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Z. Was

Publications and source records attributed to Z. Was.

125 records · Page 7Linked to original sources
arXiv2000

Final State Radiative Effects for the Exact O(alpha) YFS Exponentiated (Un)Stable W+W- Production At and Beyond LEP2 Energies

We present the LL final state radiative effects for the exact O(alpha) YFS exponentiated (un)stable WW pair production at LEP2/NLC energies using Monte Carlo event generator methods. The respective event generator, version 1.12 of the program YFSWW3, wherein both Standard Model and anomalous triple gauge boson couplings are allowed, generates n(γ) radiation both from the initial state and from the intermediate W+ W- and generates the LL final state W decay radiative effects. Sample Monte Carlo data are illustrated.

hep-ph↗
arXiv1999

Initial-Final-State Interference in the Z line-shape

The uncertainty in the determination of the Z line-shape parameters coming from the precision of the calculation of the Initial-State Radiation and Initial--Final-State Interference is 2 10**(-4) for the total cross section sigma zero(had) at the Z peak, 0.15 MeV for the Z mass M Z, and 0.1 MeV for the Z width Gamma Z. Corrections to Initial--Final-State Interference beyond \Order{α^1} are discussed.

hep-ph↗
arXiv1999

Monte Carlo Program KoralW 1.42 for All Four-Fermion Final States in e+e- Collisions

The Monte Carlo program KoralW version 1.42 is presented. It generates all four-fermion final states with multibranch dedicated Monte Carlo pre-samplers and complete, massive, Born matrix elements. The presamplers cover the entire phase space. Multiphoton bremsstrahlung is implemented in the ISR approximation within the YFS formulation with the O(alpha**3) leading-log matrix element. The anomalous WWV couplings are implemented in CC03 approximation. The standard decay libraries (JETSET, PHOTOS, TAUOLA) are interfaced. The semi-analytical CC03-type code KorWan for differential and total cross-sections is included.

hep-ph↗
arXiv1999

Coherent Exclusive Exponentiation CEEX: the Case of the Resonant e+ e- Collision

We present the first-order coherent exclusive exponentiation (CEEX) scheme, with the full control over spin polarization for all fermions. In particular it is applicable to difficult case of narrow resonances. The resulting spin amplitudes and the differential distributions are given in a form ready for their implementation in the Monte Carlo event generator. The initial-final state interferences are under control. The way is open to the use of the exact amplitudes for two and more hard photons, using Weyl-spinor techniques, without giving up the advantages of the exclusive exponentiation, of the Yennie-Frautschi-Suura type.

hep-ph↗
arXiv1999

Global Positioning of Spin GPS Scheme for Half-Spin Massive Spinors

We present a simple and flexible method of keeping track of the complex phases and spin quantization axes for half-spin initial- and final-state Weyl spinors in scattering amplitudes of Standard Model high energy physics processes. Both cases of massless and massive spinors are discussed. The method is demonstrated and checked numerically for spin correlations in tau tau-bar production and decay. Its main application will be in the forthcoming work of combining effects due to multiple photon emission (exponentiation) and spin, embodied in the Monte Carlo event generators for production and decay of unstable fermions such as the tau lepton, t-quark and hypothetical new heavy particles.

hep-ph↗
arXiv1999

How to Generate Four-Fermion Phase Space

We present a scheme for integrating the matrix element of an arbitrary e^+e^-\to f_1f_2\bar f_3\bar f_4 process over the complete four-fermion phase space, or its any part, by means of the Monte Carlo technique. The presented algorithm has been successfully implemented in the KORALW Monte Carlo code.

hep-ph↗
arXiv1998

Higher-order QED corrections to e+e- to nu bar nu gamma at LEP2

The process e+e- to nu barnu + gamma with a distinctive ``photon-plus-missing-energy'' signal can serve as an important tool to search for new physics at LEP2. It can be exploited to measure the WW gamma coupling, or to search for weakly interacting and invisible (s)particles. For meaningful comparisons of experimental data with theoretical predictions, higher-order QED corrections due to multiphoton emission must be taken into account. In the present paper we explain how the WW gamma coupling has been incorporated into the KORALZ Monte Carlo program, which can now be used for simulations of e+e- to nu bar nu + gamma events with higher-order QED corrections. The strategy of how to perform an experimental analysis in the presence of experimental cuts is proposed. The question of systematic uncertainties is addressed and some numerical results for the phenomenologically interesting case of anomalous WW gamma couplings are also given.

hep-ph↗
arXiv1997

Event Generators for WW Physics

The report summarizes the results of the activities of the Working Group on Event Generators for WW Physics at CERN during 1995.

hep-ph↗
arXiv1997

Trefoil knot and ad-hoc classification of elementary fields in the Standard Model

We present an arbitrary model based on the trefoil knot to construct objects of the same spectrum as that of elementary particles. It includes `waves' and three identical sets of sources. Due to Lorentz invariance, `waves' group into 3 types of 1, 3 and 8 objects and `sources' consists of 3 identical sets of 30+2 elements, which separate into: 1 * 1 * 2 + 1 * 2 * 2 + 3 * 2 * 2 + 3 * 1 * 2 + 3 * 1 * 2 and another 1 * 1 * 2 group (which does not match classification of the Standard Model fields). On the other hand, there is no room in this construction for objects directly corresponding to Higgs-like degrees of freedom.

hep-th↗
arXiv1997

Exact $O(α)$ Gauge Invariant YFS Exponentiated Monte Carlo for (Un)Stable for (Un)Stable $W^+W^-$ Production At and Beyond LEP2 Energies

We realize, by Monte Carlo event generator methods, the exact O}(α)$ YFS exponentiated calculation of $e^+e^- \to W^+ W^- (\to f_1\bar f'_1 + \bar f_2 f'_2)$ at and beyond LEP2 energies, where the left-handed parts of $f_i$ and $f'_i$ are the respective upper and lower components of an $SU_{2L}$ doublet, $i=1,2$. Our calculation is gauge invariant from the standpoint of its radiative effects and the respective YFS Monte Carlo event generator YFSWW3, wherein both Standard Model and anomalous triple gauge boson couplings are allowed, generates $n(γ)$ radiation both from the initial state and from the final $W^+ W^-$. Sample Monte Carlo data are illustrated.

hep-ph↗
arXiv1997

e+ e- Annihilation into Hadrons at LEP2 in the Presence of the Anomalous DESY Positron-Jet Event Phenomenon

We discuss the allowed parameter regime in the coupling-mass plane implied by the existing LEP2 data on e+ e- -> hadrons at 172.3 GeV for the leptoquark interpretation of the anomalous DESY positron-jet events for four different models of the leptoquark charges and chiral couplings to quarks, for both a loose cut on s'/s and a tight cut on s'/s. We find that this interpretation of the DESY phenomenon is still consistent with the LEP2 data although, for vector leptoquarks, a significant regime of the relevant parameter space is already excluded.

hep-ph↗
arXiv1997

Four-quark final state in W-pair production: Case of signal and background

We discuss theoretical predictions for W-pair production and decay at LEP2 and higher energies in a form suitable for comparison with raw data. We present a practical framework for calculating uncertainties of predictions given by the KORALW and grc4f Monte Carlo programs. As an example we use observables in the $s\bar s c\bar c$ decay channel: the total four-quark (four-jet) cross section and two-quark/jet invariant-mass distribution and cross section, in the case when the other two may escape detection. Effects of QED bremsstrahlung, effective couplings, running W and Z widths, Coulomb interaction and the complete tree level set of diagrams are discussed. We also revisit the question of technical precision of the new version 1.21 of the KORALW Monte Carlo code as well as of version 1.2(26) of the grc4f one. Finally we find predictions of the two programs to have an overall physical uncertainty of 2%. As a side result we show, on the example of an $s\bar s$ invariant mass distribution, the strong interplay of spin correlations and detector cut-offs in the case of four-fermion final states.

hep-ph↗