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Manuscript Title: WWGENPV: a Monte Carlo event generator for four-fermion production in e+e- -> W+W- ->4f.
Authors: G. Montagna, O. Nicrosini, F. Piccinini
Program title: WWGENPV
Catalogue identifier: ADBU_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 90(1995)141
Programming language: Fortran.
Computer: DEC VAX.
Operating system: VMS, UNIX.
RAM: 500K words
Word size: 32
Keywords: Particle physics, Elementary, Event simulation, High energy electron Positron collisions, W-pair off-shell Production, Four-fermion final state, Lep2, Corrections qed, Electron structure Functions, Experimental cuts, Monte carlo integration, Event generation, Importance sampling.
Classification: 11.2.

Nature of problem:
The precise measurement of the W-boson mass MW constitutes a primary task of the forthcoming experiments at high energy electron-positron collider LEP2 (2MW <= sqrt(s) <= 210 GeV). This experimental goal will be possible through the study of the four-fermion reaction e+e- -> W+W- -> 4f. A meaningful comparison between theory and experiment requires an accurate description of the fully exclusive process e+e- -> W+W- -> 4f, including the main effects of radiative corrections, with the final goal of providing predictions for the realistic distributions measured by the experiments [2].

Solution method:
In order to efficiently perform the high-dimensional integration (due to the four-fermion final state) in the presence of realistic cuts, the total cross section, the energy and invariant mass loss and the W invariant mass distributions are computed by means of a Monte Carlo integration for weighted events. For simulation purposes, the program can also be used as an event generator that provides a sample of unweighted events, defined as the components of the four final state fermions momenta, plus the radiative variables of the incoming electron and positron, plus sqrt(s), stored into proper n-tuples. The importance sampling technique [3] is employed to take care of the peaking behaviour of the integrand, both in the integration and in the generation mode.

Restrictions:
Electroweak processes originating the same four-fermion final state of the double-resonanting diagrams e+e- -> W+W- -> 4f (background processes) are not included in the present version of the program. The treatment of the initial state radiation is limited to the gauge invariant subset of the leading logarithmic corrections which are the most important contribution from the phenomenological point of view. Furthermore, the kinematics of the event in the presence of the initial state radiation is limited to the collinear approximation, i.e. PT/PL effects are presently neglected.

Unusual features:
none (The random number generator RANLUX [1] (included in the code) and routines from the CERN Program Library (not included) are used.)

Running time:
As integrator, the code needs about 5h of HP 9000/735 for generating 10**8 weighted events; the relative error on the corss section and the energy (invariant mass) loss are about 2*10**-4 and 6*10**-4, respectively. The generation of a sample of 10**4 unweighted events requires about 3 min on the same system.

References:
[1] F. James, Comput. Phys. Commun. 79(1994)111.
[2] G. Montagna, O. Nicrosini, G. Passarino and F. Piccinini, Phys. Lett. B 348(1995)178, and references therein.
[3] F. James, Rep. Prog. Phys. 34(1980)1145.