Kaoru Hagiwara

Probing the weak boson sector in e+e−→W+W−

A detailed analysis of W pair production in e+e− annihilation at LEP-II energies is presented by using helicity amplitudes for the process e+e−→W+W− with arbitrary WWγ and WWZ couplings. Expressing the complete angular distribution of W decay products in terms of these helicity amplitudes, we perform a systematic search for the most sensitive angular distributions or correlations for anomalous couplings. As a result precise tests of the gauge-theory cancellations between different diagrams are suggested. Angular distributions sensitive to W+W− rescattering effects and/or CP-violating vector boson couplings are studied as well. Complete helicity amplitudes for the process e+e− → W+W− → (q1q2g)(q3q4g) with arbitrary quark masses and finite W width are presented in a form convenient for their direct numerical evaluation. Amplitudes for the processes e+e− → ZZ and Zγ are also included.

Improved predictions for g−2 of the muon and αQED(MZ2)

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.

Predictions for g-2 of the muon and alpha(QED) (M**2(Z))

We calculate (g-2) of the muon and the QED coupling alpha(M_Z^2), by improving the determination of the hadronic vacuum polarization contributions and their uncertainties. We include the recently re-analysed CMD-2 data on e^+e^- -> pi^+ pi^-. We carefully combine a wide variety of data for the e^+e^- production of hadrons, and obtain the optimum form of R(s) = sigma_had^0(s)/sigma_pt(s), together with its uncertainty. Our results for the hadronic contributions to g-2 of the muon are a_mu^(had, LO) = (692.4 +- 5.9_exp +- 2.4_rad) * 10^(-10) and a_mu^(had, NLO) = (-9.8 +- 0.1_exp +- 0.0_rad) * 10^(-10), and for the QED coupling Delta alpha^(5)_had (M_Z^2)= (275.5 +- 1.9_exp +- 1.3_rad) * 10^(-4). These yield (g-2)/2 = 0.00116591763(74), which is about 2.4 sigma below the present world average measurement, and alpha(M_Z^2)^(-1) = 128.954 +- 0.031. We compare our (g-2) value with other predictions and, in particular, make a detailed comparison with the latest determination of (g-2) by Davier et al.

Amplitudes for multi-parton processes involving a current at e+e−, e±p, and hadron colliders☆

Abstract We give all tree level scattering amplitudes involving one external electroweak vector boson (γ,Z, or W) and up to five colored particles (any combination of quark and gluons). The amplitudes are presented in a form directly amenable to numerical evaluation. The handling of color factors is described in detail. The amplitudes are then used to give complete tree level cross section formulas for e+e− → n jets (n≤5), e±p → n jets (n≤4), and p(-)p → V + n jets (n≤3; V = γ, Z, W) with subsequent vector boson decay.

The SM prediction of g-2 of the muon

Abstract We calculate ( g −2)/2 of the muon, by improving the determination of the hadronic vacuum polarisation contribution, a had, LO μ , and its uncertainties. The different e + e − data sets for each exclusive (and the inclusive) channel are combined in order to obtain the optimum estimate of the cross sections and their uncertainties. QCD sum rules are evaluated in order to resolve an apparent discrepancy between the inclusive data and the sum of the exclusive channels. We conclude a had, LO μ =(683.1±5.9 exp ±2.0 rad )×10 −10 which, when combined with the other contributions to ( g −2)/2, is about 3 σ below the present world average measurement.

Low-energy constraints on electroweak three gauge boson couplings

Abstract Novel strong interactions in the electroweak bosonic sector may induce SU(2)×U(1) gauge-invariant effective interactions between the Higgs doublet field and the electroweak gauge bosons which lead to anomalous WWZ and WWγ vertices, once the Higgs field acquires a vacuum expectation value. We consider observable one-loop effects, which are induced by the dimension-six operators ( D ω Φ) † gT i W i ων (D ν Φ ) and ( D ν Φ) † g′YB ων (D ν Φ ). Gauge invariance renders such effects at most logarithmically divergent, as required by decoupling. Present bounds on the anomalous gauge boson couplings imply modest constraints on the size of new physics signals in W + W − production at LEP II.

Anomalous Higgs boson production and decay

Abstract A non-standard symmetry breaking sector may lead to derivative couplings of the Higgs boson and thereby to anomalous interactions with the electroweak gauge bosons. In the framework of gauge-invariant effective Lagrangians the resulting changes in Higgs boson production and decay mechanisms are related to anomalous triple vector boson couplings. Using low energy constraints on the dimension-six operators in the effective Lagrangian, we discuss the size of deviations from SM predictions which may be expected in Higgs boson production and decay rates. Large enhancements are allowed e.g. in the Z → Hγ and H → γγ partial decay widths, leading to Z → γγγ events at LEP.

Jet angular correlation in vector-boson fusion processes at hadron colliders

Higgs boson and massive-graviton productions in association with two jets via vector-boson fusion (VBF) processes and their decays into a vector-boson pair at hadron colliders are studied. They include scalar and tensor boson production processes via weak-boson fusion in quark-quark collisions, gluon fusion in quark-quark (qq), quark-gluon (qg) and gluon-gluon (gg) collisions, as well as their decays into a pair of weak bosons or virtual gluons which subsequently decay into l, q or gg. We give the helicity amplitudes explicitly for all the VBF subprocesses, and show that the VBF amplitudes dominate the exact matrix elements not only for the weak-boson fusion processes but also for all the gluon fusion processes when appropriate selection cuts are applied, such as a large rapidity separation between two jets and a slicing cut for the transverse momenta of the jets. We also show that our off-shell vector-boson current amplitudes reduce to the standard quark and gluon splitting amplitudes with appropriate gluon-polarization phases in the collinear limit. Nontrivial azimuthal angle correlations of the jets in the production and in the decay of massive spin-0 and -2 bosons are manifestly expressed as the quantum interference among different helicity states of the intermediate vector-bosons. Those correlations reflect the spin and the CP nature of the Higgs bosons and the massive gravitons.

The MSSM confronts the precision electroweak data and the muon g − 2

We update the electroweak study of the predictions of the Minimal Super-symmetric Standard Model (MSSM) including the recent results on the muon anomalous magnetic moment, the weak boson masses, and the final precision data on the Z boson parameters from LEP and SLC. We find that the region of the parameter space where the slepton masses are a few hundred GeV is favored from the muon g − 2 for tan β ~ 10, whereas for tan β ≃ 50 heavier slepton mass up to ~ 1000 GeV can account for the reported 3.2 σ difference between its experimental value and the Standard Model (SM) prediction. As for the electroweak measurements, the SM gives a good description, and the sfermions lighter than 200 GeV tend to make the fit worse. We find, however, that sleptons as light as 100 to 200 GeV are favored also from the electroweak data, if we leave out the jet asymme-try data that do not agree with the leptonic asymmetry data. We extend the survey of the preferred MSSM parameters by including the constraints from the b → sγ transition, and find favorable scenarios in the minimal supergravity, gauge-, and mirage-mediation models of supersymmetry breaking.

Determination of mass hierarchy with medium baseline reactor neutrino experiments

A bstractWe study the sensitivity of future medium baseline reactor antineutrino experiments on the neutrino mass hierarchy. By using the standard χ2 analysis, we find that the sensitivity depends strongly on the baseline length L and the energy resolution