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B. F. L. Ward

Publications and source records attributed to B. F. L. Ward.

At least 163 records · Page 9Linked to original sources
arXiv2000

Four-Fermion Production in Electron-Positron Collisions

This report summarises the results of the four-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the calculation of four-fermion processes in electron-positron collisions at LEP-2 centre-of-mass energies are presented, concentrating on predictions for four main reactions: W-pair production, visible photons in four-fermion events, single-W production and Z-pair production. Based on a comparison of results derived within different approaches, theoretical uncertainties on these predictions are established.

hep-ph↗
arXiv2000

The Precision Monte Carlo Event Generator KK For Two-Fermion Final States In e+e- Collisions

We present the Monte Carlo event generator KK version 4.13 for precision predictions of the Electroweak Standard Model for the process $e^+e^-\to f\bar{f} +nγ$, $f=μ,τ,d,u,s,c,b$ at centre of mass energies from $τ$ lepton threshold to 1TeV, that is for LEP, SLC, future Linear Colliders, $b,c,τ$-factories etc. Effects due to photon emission from initial beams and outgoing fermions are calculated in QED up to second order, including all interference effects, within Coherent Exclusive Exponentiation (CEEX), which is based on Yennie-Frautschi-Suura exponentiation. Electroweak corrections are included in first order, with higher order extensions, using the DIZET 6.x library. Final state quarks hadronize according to the parton shower model using JETSET. Beams can be polarized longitudinally and transversely. Decay of the tau leptons is simulated using the TAUOLA library, taking into account spin polarization effects as well. In particular the complete spin correlations density matrix of the initial state beams and final state tau's is incorporated in an exact manner. Effects due to beamstrahlung are simulated in a realistic way. The main improvements with respect to KORALZ are: (a) inclusion of the initial-final state QED interference, (b) inclusion of the exact matrix element for two photons, and (c) inclusion of the transverse spin correlations in $τ$ decays (as in KORALB).

hep-ph↗
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

Precision Calculation of Bhabha Scattering at LEP

For the small-angle Bhabha-scattering process, we consider the error budget for the calculation of the LEP/SLC luminosity in the Monte Carlo event generator BHLUMI 4.04, from the standpoint of new calculations of exact results for the respective O(alpha**2) photonic corrections in the context of the Yennie-Frautchi-Suura exponentiation. We find that an over-all precision tag for the currently available program BHLUMI 4.04 can be reduced from 0.11% to 0.061% at LEP1 and from 0.25% to 0.122% at LEP2. For the large-angle Bhabha process, we present the Monte Carlo program BHWIDE and compare its predictions with predictions of other Monte Carlo programs as well as semi-analytical calculations.

hep-ph↗
arXiv1999

New Results on the Theoretical Precision of the LEP/SLC Luminosity

We consider the error budget for the calculation of the LEP/SLC luminosity in the Monte Carlo event generator BHLUMI4.04 from the standpoint of new calculations of the exact result for the O(alpha) correction to the process e+ e- -> e+ e- + gamma in the low angle luminosity regime at SLC/LEP energies, for the double bremsstrahlung effect e+ e- -> e+ e- + gamma gamma in this regime, and for the size of the two-loop virtual correction to e+ e- -> e+ e- in this regime in context of Yennie-Frautschi-Suura exponentiation. We find that the error on the O(alpha**2) photonic correction can be reduced from the currently published value 0.1% to the value 0.027%. This leads to an over-all precision tag for the currently available program BHLUMI4.04 of 0.061%. This reduction of the precision of the calculation is important for the final LEP1 EW precision Z physics tests of the Standard Model.

hep-ph↗
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

Gauge Invariant YFS Exponentiation of (Un)stable $Z$-Pair Production At and Beyond LEP2 Energies

We present the theoretical basis and sample Monte Carlo data for the YFS exponentiated calculation of $e^+e^- \to Z Z \to f_1\bar f_1 + \bar f_2 f_2$ at and beyond LEP2 energies, where the left-handed part of $f_i$ is a component of an $SU_{2L}$ doublet, $i=1,2$. The calculation is performed for both SM couplings and for anomalous $ZZV$ triple gauge boson couplings in the conventions of Hagiwara et al.. Our formulas, which are gauge invariant, are illustrated in a proto-typical YFS Monte Carlo event generator YFSZZ.

hep-ph↗
arXiv1997

Dokshitzer-Gribov-Lipatov-Altarelli-Parisi Evolution and the Renormalization Group Improved Yennie-Frautschi-Suura Theory in QCD

We show that the recently derived renormalization group improved Yennie-Frautschi-Suura (YFS) exponentiation of soft $(k_o\to 0)$ gluons in QCD is fully compatible with the usual Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution of the structure functions of hadrons in the respective hadron-hadron hard interactions. We show how to implement the YFS exponentiation without double or over counting effects already implied by the DGLAP equation. In this way, we arrive at a theory which allows for the development of realistic, multiple gluon Monte Carlo event generators for hard hadron-hadron scattering processes in which the DGLAP evolved structure functions are correctly synthesized with the respective YFS exponentiated soft gluon effects in a rigorous way.

hep-ph↗
arXiv1997

e^+ (Anti)Quark Scattering in the Presence of the Anomalous HERA Positron-Jet Event Phenomenon

We discuss the leptoquark interpretation of the anomalous HERA positron-jet events in the context of the YFS exponentiated Monte Carlo event generator treatment of the attendant multiple photon radiative effects for both the would-be signal and the SM background, wherein finite-$p_T$ photon effects are properly taken into account and wherein infrared singularities are cancelled to all orders in $α$. We show that H1 and ZEUS data are consistent with such an interpretation for a leptoquark coupling $\lesssim 0.3g_W$, mass $\sim 200$ GeV, and width $\lesssim 2$GeV. Possible future tests are proposed.

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↗
arXiv1996

Anomalous Gauge Boson Couplings

The measurement of anomalous gauge boson self couplings is reviewed for a variety of present and planned accelerators. Sensitivities are compared for these accelerators using models based on the effective Lagrangian approach. The sensitivities described here are for measurement of generic parameters kappa_v, lambda_v, etc., defined in the text. Pre-LHC measurements will not probe these couplings to precision better than O(1/10). The LHC should be sensitive to better than O(1/100), while a future NLC should achieve sensitivity of O(1/1000) to O(1/10000) for center of mass energies ranging from 0.5 to 1.5 TeV.

hep-ph↗
arXiv1996

BHWIDE 1.00: O(alpha) YFS Exponentiated Monte Carlo for Bhabha Scattering at Wide Angles for LEP1/SLC and LEP2

We present O(alpha) YFS exponentiated results for wide angle Bhabha scattering at LEP/SLC energies using a new Monte Carlo event generator BHWIDE 1.xx. Our calculations include two options for the pure weak corrections, as presented in Beenakker et al. and in Bohm et al. From comparison with the results of Beenakker et al., Montagna et al. and Cacciari et al., we conclude that the total precision of our BHWIDE results is 0.3% (0.5%) in the LEP1/SLC regime within +-100 MeV (+2.75/-2.5 GeV) of the Z peak. For LEP2, the corresponding precision is currently estimated at 1.5%; the latter could be improved if the data in LEP2 so require. Both precision tags represent clear improvements over what is currently available in the literature.

hep-ph↗