Experimental limits on the strength of weak neutral currents in lepton pair production at PETRA energies

The processes e⁺e^? → e⁺e^? and μ⁺μ^? have been studied at PETRA using the JADE detector. The data, which were collected at s-values of up to 1300 GeV² have been analysed in terms of an electro-weak extension of QED to obtain values for the weak vector and axial vector couplings in the lepton sector. The values obtained agree with the predictions of the standard Salam-Weinberg model and the data are further analysed in terms of this model to obtain the limits 0.10 < sin²?_w < 0.40 (68% CL). The mass of the neutral weak gauge boson is deduced to be greater than 51 GeV/c².     

Search for neutral MSSM Higgs bosons at LEP

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.     

Improved measurements of the neutral current from hadron and lepton production at LEP

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine ⁺ e ^– collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ ⁰ peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ ⁰ parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.     

Search for Yukawa Production of a light neutral Higgs Boson at LEP

Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in the mass range of 4–12 GeV has been performed in the Yukawa process , using the data collected by the OPAL detector at LEP between 1992 and 1995 in collisions at about 91 GeV centre-of-mass energy. A likelihood selection is applied to separate background and signal. The number of observed events is in good agreement with the expected background. Within a CP-conserving 2HDM type II model the cross-section for Yukawa production depends on and for the production of the CP-odd A and the CP-even h, respectively, where is the ratio of the vacuum expectation values of the Higgs doublets and is the mixing angle between the neutral CP-even Higgs bosons. From our data 95% C.L. upper limits are derived for within the range of 8.5 to 13.6 and for between 8.2 to 13.7, depending on the mass of the Higgs boson, assuming a branching fraction into of 100%. An interpretation of the limits within a 2HDM type II model with Standard Model particle content is given. These results impose constraints on several models that have been proposed to explain the recent BNL measurement of the muon anomalous magnetic moment. Received: 1 November 2001 / Published online: 22 February 2002     

Precision neutral current asymmetry parameter measurements from the Tau polarization at LEP

Measurements of the lepton polarization and forward-backward polarization asymmetry near the Z resonance using the OPAL detector are described. The measurements are based on analyses of and decays from a sample of 144,810 candidates corresponding to an integrated luminosity of 151 pb. Assuming that the lepton decays according to V–A theory, we measure the average polarization near to be and the polarization forward-backward asymmetry to be , where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: These measurements are consistent with the hypothesis of lepton universality and combine to give . Within the context of the Standard Model this combined result corresponds to . Combing these results with those from the other OPAL neutral current measurements yields a value of . Received: 5 March 2001 / Published online: 6 July 2001     

Effects of new neutral currents at linear electron-positron colliders

Effects that are induced by contact four-fermion interactions in the processes e ⁺ e ^? → μ ⁺ μ ^?, $\bar bb$ , and $\bar cc$ at $\sqrt s = 0.5$ TeV linear electron-positron colliders are investigated for the case of longitudinally polarized initial beams. This analysis employs new integrated observables constructed from the polarized cross sections for the scattering of final fermions into the forward (σ _F) and the backward (σ _B) hemisphere in such a way that they single out the helicity cross sections for the processes in question. This property of the observables makes it possible to perform, in the most general form, a model-independent analysis of contact four-fermion interactions and to set constraints on their parameters. It is also shown that the sensitivity of new polarization observables to contact interactions is noticeably higher than the corresponding sensitivity of canonical observables like σ, A _FB, A _LR, and A _LR,FB.     

Possible structures of the neutral current within a gauge theory framework

We examine a class of gauge theories based on U(1)×SU(2)×G allowing for an arbitrary number of gauge bosons, while retaining the lowq ² four fermion interaction of the standard model. Measurable consequences fore ⁺ e ^?→μ ⁺ μ ^? ande ⁺ e ^?→e ⁺ e ^? at presently available as well as LEP energies are presented. Implications of the recently determined QED cut-offΛ _? ? 100 GeV on gauge boson properties and the anomalous magnetic moment of the muon are pointed out.     

Production of neutral pseudo-Goldstone bosons at LEP II and NLC in multiscale walking technicolor models

Walking technicolor (WTC) models predict the existence of heavy neutral pseudo-Goldstone bosons (PGBs), whose masses are typically expected to be larger than 100 GeV. In this paper, we investigate the production and decay of these particles at the high energy e⁺e⁻ experiments, LEP II and NLC.We find that, in WTC models, the production of neutral PGBs can be significantly enhanced, by one or two orders of magnitude, with respect to the predictions of traditional (QCD-like) TC models. The origin of such an enhancement is the existence of several low energy TC scales, that are likely to appear in WTC theories. This could allow the PGBs to be observed even at the energy and luminosity of the LEP II experiment. At LEP II, the PGBs are expected to be produced in the e⁺e⁻ → Pγ channel, and, possibly, in the e⁺e⁻ → Pe⁺e⁻ channel, with a total rate that can be of the order of several tenths per year. Due to the typical large values of PGB masses, the relative branching ratios of PGB decays, in WTC theories, are different from those predicted in traditional TC models. In particular, a large fraction of these decays can occur in the P → γγ channel. In considering the PGB production, at LEP II, we find that, in most of the final states, the distinctive signatures of WTC events should allow the Standard Model background to be reduced to a negligible level. We also find that, at a 500 GeV NLC experiment, the production of neutral PGBs can occur in several channels, and can be of the order of 10³ events per year. Instead, when we consider traditional TC models, we find that no PGB are typically predicted to be observed, both at LEP II and the NLC experiment.     

Searching for supersymmetry at LEP/LHC

For theep collider at LEP/LHC, we study the production and the signal coming from sleptons and squarks for several processes. We show how the supersymmetric signal can be extracted from the Standard Model background processes. In particular the large backgrounds due to singleW andZ production are considered. All the results are based on exact calculations of the matrix elements at tree level. Total cross sections and various kinematical distributions are presented.     

Tests of QED and elektroweak theories at PETRA

We review experimental results from PETRA on the reactions:e ⁺ e ^?→e ⁺ e ^?, μ⁺ μ^?, τ⁺ τ^?, and γγ at centre-of-mass energies up to 36.7 GeV. The data are compared with the predictions of quantum electrodynamics and electroweak theories. The implication of the total cross section measurements fore ⁺ e ^?→q \(\bar q\) →hadrons with respect to the weak mixing angle is discussed.     

Dimuon events and neutral currents

An apparent anomalous μ⁺-production, νN → μ⁺ + anything, would be observed as one of the characteristic effects of neutral current. A charm changing neutral current enhances this single muon (μ⁺) production. The μ⁺-events are caused by the same mechanism as the dimuon events, which are assumed to be produced by the semileptonic decay of charmed hadrons.     

Gauge theories and neutral currents

We consider neutral current effects in some gauge theoretic models of weak and electromagnetic interactions. Asymptotic invariance under the gauge group is proposed and used for estimating deep-inelastic cross sections. Our analysis underlines the need for a systematic search for hadronic threshold effects in the coupling of neutral currents.     

Implications of e+e- → μ+μ-, e+e- at PETRA/PEP energies on electroweak theories

The implication of measurements of μ pair production and Bhabha scattering at PETRA/PEP energies on electroweak theories are analysed in terms of single gauge boson as well as multi weak boson theories.     

Weak neutral currents and neutrino transport

Transport coefficients are evaluated for a mixture of neutrinos and complex nuclei with neutral current weak interaction as proposed by Freedman. The transport properties of this system are particularly relevant to the neutrino transport supernova model.