K. Hagiwara

Helicity Amplitudes for Heavy Lepton Production in e+ e- Annihilation

Abstract Signatures of new heavy lepton pair production in e+e− annihilation at TRISTAN/SLC/LEP energies are studied in detail. Complete helicity amplitudes for 2 → 6 process e + e − → L L → (ν L f 1 f 2 )( ν L f 3 f 4 ) are given for arbitrary masses of final fermions and for arbitrary vector and axial vector couplings. Methods to measure that L and νL masses, and the neutral- and charged-current couplings of L in terms of four-jet and one-lepton-dijet final state distributions are exemplified. Signatures of heavy neutrino-pair production are discussed briefly. A straightforward method for calculating arbitrary tree amplitudes with external fermions and vector bosons of arbitrary masses is presented for completeness.

τ polarization measurements at LEP and SLC

Abstract A precision determination of sin 2 θ w at LEP and SLC is based on observations of e − e + →τ − τ + using τ→πv decays to determine the τ polarization. We describe how the accuracy can be significantly improved by (i) observing the decays τ→ϱv and τ→a 1 v with the subsequent ϱ→ππ and a 1 →πππ decays acting as polarization analysers, and (ii) using polarization correlations measurable in the double (τ − τ + ) decay distribution.

Improved predictions for g-2 of the muon and \alpha_{\rm QED}(M_Z^2)

We update the Standard Model predictions of the anomalous magnetic moment of the muon, a_\mu = (g-2)/2, and the value of the QED coupling at the Z-boson mass, incorporating the new e^+e^- \to \pi\pi data obtained by CMD-2 and KLOE, as well as the corrected SND data, and other improvements. The prediction for a_\mu=11659180.4 (5.1) \times 10^{-10} is about 3 \times 10^{-10} lower than before, and has a smaller uncertainty, which corresponds to a 3.4\sigma deviation from the measured value. The prediction for the QED coupling is \alpha(M_Z^2)^{-1}= 128.937 \pm 0.030.

Exclusive semileptonic B-meson decays

Abstract We study the semileptonic processes B → D l ν and B → D ∗ l ν and show that the invariant hadronic form factors describing the decays can be measured directly by observing the angular correlations of the decay products. We emphasize that this allows an almost model-independent determination of the Vcb quark mixing-matrix element. We examine the theoretical models for the form factors in terms of the spectator quark approach. We present a general formalism for semileptonic decays which includes lepton mass effects, since the decay into τ-leptons may be important as background events in the search for rare decay modes involving missing particles.

Single weak boson production at TeV e+ e- colliders

We present exact tree level matrix elements for all the single weak boson production processes to order e3 in very high energy e+e− collisions; e+e− → e± v(−)cW∓, e+e−Z, vcvcZ and γγZ. The electron mass is kept finite so that the differential and total cross sections are accurate up to terms of order me2/s. The weak boson distributions are compared with those obtained by the equivalent photon/electron approximation (EPA), which works reasonably well at high vector boson transverse momenta but fails at small transverse momenta for the processes e+e−→ e+e−Z and γγZ. A simple modification of the EPA formulae is found to give a reasonable overall description of the distributions for all the single weak boson production processes. Electron helicity-flip amplitudes are found to contribute at the percent level at most, which can be reproduced accurately by a novel collinear approximation with equivalent real photon or electron distributions in an electron. Interference effects between the annihilation and non-annihilation diagrams are found to be significant only below the weak boson pair production threshold. Also explained is a straightforward method to incorporate the leptonic and parton level hadronic decays of the weak bosons into the production amplitudes.

J/ψ production from gluon jets at LEP

Abstract We calculate the cross section for J/ψ production from gluon jets at LEP via the mechanisms g→ψgg, g→χ c g and g→b b →ψX. We predict about 230 such J/ψ events for 10 6 hadronic decays of the Z, and we show the resulting J/ψ energy and angular distributions.

Higgs boson production in eγ collisions

Abstract A large number of W bosons can be produced in high energy e + e − linear colliders through the subprocess γ + e → W + v , with the photons generated by back-scattering of laser light. We have explored to which extent this process can be exploited to produce Higgs bosons through emission from the W line: γ + e → W + H + v . For very high energies, photoproduction of Higgs bosons on a W-boson target, γ + W → W + H , is the dominant fundamental subprocess.

Properties of heavy quarkonia and related states

Mass levels and widths of very heavy quarkonia and similar onium resonances of squarks, color octet and sextet constituents are studied in the mass range of 50 to 1000 GeV. The heavy quark potential with QCD two-loop improved short-distance behavior that gives a good description of the charmonium and bottomonium spectra is used, and the results are given for several values of the QCD scale parameter ΛMS(4) in the range between 0.1 and 0.4 GeV. We present parametrizations for the first ten S-wave quarkonium resonances, which are relevant if the top quark is relatively light (mt≲125 GeV) or if a heavy fourth-generation quark with suppressed weak decay rates exists. We also discuss the onset of coulombic behavior and the possible effects of Higgs boson exchange on superheavy quarkonia.

Helicity amplitude analysis of B → D∗lv decays

Abstract We show that measurements of the decay angular distributions of the process B → D ∗ l v → D π l v allow a complete determination of the B → D ∗ helicity amplitudes. We give model predictions, and emphasize that this process can potentially yield a reliable determination of the Vcb quark mixing matrix element.

Tau pair polarisation correlations as a signal for Higgs bosons

Abstract We show how the polarisation correlations of τ − τ + pairs may be used to give information on the spin of parent boson, and hence to identify scalar boson H→τ − τ + events from vector boson Z→τ − τ + decays.