arXiv subjects

M. Kuroda

Publications and source records attributed to M. Kuroda.

26 records · Page 2Linked to original sources

arXiv1998

A Simple Born-Form Approximation for e^+ e^- to W^+W^- at One Loop

A simple Born-form approximation at the one-loop level for e^+e^- to W^+ W^- at high energies is given in analytic form. The different contributions to the three invariant one-loop amplitudes, S_I^{(-)}(s,t) and S_Q^{(\pm)}(s,t), determining the Born-form helicity amplitudes are thoroughly investigated analytically and numerically. At energies above 500 GeV, the accuracy of the simple Born-form approximation for the differential production cross section is better than 1% for almost all W^+W^- production angles, independently of whether the W^+W^- polarization is summed over, or whether a longitudinal or a transverse polarization is selected for both the W^+ and the W^-.

hep-ph↗

arXiv1997

susy23 v2.0: an Event Generator for Supersymmetric Processes at $e^+e^-$ Colliders

\susy23 is a Monte-Carlo package for generating supersymmetric (SUSY) processes at $e^+e^-$ colliders. Twenty-three types of SUSY processes with 2 or 3 final state particles at tree level are included in version 2.0. {\susy23} addresses event simulation requirements at $e^+e^-$ colliders such as LEP. Matrix elements are generated by GRACE with the helicity amplitude method for processes involving massive fermions. The phase space integration of the matrix element gives the total and differential cross sections, then unweighted events are generated. Sparticle widths and decay branching ratios are calculated. Each final state particle may then decay according to these probabilities. Spin correlations are taken into account in the decays of sparticles. Corrections of initial state radiation (ISR) are implemented in two ways, one is based on the electron structure function formalism and the second uses the parton shower algorithm called QEDPS. Parton shower and hadronization of the final quarks are performed through an interface to JETSET.

hep-ph↗

arXiv1997

Algorithm for the Fermionic Lines in GRACE-SUSY

Algorithm of constructing Feynman amplitudes in the framework of minimal supersymmetic extension of the standard model is presented, which can be easily implemented in GRACE, the program of automatic generation of Feynman amplitudes. The equivalence of our method with the prescription given by Denner et al. is proved.

hep-ph↗

arXiv1997

A Simple Approximation of the One-Loop Corrected Cross Section for $e^+e^-\to W^+W^-$ at LEP 2

Using the SU(2) gauge coupling, $g_{W^\pm} (M^2_{W^\pm})$, at the high-energy scale of $M_{W^\pm}$, defined by the (theoretical value of the) leptonic W-width, rather than using the low-energy value, defined via the Fermi coupling, $G_μ$, in the Born approximation, and supplementing with Coulomb corrections and initial state radiation, errors with respect to the exact one-loop results for the differential cross section of $e^+e^-\to W^+W^-$ are below 1% at LEP 2 energies at all $W^+W^-$ production angles. A similar procedure is suggested to incorporate leading bosonic loop effects into four-fermion production in the fermion-loop scheme. The resulting accuracy below 1% is sufficient for LEP 2 experiments.

hep-ph↗

arXiv1996

Effective lagrangian description of top production and decay

We propose a rather general description of residual New Physics (NP) effects on the top quark couplings. These effects are described in terms of 20 gauge invariant $dim=6$ operators involving gauge and Higgs bosons as well as quarks of the third family. We compute their implications for the $γt\bar t$, $Z t \bar t$ and $tbW$ vertices and study their observability in the process $e^-e^+ \to t\bar t$ with $t\to bW \to b\ell^+ν_\ell$. We present results for the integrated cross section, the angular distribution and various decay distribution and polarization asymmetries for NLC energies of $0.5-2~TeV$. Observability limits are discussed and interpreted in terms of the NP scales associated to each operator through the unitarity constraints. The general landscape of the residual NP effects in the heavy quark and bosonic sectors is also presented.

hep-ph↗

arXiv1995

Refined Analysis of the Electroweak Precision Data

We refine our recent analysis of the electroweak precision data at the \PZO\ pole by including the hadronic decay modes of the \PZO. Within the framework of an effective Lagrangian we parametrize $SU(2)$ violation by the additional process-specific parameters $\De y_ν$, $\De\yh$, and $\De\yb$ (for the $\PZOν\barν$, $\PZO\Pq\bar\Pq$, and $\PZO\Pb\bar\Pb$ vertices) together with the previously introduced parameters $\De x$, $\De y$, and $\eps$. We find that a six-parameter analysis of the experimental data is indeed feasible, and it is carried out in addition to a four-parameter fit for $\De x$, $\De y$, $\eps$, and $\De\yb$ only. We reemphasize that the experimental data have become sensitive to the (combined) magnitude of the vertex corrections at the $\PWp\Pl\barν$ ($\PWmν\bar\Pl$) and $\PZO\Pl\bar\Pl$ vertices, $\De y$, which is insensitive to the notion of the Higgs mechanism but dependent on the non-Abelian, trilinear vector-boson coupling. Full explicit analytical results for the standard one-loop predictions for the above-mentioned parameters are given, and the leading two-loop top-quark effects are included. The analytic formluae for the analysis of the experimental data in terms of the parameters $\De x$, $\De y$ etc.\ are presented in order to encourage experimentalists to persue such an analysis by themselves with future data.

hep-ph↗

arXiv1994

On the Significance of the Electroweak Precision Data

We elaborate on a recently suggested effective Lagrangian for charged-current and neutral-current electroweak interactions which in comparison with the standard electroweak theory contains three free parameters $Δx, Δy$, $\varepsilon$ which quantify different sources for violations of $SU(2)$ symmetry. Within the standard $SU(2)_I \times U(1)_Y$ electroweak theory, we present both exact and very much refined approximate analytical one-loop expressions for these parameters in terms of the canonical input, $G_μ, \MZ$, $\alpz$, the top-quark mass, $\Mt$, and the Higgs-boson mass, $\MH$. We reemphase the importance of discriminating between the {\it empirically well-known purely fermionic} (vacuum polarization) contributions to $Δx, Δy$, $\varepsilon$ and the {\it empirically unknown bosonic} ones with respect to present and future electroweak precision tests. The parameters $Δx$ and $\varepsilon$ are hardly affected by standard bosonic corrections, while the full one-loop results for $Δy$ differ appreciably from the ones obtained by taking into account fermion loops only. A detailed comparison with the experimental data on $\MWpm/\MZ$, $\bar\sw^2$, $Γ_l$ shows that these data start to become accurate enough to be sensitive to standard (bosonic) contributions to $Δy$ beyond fermion loops.

hep-ph↗

arXiv1993

On the Interpretation of the Electroweak Precision Data

The recent precision electroweak data on $Γ^l, \bar s^2_W$ and $M_W/M_Z$ are compared with the tree-level and the dominant-fermion-loop as well as the full one-loop standard-model predictions. While the tree-level predictions are ruled out, the dominant-fermion-loop predictions, defined by using $α(M^2_Z)\cong 1/128.9$ in the tree-level formulae, as well as the full one-loop predictions are consistent with the experimental data. Deviations from the dominant-fermion-loop predictions are quantified in terms of an effective Lagrangian containing three additional parameters which have a simple meaning in terms of $SU(2)$ symmetry violation. The effective Lagrangian yields the standard one-loop predictions for specific values of these parameters, which are determined by $m_t$ and $m_H$.

hep-ph↗