arXiv subjects

B. F. L. Ward

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

At least 145 records · Page 8Linked to original sources

arXiv2002

Precision W-pair physics with the YFSWW3 and KoralW Monte Carlos

We present the recent developments in the precision studies of W-pair and single-W processes in e+e- collisions achieved with the help of the KoralW and YFSWW3 Monte Carlo generators. We focus on the theoretical precision of the measurements of M_W and anomalous couplings on the example of lambda coupling. We present the mechanism of running these two independent codes in the form of one Concurrent Monte Carlo code. We describe also the extensions of KoralW necessary to emulate the kinematical region of single-W process.

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arXiv2002

Electric Charge Screening Effect in Single-W Production with the KoralW Monte Carlo

Any Monte Carlo event generator in which only initial state radiation (ISR) is implemented, or ISR is simulated independently of the final state radiation (FSR), may feature too many photons with large transverse momenta, which deform the topology of events and result in too strong an overall energy loss due to ISR. This overproduction of ISR photons happens in the presence of the final state particle close to the beam particle of the same electric charge. It is often said that the lack of the electric charge screening effect between ISR and FSR is responsible for the above pathology in ISR. We present an elegant approximate method of curing the above problem, without actually reinstalling FSR. The method provides theoretical predictions of modest precision: < 2%. It is, however, sufficient for the current 1W data analysis at the LEP2 collider. Contrary to alternative methods implemented in other MC programs, our method works for the ISR multiphotons with finite p_T. Although this method is not an exact implementation of the complete/exact ISR, FSR and their interference, it is very closely modelled on it. We present a variety of numerical results obtained with the newest version of the KoralW Monte Carlo, in which this method is already implemented.

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arXiv2002

YFS MC Approach to QCD Soft Gluon Exponentiation

We present two things in this discussion. First, we develop and prove the theory of the extension of the YFS Monte Carlo approach to higher order SU_{2L} x U_1 radiative corrections to the analogous higher order QCD radiative corrections. Contact is made with other pioneering soft gluon resummation theories in the literature. Second, semi-analytical results and preliminary explicit Monte Carlo data are presented for the specific example of the processes p-bar p -> t-bar t +n(G)+X at FNAL energies, where G is a soft gluon and the respective event generator, ttp1.0, features realistic, event-by-event simulation of multiple, soft, finite p_T gluon effects in which the infrared singularities are canceled to all orders in alpha_s. We comment briefly on the implications of our results on the CDF/D0 observations and on their possible applications to RHIC physics and to LHC physics.

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arXiv2002

On Theoretical Uncertainties of the W Angular Distribution in W-Pair Production at LEP2 Energies

We discuss theoretical uncertainties of the distribution in the cosine of the W polar angle projected into a measurement of the anomalous triple gauge-boson coupling λ=λ_γ=λ_Z at LEP2 energies for the tandem of the Monte Carlo event generators KoralW and YFSWW3 and for the Monte Carlo event generator RacoonWW. Exploiting numerical results of these programs and cross-checks with experimental fitting procedures, we estimate that the theoretical uncertainty of the value of λdue to electroweak corrections, as obtained at LEP2 with the help of these programs, is ~0.005, about half of the expected experimental error for the combined LEP2 experiments (~0.010). We use certain idealized event selections; however, we argue that these results are valid for realistic LEP2 measurements.

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arXiv2002

Exact Differential O(alpha**2) Results for Hard Bremsstrahlung in e+e- Annihilation to 2f At and Beyond LEP2 Energies

We present the exact O(alpha) correction to the process e+ e- -> f f-bar + gamma, f neq e, for ISR oplus FSR at and beyond LEP2 energies. We give explicit formulas for the completely differential cross section. As an important application, we compute the size of the respective sub-leading corrections of O(alpha L) to the f f-bar cross section, where L is the respective big logarithm in the renormalization group sense so that it is identifiable as L = ln |s|/m_e^2 when s is the squared e+e- cms energy. Comparisons are made with the available literature. We show explicitly that our results have the correct infrared limit, as a cross-check. Some comments are made about the implementation of our results in the framework of the Monte Carlo event generator KK MC.

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arXiv2001

Precision Predictions for (Un)Stable W+W- Pair Production At and Beyond LEP2 Energies Beyond LEP2 Energies

We present precision calculations of the processes e+e- -> 4-fermions in which the double resonant W+W- intermediate state occurs. Referring to this latter intermediate state as the 'signal process', we show that, by using the YFS Monte Carlo event generators YFSWW3-1.14 and KORALW1.42 in an appropriate combination, we achieve a physical precision on the signal process, as isolated with LEP2 MC Workshop cuts, below 0.5 per cent. We stress the full gauge invariance of our calculations and we compare our results with those of other authors where appropriate. In particular, sample Monte Carlo data are explicitly illustrated and compared with the results of the program RacoonWW of Dittmaier {\it et al.}. In this way, we show that the total (physical plus technical) precision tag for the WW signal process cross section is 0.4 per cent for 200 GeV, for example. Results are also given for 500 GeV with an eye toward the LC.

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arXiv2001

On Theoretical Uncertainties of the W Boson Mass Measurement at LEP2

We discuss theoretical uncertainties of the measurement of the $W$ boson mass at LEP2 energies, reconstructed with the help of the tandem of the Monte Carlo event generators KoralW and YFSWW3. Exploiting numerical results obtained with these programs, and the existing knowledge in the literature, we estimate that the theoretical uncertainty of the $W$ mass due to electroweak corrections, as reconstructed at LEP2 with the help of these programs, is $\sim 5 $MeV. Since we use certain idealized event selections and a simple $M_W$-fitting procedure, our numerical exercises can be (should be) repeated for the actual ``$M_W$ extraction methods'' of the LEP2 measurements, using KoralW and YFSWW3 or other Monte Carlo programs.

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arXiv2001

CP Violation in Exclusive B Decays: Recoil Phase Effect

In the perturbative QCD approach to exclusive B decays to two light mesons, the leading twist contribution corresponds to those diagrams in the Lepage-Brodsky expansion in which the would be spectator quark receives its recoil momentum via one gluon exchange. We show that the resulting amplitude, which in the spectator model is real, acquires an imaginary part which may be comparable in size to its real part. Thus, this source of the strong interaction phase in the amplitude must be taken into account in general to discuss, reliably, the expectations for CP violation in B decays at any B-Factory type scenario.

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arXiv2001

Size of Penguin Pollution of the CKM CP Violating Phase in B-bar_s -> rho K_s

We use the perturbative QCD methods of Lepage and Brodsky to calculate the rate for B-bar_s -> rho K_s, with an eye toward the CP violating unitarity triangle angle gamma. We show that, although the penguins are large, there are regions of the allowed parameter space of the Cabibbo-Kobayashi-Maskawa mixing matrix wherein gamma is measurable in the sense that penguins change the value of sin(2gamma) one would extract from the attendant time dependent asymmetry measurement by less than 29 per cent, so that a 3sigma measurement of sin(2gamma) as being different from 0 is allowed by the corresponding theoretical uncertainty. This would establish CP violation in B_s decays. The rates which we find tend to favor the type of luminosities now envisioned for hadron-based B-factories.

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arXiv2001

The Monte Carlo Program KoralW version 1.51 and The Concurrent Monte Carlo KoralW&YFSWW3 with All Background Graphs and First Order Corrections to W-Pair Production

The version 1.51 of the Monte Carlo (MC) program KoralW for all $e^+e^-\to f_1\bar f_2 f_3\bar f_4$ processes is presented. The most important change since the previous version 1.42 is the facility for writing MC events on the mass storage device and re-processing them later on. In the re-processing one may modify parameters of the Standard Model in order to fit them to experimental data. Another important new feature is a possibility of including complete ${\cal O}(α)$ corrections to double-resonant W-pair component-processes in addition to all background (non-WW) graphs. The inclusion is done with the help of the YFSWW3 MC event generator for fully exclusive differential distributions (event-per-event). Technically, it is done in such a way that YFSWW3 runs concurrently with KoralW as a separate slave process, reading momenta of the MC event generated by KoralW and returning the correction weight to KoralW. KoralW introduces the ${\cal O}(α)$ correction using this weight, and finishes processing the event (rejection due to total MC weight, hadronization, etc.). The communication between KoralW and YFSWW3 is done with the help of the FIFO facility of the UNIX/Linux operating system. This does not require any modifications of the FORTRAN source codes. The resulting Concurrent MC event generator KoralW&YFSWW3 looks from the user's point of view as a regular single MC event generator with all the standard features.

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arXiv2001

The Monte Carlo Event Generator YFSWW3 version 1.16 for W-Pair Production and Decay at LEP2/LC Energies

We present the Monte Carlo event generator YFSWW3 version 1.16 for theprocess of W-pair production and decay in electron-positron collisions. It includes ${\cal O}(α)$ electroweak radiative corrections in the WW production stage together with the ${\cal O}(α^3)$ initial-state-radiation (ISR) corrections in the leading-logarithmic (LL) approximation, implemented within the Yennie-Frautschi-Suura (YFS) exclusive exponentiation framework. The photon radiation in the W decays is generated by the dedicated program PHOTOS up to ${\cal O}(α^2)$ LL, normalised to the W branching ratios. The program is interfaced with the $τ$ decay library TAUOLA and the quark fragmentation/hadronization package JETSET. The semi-analytical code KORWAN for the calculations of the differential and total cross-sections at the Born level and in the ISR approximation is included.

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arXiv2001

Precision Predictions for (Un)Stable WW/4f Production in e +e- Annihilation: YFSWW3/KoralW-1.42/YFSZZ

We present precision calculations of the processes e+ e- -> 4-fermions in which the double resonant W+ W- and ZZ intermediate states occur. Referring to these latter intermediate states as the 'signal processes', we show that, by using the YFS Monte Carlo event generators YFSWW3-1.14 and KoralW-1.42 in an appropriate combination, we achieve a physical precision on the WW signal process, as isolated with LEP2 MC Workshop cuts, below 0.5 per cent. We stress the full gauge invariance of our calculations and we compare our results with those of other authors where appropriate. In particular, sample Monte Carlo data are explicitly illustrated and compared with the results of the program RacoonWW of Denner et al. In this way, we cross check that the total (physical oplus technical) precision tag for the WW signal process cross section is 0.4 per cent for 200 GeV, for example. Results are also given for 500 GeV with an eye toward the LC. For the analogous ZZ case, we cross check that our YFSZZ calculation yields a total precision tag of 2 per cent, when it is compared to the results of ZZTO and GENTLE of Passarino and Bardin et al., respectively.

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arXiv2001

Coherent Exclusive Exponentiation of 2f Processes in e+e- Annihilation

In the talk we present the Coherent Exclusive Exponentiation (CEEX) which is implemented in the KK MC event generator for the process e+e- to f bar f +n gamma, f=mu,tau,d,u,s,c,b for center of mass energies from tau lepton threshold to 1TeV, that is for LEP1, LEP2, SLC, future Linear Colliders, b,c,tau-factories etc. We will attempt a short discussion of the theoretical concepts necessary in our approach, in particular the relations between the rigorous calculation of spin amplitudes (perturbation expansion), phase space parametrisation and exponentiation. In CEEX effects due to photon emission from the initial beams and outgoing fermions are calculated in QED up to second-order, including all interference effects. Electroweak corrections are included in first-order, at the amplitude level. The beams can be polarised longitudinally and transversely, and all spin correlations are incorporated in an exact manner. Precision predictions, in particular the photon emission at LEP2 energies, are also shown.

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arXiv2000

Coherent Exclusive Exponentiation for Precision Monte Carlo Calculations

In this contribution we give a short overview of the new Coherent Exclusive Exponentiation (CEEX) which is implemented in the new \KK MC event generator for the process $e^+e^-\to f\bar{f} +nγ$, $f=μ,τ,d,u,s,c,b$ with validity for center of mass energies from $τ$ lepton threshold to 1TeV, that is for LEP1, LEP2, SLC, future Linear Colliders, $b,c,τ$-factories etc. In CEEX effects due to photon emission from initial beams and outgoing fermions are calculated in QED up to second-order, including all interference effects. Electroweak corrections are included in first-order, at the amplitude level. Beams can be polarized longitudinally and transversely, and all spin correlations are incorporated in an exact manner. We describe briefly the essence of CEEX as compared with older exclusive variants of the exponentiation (EEX) and present samples of the numerical results, concentrating on the question of the technical and physical precision for the total cross section and for the charge asymmetry. New results on the interference between initial and final state photon emission at LEP2 energies are also shown.

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arXiv2000

W-Pair Production with YFSWW/KoralW

A theoretical description of W-pair production in terms of two complementary Monte Carlo event generators YFSWWand KoralW is presented. The way to combine the results of these two programs in order to get precise predictions for WW physics at LEP2 and LC energies is discussed.

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arXiv2000

Coherent Exclusive Exponentiation for Precision Monte Carlo Calculations of Fermion Pair Production / Precision Predictions for (Un)stable W+W- Pairs

We present the new Coherent Exclusive Exponentiation (CEEX), in comparison to the older Exclusive Exponentiation (EEX) and the semi-analytical Inclusive Exponentiation (IEX), for the process e+e- -> f-bar f + n(gamma), f=mu,tau,d,u,s,c,b, with validity for centre of mass energies from tau lepton threshold to 1 TeV. We analyse 2f numerical results at the Z-peak, 189 GeV and 500 GeV. We also present precision calculations of the signal processes e+e- -> 4f in which the double resonant W+W- intermediate state occurs using our YFSWW3-1.14 MC. Sample 4f Monte Carlo data are explicitly illustrated in comparison to the literature at LEP2 energies. These comparisons show that a TU for the signal process cross section of 0.4 percent is valid for the LEP2 200 GeV energy. LC energy results are also shown.

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arXiv2000

Two-Fermion Production in Electron-Positron Collisions

This report summarizes the results of the two-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the theoretical calculations of the two fermion production process in the electron-positron collision at LEP2 center of the mass energies are reported. The Bhabha process and the production of muon, tau, neutrino and quark pairs is covered. On the basis of comparison of various calculations, theoretical uncertainties are estimated and compared with those needed for the final LEP2 data analysis. The subjects for the further studies are identified.

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arXiv2000

Coherent Exclusive Exponentiation For Precision Monte Carlo Calculations

We present the new Coherent Exclusive Exponentiation (CEEX), the older Exclusive Exponentiation (EEX) and the semi-analytical Inclusive Exponentiation (IEX) for the process $e^+e^-\to f\bar{f} +nγ$, $f=μ,τ,d,u,s,c,b$ with validity for centre of mass energies from $τ$ lepton threshold to 1TeV, that is for LEP1, LEP2, SLC, future Linear Colliders, $b,c,τ$-factories etc. They are based on Yennie-Frautschi-Suura exponentiation. In CEEX effects due to photon emission from initial beams and outgoing fermions are calculated in QED up to second-order, including all interference effects. Electroweak corrections are included in first-order, at the amplitude level. Beams can be polarized longitudinally and transversely, and all spin correlations are incorporated in an exact manner. EEX is more primitive, lacks initial-final interferences, but it is valuable for testing the newer CEEX. IEX provides us set of a sophisticated semi-analytical formulas for the total cross section and selected inclusive distributions which are mainly used for cross-checks of the MC results. We analyse numerical results at the Z-peak 189 GeV and 500 GeV for simple kinematical cuts (comparisons with IEX) and for realistic experimental cuts. Physical precision and technical precision are determined for the total cross section and for the charge asymmetry.

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