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.Comment: LateX file, 6 pages, conference contributio

Jadach, S.

Placzek, W.

Skrzypek, M.

Ward, B. F. L.

Was, Z.

English

arXiv

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.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.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.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.A theoretical description of W-pair production in terms of two complementary Monte Carlo event generators YFSWW and 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

Ward, B.F.L.

CERN Document Server

W-Pair Production withYFSWW/KoralW1W. P laczeka,b, S. Jadachc,d,b, M. Skrzypekd,b B.F.L. Warde,f,band Z. Wasd,ba Institute of Computer Science, Jagellonian University,ul. Nawojki 11, 30-072 Cracow, Poland,bCERN, CH-1211 Geneva 23, Switzerland,cDESY Zeuthen, Platanenallee 6, D-15738 Zeuthen, GermanydInstitute of Nuclear Physics, ul. Kawiory 26a, 30-055 Cracow, Poland,eDepartment of Physics and Astronomy,The University of Tennessee, Knoxville, TN 37996-1200, USA,fSLAC, Stanford University, Stanford, CA 94309, USA.Abstract. A theoretical description of W-pair production in terms of two com-plementary Monte Carlo event generators YFSWW and KoralW is presented. Theway to combine the results of these two programs in order to get precise predic-tions for WW physics at LEP2 and LC energies is discussed.The process of W-pair production in electron{positron colliders is very im-portant for testing the Standard Model (SM) and searching for signals ofpossible \new physics"; see e.g. Ref [1]. One of the main goals of investigatingthis process at present and future e+e− experiments is to measure preciselythe basic properties of the W boson, such as its mass MW and width ΓW . Thisprocess also allows a study, at the tree level, of triple and quartic gauge bosoncouplings, where small deviations from the subtle SM gauge cancellations canlead to signicant eects on physical observables { these can be signals of\new physics".Since the W’s are unstable and short-lived particles, the W-pairs are notobserved directly in the experiments but through their decay products: four-fermion (4f) nal states (which may then also decay, radiate gluons/photons,hadronize, etc.). As high energy charged particles are involved in the process,one can also observe energetic radiative photons. So, at the parton level, onehas to consider a general process:1) Work partly supported by the Maria Sk lodowska-Curie Joint Fund II PAA/DOE-97-316 and by the US Department of Energy Contracts DE-FG05-91ER40627 and DE-AC03-76ER00515.e+ + e− −! 4f + nγ; (n = 0; 1; 2; : : :); (1)where also some background (non-WW) processes contribute. In a theoreticaldescription of this process { according to quantum eld theory { one alsohas to include virtual eects, the so-called loop corrections. This generalprocess is very complicated since it involves  80 dierent channels (4f nalstates) with complex peaking behaviour in multiparticle phase space and alarge number of Feynman diagrams to be evaluated. Even in the massless-fermion approximation the number of Feynman graphs grows up from 9{56per channel at the Born level to an enormous 3579{15948 at the one-looplevel [2]. The full one-loop calculations have not been nished yet, even forthe simplest case (doubly plus singly W-resonant diagrams) [3]. But even ifthey existed one would be faced with problems in their numerical evaluationsin practical applications, particularly within Monte Carlo event generators{ they would be extremely sizeable and very slow. These are the reasonswhy ecient approximations in the theoretical description of this process arenecessary. These approximations should be such that on the one hand theywould include all contributions/corrections that are necessary for the requiredtheoretical accuracy (dependent on experimental precision) and on the otherhand they would be ecient enough for numerical computations. Given thecomplicated topologies of the 4f (+nγ) nal states, such calculations should be,preferably, given in terms of a Monte Carlo event generator that would allowone to simulate the process directly [4,5]. Here we present such a solution forthe W-pair production process, which consists of two complementary MonteCarlo event generators: YFSWW3 and KoralW. More details on YFSWW3 can befound in Refs. [6{9] and on KoralW in Refs. [10{12].KoralW includes the full lowest-order e+e− ! 4f process but with simpliedradiative corrections { the universal ones such as initial-state radiation (ISR),the Coulomb eect, etc. In YFSWW3, on the other hand, the lowest-order pro-cess is simplied { only the doubly W-resonant contributions are taken intoaccount, but inclusion of the radiative corrections in this process goes beyondthe universal ones. In the current version of YFSWW3 only those non-universal(non-leading) corrections are included that are necessary to achieve the the-oretical precision for the total WW cross section of 0:5% required at LEP2.For the future linear colliders (LC) this may not be sucient, so even somehigher-order corrections would have to be added, which is possible within theframework of YFSWW3. The important thing is that the two programs have awell established common part, which is the doubly W-resonant (WW) processwith the same universal radiative corrections. This, as will be shown later,allows us to combine the results of the two programs to achieve the desiredtheoretical precision for WW observables. The ISR eects in both programsare based on the Yennie{Frautschi{Suura (YFS) exclusive exponentiation pro-cedure [13,14], with an arbitrary number of non-zero pT radiative photons.The Coulomb correction is implemented in the standard version according toRef. [15] and also in the form of the \screened" Coulomb ansatz of Ref. [16],which is an ecient approximation of non-factorizable corrections. The full4f matrix element with non-zero fermion masses for KoralW has been gener-ated using the GRACE system of the MINAMI-TATEYA collaboration [17].For an ecient event generation, two independent 4f phase-space presamplershave been developed [18]. In this way KoralW is able to provide the impor-tant 4f-background correction to the WW-process in the form of MC events.However, as was already shown in Ref. [2], the pure universal radiative correc-tions and the 4f-background corrections are not sucient for a nal theoreticalprecision tag of 0:5% for LEP2 experiments. By using the exact O() calcu-lations of Refs. [19,20] for on-shell W-pair production, it was shown that thenon-leading electroweak (EW) corrections can be as large as 1{2% at LEP2energies (as will be seen later, they are even larger at LC energies). These cal-culations were done, however, in the on-shell-W approximation (stable W), sothe question was how to implement (or extend) them in the realistic o-shellWW production. A workable solution to this turned out to be the so-calledleading-pole approximation (LPA). The LPA was also needed for other rea-sons. Namely, the matrix element for the WW production and decay basedon three double-resonant Feynman graphs (so-called CC03) is not SU(2)LU1gauge-invariant, and the simplest way to achieve the full gauge invariance isto use the LPA.There are two approaches within the LPA: the one already discussed inRef. [2] and employed in the actual calculations for the WW process inRef. [21], and the second advocated by R. Stuart in Ref. [22]. In the rstapproach, the whole matrix element is expanded in Laurent series about com-plex poles corresponding to two resonant W’s; then in the LPA only the leadingterms of this expansion are retained. In this approach one gets a direct cor-respondence to the on-shell W-pair production and decay, but the results candier from the realistic process by several per cent. This can be corrected byadding the dierence between the predictions of the full 4f process and this ap-proximation, at least at the Born level; however, it is not obvious how to do iton an event-by-event basis. We have implemented in YFSWW3 this solution andit is called the LPAb option { it can be useful for some tests/cross-checks. Inthe second approach, the gauge-invariant matrix element is rst decomposedinto a sum of Lorentz scalar functions multiplied by spinor and Lorentz-tensorfactors according to the standard S-matrix theory [23]. Then, only the Lorenzscalar functions, which describe the nite-range W propagation, are expandedabout their complex poles. In the LPA, as previously done, only the leadingterms in (ΓW=MW ) are retained. In this approach the results are very closeto the predictions based on the minimum gauge-invariant subset of Feynmandiagrams including the WW production (so-called CC11), e.g. for the to-tal cross section the dierences are below 0:1% at 200 GeV and  0:5% at500 GeV. This solution is implemented in YFSWW3 as the LPAa option andit is recommended for the event generation. The non-universal (non-leading)corrections are included in both LPAs through the YFS exponentiation for theWW production stage including photon radiation o the W bosons (split ina gauge-invariant way into the radiation in the production and decay stages).Here we employ the exact O() calculations for the on-shell WW produc-tion of Ref. [20]. In the on-pole LPA residuals we make the approximationsp  M2W , where sp is the complex pole position and MW is the on-shell Wmass, which means neglecting terms  (=)(ΓW=MW ) { unimportant for theaimed theoretical accuracy. For the radiation in the W decays, we use in thecurrent version of YFSWW3 the leading-log-type program PHOTOS [24], nor-malized to the radiatively corrected W branching ratios; however, the YFSexponentiation for this process is in progress. The non-factorizable correc-tions (interferences between various stages of the process) have been includedonly via the so-called screened Coulomb ansatz [16] (which is a sucient ap-proximation for LEP2), but can be implemented to their full extent in thefuture.Having these two MC event generators, we can combine their results, inorder to obtain precise predictions for the WW process, in two ways. Either wecan take the best prediction from YFSWW3 and correct it for the 4f backgroundusing KoralW, which can be symbolically denoted by:Y/K = Y  4fK ; (2)or we can take the best prediction from KoralW and correct it for the non-leading (NL) eects to the \signal" process from YFSWW3, which we can writesymbolically as:K/Y = K  NLY : (3)This can be done easily at the level of the total cross section as well as forthe dierential distributions. Recently, reweighting interfaces have been de-veloped for the two programs so that it can also be done on an event-by-eventbasis [25,26]. All this is possible because both programs have some commonbasic distribution, which is the WW signal process with the universal radiativecorrection, and it has been checked that they agree very well at this level [9].YFSWW3 was also compared with an independent MC program,RacoonWW [27], which includes the non-universal O() corrections for theW-pair production. The two programs were found to agree for the total WWcross section < 0:4% at LEP2 energies [5] and < 0:5% at 500 GeV [9]. Numer-ically, the non-universal O() corrections as calculated by YFSWW3 are  1{2%at LEP2 energies and  5{10% at LC energies (0:5{1:5 TeV), and they arealways negative. On the other hand the ISR corrections change their signfrom being large negative near the WW threshold to being large positive atLC energies (thus cancelling partially the eects of non-universal corrections).AcknowledgementsOne of us (W.P.) thanks the organizers of the LCWS 2000 at Fermilab fortheir kind hospitality and their nancial support. We also acknowledge thesupport of the CERN Theory Division, all the LEP Collaborations and theDESY Directorate.REFERENCES1. Physics at LEP2, edited by G. Altarelli, T. Sjo¨strand and F. Zwirner (CERN96-01, Geneva, 1996), 2 vols.2. W. Beenakker et al., WW Cross-Sections and Distributions, in [1], Vol. 1, p. 79.3. A. Vicini, Acta Phys. Polon., B29 (1998) 2847.4. D. Bardin et al., WW Event Generators for WW Physics, in [1], Vol. 2, p. 3.5. M. Gru¨newald et al., Four-Fermion Production in Electron-Positron Collisions,in: Reports of the Working Groups on Precision Calculations for LEP2 Physics,edited by S. Jadach, G. Passarino and R. Pittau (CERN 2000-009, Geneva,2000), p. 1.6. S. Jadach, W. P laczek, M. Skrzypek and B.F.L. Ward, Phys. Rev. D54 (1996)5434.7. S. Jadach, W. P laczek, M. Skrzypek, B.F.L. Ward and Z. Was, Phys. Lett.B417 (1998) 326.8. S. Jadach, W. P laczek, M. Skrzypek, B.F.L. Ward and Z. Was, Phys. Rev.D61 (2000) 113010; preprint CERN-TH/99-222; hep-ph/9907436.9. S. Jadach, W. P laczek, M. Skrzypek, B.F.L. Ward and Z. Was, preprint CERN-TH/2000-337; hep-ph/0007012; to be submitted to Phys. Lett. B.10. M. Skrzypek, S. Jadach, W. P laczek and Z. Was, Comput. Phys. Commun. 94(1996) 216.11. M. Skrzypek et al., Phys. Lett. B372 (1996) 289.12. S. Jadach, W. P laczek, M. Skrzypek, B.F.L. Ward and Z. 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Eden, P.V. Landsho, D.I. Olive, and J.C. Polkinghorne, The AnalyticS-Matrix (Cambridge University Press, Cambridge, 1966).24. E. Barberio and Z. Was, Comput. Phys. Commun. 79 (1994) 291.25. S. Jadach, W. P laczek, M. Skrzypek, B.F.L. Ward and Z. Was, YFSWW3version 1.15, available at: http://cern.ch/placzek/.26. S. Jadach, W. P laczek, M. Skrzypek, B.F.L. Ward and Z. Was, KoralW, avail-able at: http://hpjmiady.ifj.edu.pl/.27. A. Denner, S. Dittmaier, M. Roth and D. Wackeroth, Nucl. Phys. B587 (2000)67.

W-Pair Production with YFSWW/KoralW

W. Pl̸aczek

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W-pair production with YFSWW/KoralW

W-Pair Production with YFSWW/KoralW

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