Electroproduction of η-meson at the S11 (1535) resonance

The reaction e^? + p → e^? + p + η has been studied in the region of the S₁₁ (1535)-resonance by detecting the recoil proton in coincidence with the scattered electron. The reaction has been observed at three four-momentum transfers of the virtual photon: q² = 0.2, 0.28 and 0.4 (GeV/c). First results of the differential cross section measurements are given and compared with quark model calculations.     

CompleteO(α) corrections to polarized Compton scattering

General expressions for Compton scattering and itsO(α) real and virtual corrections are given for electron and photon beams with arbitrary polarization. The expressions obtained are suitable for use in Monte Carlo simulations of Compton polarimeters planned for polarizede ⁺ e ^? colliders. The basis for the calculation is the spinor techniques recently applied to photon and gluon bremsstrahlung. The practical application of these techniques for massive fermions is discussed.     

Upper limit to the cross-section for the process νμ + e− → νμ + e−

An upper limit of 3 × 10^?42E_ν cm² per electron, at 90% confidence level, has been obtained for the cross section of the process ν_μ + e^? → ν_μ + e^?, interpreted in the usual V-A framework, for an electron recoil energy larger than 0.3 GeV. This estimate is based on the observation of one candidate event in exposures of Gargamelle to the CERN PS neutrino beam.     

Photon dispersion in a strong magnetic field

In an external magnetic field analytical properties are studied of the photon polarization tensor calculated as the electron-positron loop in the Furry picture. The polarization tensor is reexpressed as a sum over singular contributions coming from e⁺e^?-pair photocreation in semidiscrete Landau states. The solutions of the photon dispersion equation (i.e., the one for the poles of the photon propagator) are described. The shape of the photon dispersion curve obtained is responsible for the effect of photon deflection by a strong magnetic field. No physically reliable solutions, apart from spurions, are found for the longitudinal photon mode. An infinite number of solutions with complex space-momentum are found, with no apparnet ghosts among them. An attempt is made to interprete the former in terms of possible bound and quasibound states of electron and positron pairs.     

Search for elastic muon-neutrino electron scattering

One possible event of the process ν^?_μ + e^? → v^?_μ + e^? has been observed. The various background processes are discussed and the event interpreted in terms of the Weinberg theory. The 90% confidence limits on the Weinberg parameter are 0.1 < sin²θ_W < 0.6.     

Evidence for the τ → νϱπ decay mode

Annihilation of e⁺e^? into final states with a single electron has been studied with the PLUTO detector at the DORIS storage ring at CMS energies from 3.6 to 5 GeV. In the sample of 4-prong events without any detected photon we observe 21 events which we assign to the reaction e⁺e^? → τ⁺τ^? → ννe + ν?⁰π. We obtain a branching ratio for τ⁺ → ν?⁰π⁺ of 0.050 ± 0.015 with an overall systematic uncertainty of 30%. The data are consistent with the ?π coming from an A₁ meson.     

Perturbative QCD and the exclusive decay of heavy quarkonium into a photon and two mesons

We consider the radiative decays of the ³S₁ ground state Q$\text{Q}$ into a real photon and two light pseudoscalar or vector mesons. For sufficiently large mass of the heavy quark Q, the decay, e.g. ? → γππ, can be calculated in perturbative QCD, provided that the photon does not carry away a predominant fraction of the available energy. We present differential and integrated Dalitz plot distributions and, for quarkonium states produced with transverse polarization in e⁺e^? annihilation, angular distributions of the orientation of the decay triangle relative to the beam direction.     

Higher order QCD contributions to electron structure functions

We study deep inelastice ⁺ e ^? scattering with untagged throughgoing target electrons. The corresponding electron structure functions are not trivially related to photon structure functions. Both can be calculated in QCD perturbation theory but the first ones can be measured more easily. In studying next-to-leading log contributions to electron structure functions we find that the leading logQ ² approximation completely fails at present energies. This is due to different scales λ andm _e . A modified leading-log approximation is introduced. Higher order corrections to this approximation are small away fromx=0 or 1.     

Hadronic contributions to (g−2) of the leptons and to the effective fine structure constant α(M Z 2 )

The new experiment planned at Brookhaven to measure the anomalous magnetic moment of the muona _μ≡(g _μ?2)/2 will improve the present accuracy of 7 ppm by about a factor of 20. This requires a careful reconsideration of the theoretical uncertainties of theg?2 predictions, which are dominated by the error of the contribution from the light quarks to the photon vacuum polarization. This issue is cruicial also for the precise determination of the running fine structure constant at theZ-peak as LEP/SLC experiments continue to increase their precision. In this paper we present an updated analysis of the hadronic vacuum polarization using all presently availablee ⁺e^? data. This seems to be justified because previous work on the subject was based to some extent on preliminary or incomplete experimental data. Contributions from different energy ranges are presented separately forg?2 of the muon and the τ-lepton and for α(M _Z ² ). We obtain the resultsa _μ ^had* =(725±16)×10^?10 anda _τ ^had* =(351±10)×10^?8, where the asterisk indicates the dressed (renormalization group improved) value. For the effective fine structure constant atM _Z=91.1888 GeV we obtainΔα _had ⁽⁵⁾ =0.0280±0.0007 and α(M _Z ² )^?1=128.896±0.090. Further improvement in the accuracy of theoretical predictions which depend on the hadronic vacuum polarization requires more precise measurements ofe ⁺e^? cross-sections at energies below about 12 GeV in future experiments.     

Jet production in e+e− collisions to order α3 and α4 and including QCD corrections

Cross sections for e⁺e^?→jet₁ + jet₂ + X are calculated to order α³ and α⁴ in the electromagnetic coupling and to arbitrary order in α_s in the leading logarithm approximation of QCD. Physically the processes included are the emission of hard protons from the incident e^± followed by annihilation, photon-proton scattering and deep inelastic scattering on an electron. The experimental separation of the mechanisms on the basis of their different event structure is discussed.     

Lepton pair production in high-frequency laser fields

The production of electron-positron and muon-antimuon pairs in high-frequency laser fields via few-photon absorption is considered. It is assumed that an intense X-ray laser beam collides either with a relativistic ion beam or with a second, equally intense laser beam. We study the generation of free e ⁺ e ^? pairs, free μ⁺μ^? pairs, and bound-free e ⁺ e ^? pairs where in the latter case the electron is born in a low-lying atomic orbit of the projectile nucleus. Effects resulting from the finite nuclear size, the laser’s polarization state, and its magnetic field component are examined, which are testable experimentally by virtue of upcoming X-ray free-electron laser (XFEL) devices.     

Leptonic CP violation in leptoquark models

We study the leptonic CP violation effects in leptoquark models. We show that in general the leptoquark contribution to the longitudinal muon polarization effect inη→μ ⁺ μ ^? decay is very small and in a model which has no couplings induced by the leptoquark between different generations, both the electric dipole moment of electron or muon and longitudinal polarization of electron in π⁰,η→e ⁺ e ^? decays can be large.     

WWψ couplings from singleW-production in polarizede + e − collisions

Using the processeψ→Wv we study theWWψ vertex ine ⁺ e ^? collisions with polarized beams at a future 500 GeV collider. The equivalent photon approximation and its accuracy are discussed for the case of polarized electron beams. The resulting photon spectrum is then used to compute the total cross section as well as verious differential distributions for the processe ⁺ e ^?→e ⁺ W ^? v. Special emphasis is put on the effects of using polarized electron beams and their influence on the sensitivity to anomalous couplings.     

Longitudinal and transverse contributions to the threshold cross-section slope of single-pion electroproduction by a proton

The cross sections of inelastic electron scattering are measured near the threshold of single-pion production by a proton at k²=2.5, 3.5, 5.0 and 8.0 fm^? in the polarization range of the virtual photon ?=0.3–0.9. The separation of the |E₀₊|² and k²|S₀₊|² contributions to the cross-section slope at threshold is carried out and thus a new method of determining the F_A and F_π form factors is realized. The values for the axial mass and mean-square radius of the pion are obtained.     

QED polarization asymmetries fore + e − scattering due to helicity flips

The polarization asymmetries for thee ⁺ e ^? scattering with polarized incoming and outgoing beams, which are proportional to the amplitudes? ₅ describing one helicity flip and? ₅ describing two helicity flips, have been calculated including their pure QED radiative corrections. These asymmetries are partly large and can be observed well at low energies.     

e+e− → γ + Hadrons

We give the spectrum in momentum and angle of directly produced γ's in e⁺e^? → γ + hadrons, and show that in QCD if p_⊥² of the recoil hadron jet relative to the photon is large, the leading logarithmic corrections to the lowest order result are absent. This process is therefore of great value in learning whether QCD actually governs quark dynamics.     

Soft gluon corrections to large-pT direct photon and dilepton production

QCD O(α_s²) corrections to large-p_T real and virtual photon production from scattering in the quark-gluon channel are studied in the soft gluon limit. In this limit we carefully preserve the analytic structure of amplitudes in the small-momenta integration regions. The result is a rather large correction to the Born term which significantly improves the agreement with data on p + p → γ + X. The application of the approach to other processes is discussed and the dominant terms of the O(α_s²) correction to e⁺e^? → three jets are reproduced.     

Determination of the longitudinal and the transverse part in π+ electroproduction

We report on an experiment where the different contributions from the transverse and longitudinal polarization of the virtual photon are measured separately for the reaction e^?p→e^?π⁺n. The data taken above the resonance region at small ∣t∣ values in the q² range of ∣q²∣ < 0.5 GeV² show a clear dominance of the longitudinal part of the cross section and are well described by a generalized Born-term model. Using this model the electromagnetic form factor of the pion is determined. At q² = ?0.35 GeV² one gets F_π = 0.598 ± 0.021.     

Leptonic polarization asymmetries ate + e − colliders inE 6 theories

We examine the left-right and azimuthal polarization asymmetries of lepton pairs produced ine ⁺ e ^? ande ^? e ^? interactions as probes of new physics resulting fromE ₆ theories. The asymmetries are calculated and compared for the processese ⁺ e ^?→µ ⁺ µ ^?,e ⁺ e ^?→e ⁺ e ^?,e ^? e ^?→e ^? e ^?, ande ⁺ e ^?→L ⁺ L ^? whereL is either anE ₆ exotic, mirror, or fourth generation heavy lepton. Our results show that lepton polarization asymmetries can be used to easily distinguish differentE ₆ models and can be used to distinguish among possible new heavy leptonsL.     

Electroweak interference effects in the high energye ++e −→e ++e −+hadrons process

We examine interference effects of the electrowcak interaction in the high energye ⁺+e ^?→e ⁺ +e ^?+hadrons process. The effect provides a further test of the G-W-S model [1] and may lead to a better understanding of the photon structure functions.