couplings, is mapped out. The line of monopole condensation transitions is separate from the line of chiral symmetry restoration as long as the four-Fermi coupling is not too small. The simulation results indicate that the monopole condensation transition is first order while the chiral transition is second order. The challenges in determining the universality class of the chiral transition are discussed. If the scaling region for the chiral transition is sufficiently wide, the ${16}^{4}$ simulations predict critical indices far from mean field values. We briefly discuss a speculative scenario in which antiscreening provided by strands of bound monopole and antimonopole loops is the agent that balances the screening of fermion-antifermion pairs to produce ultraviolet stable fixed points.

Chargino and neutralino decays revisited

We perform a comprehensive analysis of the decays of charginos and neutralinos in the Minimal Supersymmetric Standard Model where the neutralino $\chi_1^0$ is assumed to be the lightest supersymmetric particle. We focus, in particular, on the three-body decays of the next-to-lightest neutralino and the lightest chargino into the lightest neutralino and fermion-antifermion pairs and include vector boson, Higgs boson and sfermion exchange diagrams, where in the latter contribution the full mixing in the third generation is included. The radiative corrections to the heavy fermion and SUSY particle masses will be also taken into account. We present complete analytical formulae for the Dalitz densities and the integrated partial decay widths in the massless fermion case, as well as the expressions of the differential decay widths including the masses of the final fermions and the polarization of the decaying charginos and neutralinos. We then discuss these decay modes, in particular in scenarios where the parameter $\tan \beta$ is large and in models without universal gaugino masses at the Grand Unification scale where some new decay channels, such as decays into gluinos and $q\bar{q}$ pairs, open up.

Logarithmic supersymmetric electroweak effects on four-fermion processes at TeV energies

We compute the minimal supersymmetry model one-loop contributions to the asymptotic energy behavior of fermion-antifermion pair production at future lepton-antilepton colliders. In addition to the conventional logarithms of renormalization group origin, extra supersymmetry linear logarithmic terms appear of ``Sudakov-type.'' In the TeV range their overall effect on a variety of observables can be quite relevant and drastically different from that obtained in the standard model case.

NONPERTURBATIVE DISPERSIVE SECTOR IN STRONG (QUASI-)ABELIAN FIELDS

In strong (quasi-)Abelian fields, even at the one-loop level of the coupling constant, quantum fluctuations of fermions induce an effective Lagrangian density whose imaginary (absorptive) part is purely nonperturbative and known to be responsible for the fermion–antifermion pair creation. On the other hand, the induced real (dispersive) part has perturbative and nonperturbative contributions. In the one-loop case, we argue how to separate the two contributions from each other for any strength of the field. We show numerically that the nonperturbative contributions are in general comparable with or larger than the induced perturbative ones. We arrive at qualitatively similar conclusions also for the induced energy density. Further, we investigate numerically the quasianalytic continuation of the perturbative results into the nonperturbative sector, by employing (modified) Borel–Padé. It turns out that in the case at hand, we have to integrate over renormalon singularities, but there is no renormalon ambiguity involved.

Subleading Sudakov logarithms in electroweak processes

Recent results for the asymptotic behavior of fermion scattering amplitudes in the Sudakov limit are presented including next-to-leading logarithmic corrections. These are used for the analysis of the dominant electroweak corrections to the fermion-antifermion pair production in e + e −annihilation at high energy.

Nonperturbative dispersive sector in strong (quasi-)Abelian fields

In strong (quasi-)Abelian fields, even at the one-loop level of the coupling constant, quantum fluctuations of fermions induce an effective Lagrangian density whose imaginary (absorptive) part is purely nonperturbative and known to be responsible for the fermion-antifermion pair creation. On the other hand, the induced real (dispersive) part has perturbative and nonperturbative contributions. In the one-loop case, we argue how to separate the two contributions from each other for any strength of the field. We show numerically that the nonperturbative contributions are in general comparable with or larger than the induced perturbative ones. We arrive at qualitatively similar conclusions also for the induced energy density. Further, we investigate numerically the quasianalytic continuation of the perturbative results into the nonperturbative sector, by employing (modified) Borel-Pade. It turns out that in the case at hand, we have to integrate over renormalon singularities, but there is no renormalon ambiguity involved.

STRUCTURE OF THE PHOTON

The photon is of course the gauge boson of QED, and is as far as we know elementary. When we measure photon “structure”, what we are in fact doing is probing the quantum fluctuations of the field theory. The photon couples - via a splitting into virtual, charged fermion-antifermion pairs - into the electroweak and strong interactions. Such behaviour is an important aspect of quantum field theories, and similar phenomena arise in many different situations. For example, the gluon splitting to quarks drives the scaling violations in hadronic structure. Studies of photon structure test our understanding of this behaviour. In this talk I give an overview of the current data and phenomenology, and outline some opportunities available in the medium term future.

Bounds on universal new physics effects from fermion-antifermion production at LEP2

We consider lepton-antilepton annihilation into a fermion-antifermion pair at variable c.m. energy. We propose for this process a simple parametrization of the virtual effects of the most general model of new physics of universal type. This parametrization is based on a recent approach, that uses the experimental results of LEP1, SLC as theoretical input. It introduces three functions whose energy dependence is argued to be smooth and, in first approximation, negligible. A couple of representative models of new physics are considered, as a support of the previous claim. Explicit bounds are then derived for this type of new physics from the available LEP2 data, and a discussion is given of the relevance in this respect of the different experimental measurements. The method is then extended to treat the case of two particularly simple models of non universal type, for which it is possible to draw analogous conclusions.

Summing up subleading Sudakov logarithms

We apply the strategy of regions within dimensional regularization to find functions involved in evolution equations which govern the asymptotic dynamics of the Abelian form factor and four-fermion amplitude in the SU(N) gauge theory in the Sudakov limit up to the next-to-leading logarithmic approximation. The results are used for the analysis of the dominant electroweak corrections to the fermion-antifermion pair production in the $e^+e^-$ annihilation at high energy.

BrokenRparity contributions to flavor changing rates andCPasymmetries in fermion pair production at leptonic colliders

We examine the effects of the R parity odd renormalizable interactions on flavor changing rates and $\mathrm{CP}$ asymmetries in the production of fermion-antifermion pairs at leptonic (electron and muon) colliders. In the reactions ${l}^{\ensuremath{-}}{+l}^{+}\ensuremath{\rightarrow}{f}_{J}+{f}_{{J}^{\ensuremath{'}}} (l=e,$ \ensuremath{\mu}; $J\ensuremath{\ne}{J}^{\ensuremath{'}})$ the produced fermions may be leptons, down quarks, or up quarks, and the center of mass energies may range from the Z-boson pole up to $1000$ GeV. Off the Z-boson pole, the flavor changing rates are controlled by tree level amplitudes and the $\mathrm{CP}$ asymmetries by interference terms between tree and loop level amplitudes. At the Z-boson pole, both observables involve loop amplitudes. The lepton number violating interactions, associated with the coupling constants ${\ensuremath{\lambda}}_{\mathrm{ijk}},$ ${\ensuremath{\lambda}}_{\mathrm{ijk}}^{\ensuremath{'}},$ are only taken into account. The consideration of loop amplitudes is restricted to the photon and Z-boson vertex corrections. We briefly review flavor violation physics at colliders. We present numerical results using a single, species and family independent, mass parameter $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{m}$ for all the scalar superpartners and considering simple assumptions for the family dependence of the R parity odd coupling constants. Finite nondiagonal rates $(CP$ asymmetries) entail nonvanishing products of two (four) different coupling constants in different family configurations. For lepton pair production, the Z-boson decays branching ratios ${B}_{{\mathrm{JJ}}^{\ensuremath{'}}}=B(\stackrel{\ensuremath{\rightarrow}}{Z}{l}_{J}^{\ensuremath{-}}{+l}_{{J}^{\ensuremath{'}}}^{+})$ scale in order of magnitude as ${B}_{{\mathrm{JJ}}^{\ensuremath{'}}}\ensuremath{\approx}(\ensuremath{\lambda}{/0.1)}^{4}(100\mathrm{GeV}/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{m}{)}^{2.5}{10}^{\ensuremath{-}9},$ with coupling constants $\ensuremath{\lambda}={\ensuremath{\lambda}}_{\mathrm{ijk}}$ or ${\ensuremath{\lambda}}_{\mathrm{ijk}}^{\ensuremath{'}}$ in appropriate family configurations. The corresponding results for d- and u quarks are larger, due to an extra color factor ${N}_{c}=3.$ The flavor nondiagonal rates, at energies well above the Z-boson pole, slowly decrease with the center of mass energy and scale with the mass parameter approximately as ${\ensuremath{\sigma}}_{{\mathrm{JJ}}^{\ensuremath{'}}}\ensuremath{\approx}(\ensuremath{\lambda}{/0.1)}^{4}(100\mathrm{GeV}/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{m}{)}^{2\ensuremath{-}3}(1\ensuremath{-}10)\mathrm{fbarn}.$ Including the contributions from an sneutrino s-channel exchange could raise the rates for leptons or d quarks by one order of magnitude. The $\mathrm{CP}$-odd asymmetries at the Z-boson pole, ${\mathcal{A}}_{{\mathrm{JJ}}^{\ensuremath{'}}}{=(B}_{{\mathrm{JJ}}^{\ensuremath{'}}}\ensuremath{-}{B}_{{J}^{\ensuremath{'}}J}{)/(B}_{{\mathrm{JJ}}^{\ensuremath{'}}}{+B}_{{J}^{\ensuremath{'}}J}),$ vary inside the range ${(10}^{\ensuremath{-}1}\ensuremath{-}{10}^{\ensuremath{-}3})\mathrm{sin}\ensuremath{\psi},$ where $\ensuremath{\psi}$ is the $\mathrm{CP}$-odd phase. At energies higher than the Z-boson pole, $\mathrm{CP}$-odd asymmetries for leptons, d-quark and u-quark pair production lie approximately at ${(10}^{\ensuremath{-}2}\ensuremath{-}{10}^{\ensuremath{-}3})\mathrm{sin}\ensuremath{\psi},$ irrespective of whether one deals with light or heavy flavors.

SU(3) gauge theory with adjoint fermions

We analyze the finite temperature phase diagram of QCD with fermions in the adjoint representation. The simulations performed with four dynamical Majorana fermions, which is equivalent to two Dirac fermions, show that the deconfinement and chiral phase transitions occur at two distinct temperatures, T chiral ⋍ 6.65 T deconf. While the deconfinement transition is first order we find evidence for a continuous chiral transition. We also present potentials for T < T deconf and T deconf < T < T chiral both for fundamental and adjoint fermion-antifermion pairs.

Measurement of W-pair cross sections in e +e − interactions at [formula omitted] and W-decay branching fractions

We report on the measurement of W-boson pair-production with the L3 detector at LEP. In a data sample corresponding to a total luminosity of 55.47 pb −1 collected at an average centre-of-mass energy of 182.68 GeV , we select 824 four-fermion events with pairs of hadronic jets or pairs of leptons with high invariant masses. Branching fractions of W decays into different fermion-antifermion pairs are determined. Assuming charged-current lepton universality, the branching fraction for hadronic W decays is measured to be: B( W→hadrons)=70.1±1.3 (stat.)±0.4 (syst.) % . Combining all final states the total cross section for W-pair production is measured to be: σ WW =16.53±0.67 (stat.)±0.26 (syst.) pb.

Search for the standard model Higgs boson in ${\rm e^+e^-}$ collisions at $\sqrt s = 161$ –172 GeV

This paper describes a search for the Standard Model Higgs boson using data from \({\rm e^+e^-}\) collisions collected at center-of-mass energies of 161, 170 and 172 GeV by the OPAL detector at LEP. The data collected at these energies correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb\(^{-1}\), respectively. The search is sensitive to the main final states from the process in which the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electron, muon or tau leptons. One candidate event is observed, in agreement with the Standard Model background expectation. In combination with previous OPAL searches at center-of-mass energies close to the Z\(^0\) resonance and the revised previous OPAL searches at 161 GeV , we derive a lower limit of 69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence level.

Two-loop corrections to the electromagnetic vertex for energies close to threshold

Two-loop contributions to the electromagnetic form factors are calculated in the kinematic regime close to the fermion-antifermion threshold. The results are presented in an expansion in the velocity $\beta$ of the fermions in the c.m. frame up to next-to-next-to leading order in $\beta$. The existence of a new Coulomb singularity logarithmic in $\beta$, which is closely related to the $O(\alpha^2\ln\alpha)$ corrections known from positronium decays, is demonstrated. It is shown that due to this Coulomb singularity $O(\alpha^2)$ relativistic corrections to the non-relativistic cross section of heavy fermion-antifermion pair production in $e^+e^-$ annihilation cannot be determined by means of conventional multi-loop perturbation theory.

Axion decay in a strong magnetic field

The axion decay into charged fermion-antifermion pair $a ~\to f ~\tilde f$ is studied in external crossed and magnetic fields. The result we have obtained can be of use to re-examine the lower limit on the axion mass in case of possible existence of strong magnetic fields at the early Universe stage.

Measurement of W-pair cross sections in e+e− interactions at =172 and W-decay branching fractions

Abstract We report on the measurement of W-boson pair-production with the L3 detector at LEP at an average centre-of-mass energy of 172.13 GeV. In a data sample corresponding to a total luminosity of 10.25 pb−1 we select 110 four-fermion events with pairs of hadronic jets or leptons with high invariant masses. Branching fractions of W decays into different fermion-antifermion pairs are determined with and without the assumption of charged-current lepton universality. The branching fraction for hadronic W decays is measured to be: B(W → hadrons) = 64.2−3.8+3.7 (stat.) ± 0.5 (syst.) %. Combining all final states the total cross section for W-pair production is measured to be: σWW = 12.27−1.32+1.41 (stat.) ± 0.23 (syst.) pb. The results are in good agreement with the Standard Model.

Search for the Standard Model Higgs boson in e +e − collisions at [formula omitted

This letter describes a search for the Standard Model Higgs boson using data from e +e − collisions collected at a centre-of-mass energy of 161 GeV by the OPAL detector at LEP. The ata correspond to an integrated luminosity of 10.0 pb −1. The search is sensitive to the main final states from the process where the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electrons, muons or tau leptons. Two candidate events are observed, in agreement with Standard Model background expectations. In combination with previous OPAL searches at centre-of-mass energies close to the Z 0 resonance, we derive a lower limit of 65.0 GeV for the mass of the Standard Model Higgs boson, at the 95% confidence level.

Fluctuons—III. Gravity

The fluctuon principle provides a model of gravity. Mass and field are interpreted in terms of density depression in the full sea of vacuum fermions. Gravitons are interpreted as self-perpetuating chains of transient fermion-antifermion pairs (propagating gravitational fluctuons) and gravitational waves as compression-expansion waves of vacuum density. The direct gravitational interaction between absorbing particles is repulsive (whether the absorbers are permanent positive energy particles or trapped fluctuons). The attractive gravitational interaction arises from the ‘pushing’ effect of propagating gravitational fluctuons originating in the low mass regions of space, where the density of trapped fluctuons is highest. The model implies the principle of equivalence and through this the general relativistic equations of gravitation.