F. Schrempp

Constraints on anomalous WWγ and WWγγ couplings at a 500 GeV linear e+e− collider☆

Abstract We examine the potential of a future 500 GeV linear e + e − collider (NLC) to probe the “minimal” set ( ϰ γ , λ γ ) of anomalous WW γ and WW γγ couplings via the (sub) processes γ →W + W − , e − γ →W − v and e + e − →W + W − , for comparison. Photon beams both from classical Bremsstrahlung and, notably, from backscattering laser light off e ± beams are considered. The differential cross sections d σ /d cos θ of the three reactions are calculated analytically and used as observables in a χ 2 analysis under identical assumptions on machine parameters. The constraints emerging from the use of laser photon beams appear very encouraging, are independent of ϰ Z and λ Z and impressively underline the importance of realizing laser photon beams at the NLC.

Instantons in deep-inelastic scattering: The simplest process

Instanton calculations in QCD are generically plagued by infrared divergencies associated with the integration over the instanton size ϱ. Here, we demonstrate explicitly that the typical inverse hard momentum scale Q−1 in deep-inelastic scattering provides a dynamical infrared cutoff for the size parameter ϱ. Hence, deep-inelastic scattering may be viewed as a distinguished process for studying manifestations of QCD instantons. For clarity, we restrict the explicit discussion to the simplest chirality-violating process, y* + g → qL + qR. We calculate the corresponding fixed-angle cross section as well as the contributions to the gluon structure functions, F2g andFLg, within standard instanton perturbation theory in leading semi-classical approximation. To this approximation, fixed-angle scattering processes at high Q2 are reliably calculable. In the Bjorken limit, the considered instanton-induced process gives a scaling contribution to F2g(x, Q2) and the Callan-Gross relation holds.

Finite-energy sum rules and the reactions ee → eeϵ (750) and ee → eef (1260)

Abstract The ϵγγ and f γγ coupling constants are calculated from finite-energy sum rules for γπ → πγ . The cross sections for ϵ and f production in colliding electron beam reactions are found to be significant. There is reasonable agreement with the tensor meson dominance estimates for the f γγ couplings.

High-energy reactions seen from the s-channel: a complex pole in the impact parameter plane

Abstract An s -channel model for inelastic reactions at high energies is presented. The scattering amplitude is split up into the contribution from the right-hand cut ( T r.h.c. ) and that from the left-hand cut ( T l.h.c. ) in a fixed- t dispersion relation; T r.h.c. is assumed to be dominated by the peripheral s -channel resonances associated with a complex pole b 0 ( s ) in the impact-parameter plane. It yields the rotating-phase part of the amplitude; T l.h.c. is assumed to be the analytic continuation of the analogous term in the u -channel. It provides a real, non-peripheral background in the s -channel. The predictions of this model for the t -dependence of high-energy amplitudes are confronted with experiment. A very good agreement is found in all cases. Moreover, we present an analytic expression for the complex pole trajectory b 0 ( s ), which agrees with the experimentally determined one and with analyticity requirements. This crucially interlocks the two terms T r.h.c. (being a function of b 0 ( s )) and T l.h.c. (being a function of b 0 ( u )).

Composite scalars in e+e- collisions and radiative Z decays

Abstract Starting point is the hypothesis that the observed Z →e + e − γ decays are mediated by a (composite) spin 0 boson X with 40≲ m X ≲50 GeV. The consequences for e + e − →e + e − , e + e − → γγ and e + e − → hadrons at PETRA are explored. PETRA experiments turn out to be sensitive up to masses m X ∼50 GeV; the best indicator for m X ≳ 48 GeV is the angular distribution of Bhabha scattering.

Zooming-in on Instantons at HERA

Abstract In view of the intriguing, preliminary search results for instanton-induced events at HERA from the H1 Collaboration, some important remaining theoretical issues are discussed. Notably, the question is addressed, to which extent the H1 analysis may be directly compared to our original predictions from instanton-perturbation theory, since certain fiducial cuts are lacking in the H1 data. Various theoretical uncertainties are evaluated and their impact on the observed excess is discussed. An improved understanding of the experimental findings along with an encouraging over-all agreement with our original predictions seems to emerge.

A confining SU(2)L × SU(2)R gauge model of the weak interactions

With the aim of understanding weak interactions as residual hypercolor interactions among composite quarks and leptons, we investigate a confining SU(2)L × SU(2)R hypercolor gauge theory with only fundamental fermions (i.e. without fundamental scalars). A scenario with two different confinement scales, ΛL ∼ GF−12 ∼ 300 GeV and ΛR > ΛL, is systematically analyzed as a function of the parameter ξ = ΛR/ΛL. The general requirement of anomaly saturation is combined with effective lagrangian techniques—a powerful procedure also of interest for more general confining product groups. Two solutions, separated by a phase boundary, emerge. Phase I for 1 ⩽ ξ ξcrit: only left-handed massless composite quarks and leptons (of the Abbott-Farhi type). Phase II results from separate anomaly saturation at the two scales ΛR and ΛL, Phase I from joint anomaly saturation (matching). Dynamical insight comes from treating confinement in two steps, at ΛR and ΛL consecutively, which amounts to a complementary version of the technicolor mechanism; it necessarily entails the appearance of Goldstone bosons in the intermediate momentum range, Λ < p < ΛR, which play the role of dynamical scalars being subject to SU(2)L confinement.

Unitarized finite energy sum rules and their application to π N-charge exchange

Abstract The problem of ‘unitarizing’ finite energy sum rules (FESR) by the inclusion of Regge cuts is investigated. Instead of considering Regge cuts in FESR for the full scattering amplitude T fi , new FESR are proposed for that part of T fi , which has just pure Regge pole asymptotic behavior if the Regge cuts are included in T fi . The Regge cuts in the high-energy region are evaluated according to the ‘weak-cut’ version of the Reggeized K -matrix model. The new FESR then look like the conventional ones, however, with T fi replaced by the corresponding two-particle K -matrix element. With these K -matrix FESR the serious difficulties arising in conventional approaches to the problem disappear. With the new K -matrix FESR a detailed analysis of πN-charge exchange is performed. The K -matrix FESR turn out to be satisfied with only the ϱ pole on the high energy side much better and more locally than the corresponding conventional FESR. The ϱ residues in the K -matrix are predicted to choose ‘nonsense’ at α ϱ = 0. If reduced ϱ residues are defined as in the Regge limit of a πN-Veneziano formula, they are found to be practically constant between t = m ϱ 2 and t ≈ −0.4 (GeV/ c ) 2 . The values agree well with those known at t = m ϱ 2 and with results from high-energy fits.

QCD-INSTANTONS AND SATURATION AT SMALL X

We briefly report on the contribution of QCD-instantons to the phenomenon of saturation in deep-inelastic scattering (DIS) at small Bjorken-x. The explicitly known instanton gauge field serves as a concrete realization of an underlying non-perturbative saturation mechanism associated with strong classical fields. Two independent strategies were pursued, with consistent results: On the one hand, an approach starting from instanton-perturbation theory was considered and on the other hand, the scattering of a Wilson loop in an instanton gauge field background. In both cases, the conspicuous, intrinsic instanton size scale ~ 0.5 fm, as known from the lattice, turns out to determine the saturation scale.

An estimate for the Higgs mass

Abstract The high energy behaviour of weak interactions is assumed to be softened (by some unspecified strong interactions) to the extent that the W L W L → W L W L amplitude with “exotic” t -channel isospin I t = 2 satisfies a superconvergence sum rule. At low energies, E ≲ ( 2 G F ) −1 2 , its saturation in terms of the standard model tree amplitude turns out to work amazingly well, determining the only unknown parameter, the Higgs mass, to m H ⋍ 2.3 m W ⋍ 190 GeV.