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Physics of W Bosons and Four- Processes at LEP2/ Energies S. Jadach, Z. Wa.s, and M. Skrzypek

The calculative framework for predictions of 4-fermion final states at LEP including complete first order radiative corrections for dominant diagrams was established.

• KORALW — Version 1.42, featuring the simulation of all four-fermion processes at LEP2, is now in full use at LEP2, making it the only available calculation featuring ^(7) radiation for the initial state with finite px photons in the YFS-style of exclusive exponentiation. It is published in Comp. Phys. Commun. 119 (1999) 272. We have recently begun the development of its tandem use with the YFSWW3 exact O(a) exponentiated calculation of the CC03 signal process for the WW pair production and decay. This tandem is the only MC event generator calculation with the demonstrated capability to reach the 0.5% tag called-out by the 1996 CERN Yellow Book Report on LEP2 Physics for the WW pair production cross section.

• YFSZZ — We continued with the support, of LEP2 users, for our work on YFS exponentiated MC for ZZ pair production. Its results continue to be used for setting limits on anomalous couplings as presented by the LEP collaborations at the 1998 Rochester Conference. Issues such as the proper SM coupling constant inputs at LEP2, etc., have been resolved by direct interaction with LEP users, so that the results quoted by the LEP collaborations based on this calculation are in fact correct.

• YFSWW3 — We continued the development of the tandem use, with KORALW1.42, of our published exact O(a) YFS exponentiated CC03 MC for the signal WW processes at LEP2, which is the only such four-fermion MC with these corrections in existence. With its version 3-1.12, we cross-checked the semi-analytical work of Beenakker et al. (Nucl. Phys. B548 (1999) 3, Phys. Lett. B435 (1999) 233) on the effect of FSR in the WW pair production and decay - we find good correspondence between our results and this semi-analytical work. We have formulated a precise statement of the gauge-invariance and renormalization scheme independence of our approach to the tandem use of YFSWW3 and KORALW1.42, which we refer to as the leading pole approximation (LPA). In this way, we have the only scheme realized by MC event generators with the proven capability to reach the 0.5% tag called-out by the 1996 CERN Yellow Book Report on LEP2 Physics for the WW pair production cross section and which is manifestly gauge-invariant and independent of the renormalization scheme.

Two-Fermion Production at -Positron UBS? S. Jadach and Z. Wa,s == o H§ A new form of exponentiation based on spin amplitudes was established. Predictions for LEPS = Q_ observables at precision of 0.2% were calculated thanks to the control of effects such as initial-final ^^ state interference. The results were incorporated into public packages for Monte Carlo simulations. ^= • KK - as a replacement for Monte Carlo packages such as KORALZ: we have published (Phys. Lett. B449 (1999) 97 and hep-ph/9912214) our new approach to YFS exponentiated MC's based on a new realization of the cancellation of infrared singularities to all orders in a in which the real emission is also isolated at the amplitude level. We refer to these new series of calculations as ICIC. The applications to the Z-return events at LEP2 and to the precision Z line-shape analysis at LEP1 (Phys. Lett. B456 (1999) 77; ibid B465 (1999) 254), wherein the initial-final interference radiative effects are needed in the presence of n(-y) radiation, are already well under way. 146

• KORALZ - We have also released the version 4.04 of KORALZ ( hep-ph/9905205 ) for use at LEP2 with appropriate upgrades and extensions of its various modules. For example the LEP2- compatible DIZET6.X EW library has been implemented. For, while KK.f is the calculation which will ultimately replace KORALZ, the KORALZ 4.04 version will continue to be used in LEP1 and in LEP2 future data analysis. Special tools for study of anomalous W-W-7 (hep- ph/9905225) and r-r-7 couplings necessary in experimental search for new phenomena were also published (hep-ph/9905207).

PL0002440 Old and New Physics at Hadron Colliders S. Jadach and E. Richter-Was

Our participation in the LHC program consists both of rather more theoretical activities related to techniques of summation of infrared QCD singularities and some which are rather more practical, being related to the study of possible signatures of new phenomena and their backgrounds in the explicit context of the future detector properties. The following activities should be mentioned:

• LHCYFS - We published (Mod. Phys. Lett. A14 (1999) 491) the recently developed procedure for realising n(G) radiative effects in our new YFS exponentiated approach to QCD without double counting the LL infrared effects already present in DGLAP evolved structure functions. This is a crucial step for developing hadron level MC-s of the LHCYFS-type in which multiple soft gluons are produced on an event-per-event basis for incoming hadron beams as opposed to the incoming fundamental particle beams studied in our earlier works. Our work to realize this DGLAP-YFS synthesization by MC methods continues.

• ATLFAST - the ATLFAST package including fast parametrisation of the charged tracks recon- struction in the Inner Detector and several algorithms for the reconstruction of jets is in wide use by the ATLAS collaboration. It was especially instrumental in the preparation of the AT- LAS Overall Performance TDR (CERN/LHCC/99-15) and is available together with the long write-up from the ATLAS www page http://atlasinfo.cern.ch/Atlas/GROUPS/PHYSICS/physics.html under entry: Tools for physics simulation. • LHC versus Tevatron potential for the Higgs searches — The discovery potentials for the Teva- tron and the LHC for the Higgs in the mass range 100-130 GeV have been studied intensively using the ATLFAST and related programs. We have estimated that, as concerns the comparison of the Tevatron and LHC for the 100-130 GeV Higgs mass range in the WH and ZH channels, the LHC has a signal 5 times larger than the Tevatron in the WH channel for a given integrated luminosity. The Tevatron, however, has a more favourable background. For the ZH channel, the key issue is the ability to reduce the QCD background and this applies to both the Tevatron and the LHC experiments, although it has been neglected in Tevatron studies to date. For the Tevatron, which relies on a combination of several channels for its Higgs discovery potential, confirmation of the feasibility of the ZH channel in view of the QCD background is crucial and our work in this arena continues accordingly. The results of these studies become an important factor in the discussion between the Tevatron and LHC physicists. Several very interesting stud- ies related to these topics were performed, concerning reliability of the parton shower Monte Carlo in predicting heavy flavour content of multijet events and W/Z + multi b- production.