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Manuscript Title: Wphact 2.0: a fully massive monte carlo generator for four fermion physics at e+e- colliders.
Authors: E. Accomando, A. Ballestrero, E. Maina
Program title: WPHACT, version 2.0
Catalogue identifier: ADEN_v2_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 150(2003)166
Programming language: Fortran.
Computer: Compaq ALPHA workstation, HP workstation, Linux Intel PC.
Operating system: UNIX, LINUX, VMS.
RAM: 500K words
Word size: 32
Keywords: Elementary particle physics, Event simulation, High energy electron positron collisions, Four-fermion final state, W-pair production, Higgs, Z-pair production, LEP2, NLC, QED corrections, Electron structure, Functions, Coulomb corrections, Anomalous couplings, Massive matrix elements, Gauge invariance, Fermion loop, Single W, Single Z, Running alphaQED, CKM mixing, Resonances.
Classification: 11.2.

Nature of problem:
All e+e- Standard Model (SM) processes with four fermion final states are necessary for detailed studies of the properties of the W and of the Higgs bosons, for measuring double and single boson production, for high precision tests of the SM and for evaluating backgrounds to searches. One wants therefore a Monte Carlo event generator which provides an accurate description of all four fermion processes in the largest possible region of phase space and which can be used for realistic experimental simulations.

Solution method:
Full tree level matrix elements for all processes are computed by means of subroutines which make use of the helicity formalism of ref. [1]-[2]. The speed in computing these amplitudes, that the above mentioned method allows, is essential to take exactly into account fermion masses and to obtain high precision in a reasonable amount of CPU time also in regions with large and possibly overlapping enhancements.

Reasons for new version:
Extend the code to cover in a reliable way all t-channel dominated processes and low ffbar invariant mass regions. Introduce the possibility to simultaneously generate unweighted events for all processes or any user-selected subset. Include photon pt and CKM mixing.

Summary of revisions:
Fully massive matrix elements, new unweighted event generation for any subset of four fermion final states in a single run, implementation of the Imaginary Fermion Loop gauge restoring scheme, new phase space mappings, new input system, possibility to compute subsets of Feynman diagrams and options for including ISR via QEDPS, running alphaQED, CKM mixing, resonances in qqbar channels.

Restrictions:
QED radiative corrections are implemented only in Initial State Radiation and in Coulomb corrections. Final state radiation is not computed when unweighted events are not requested. Only the imaginary part of Fermion Loop corrections is implemented. QCD corrections are introduced in an approximate way.

Unusual features:
STRUCTURE declarations are used.

Additional comments:
Computer: Any computer with FORTRAN77 compiler which admits structures. WPHACT has been tested on Compaq ALPHA and HP stations as well as on Linux Intel PC. For this last type of machine the Portland Group pgf77 compiler has been employed, while the GNU compiler cannot be used as it does not have the STRUCTURE extension. Compaq and Portland Group FORTRAN90 compilers can also be used.

Running time:
The running time strongly depends on the process considered and on the precision requested. The example reported in the test run took about 90 minutes on a 500 MHz ALPHA computer, which corresponds to effectively generating more than 1500 weighted e-e+mu-mu+ events per second.

References:
[1] A. Ballestrero and E. Maina, Phys. Lett. B350 (1995) 225
[hep-ph/9403244] .
[2] A. Ballestrero, 'PHACT 1.0 - Program for Helicity Amplitudes Calculations with Tau matrices
[hep-ph/9911318] in 'Proceedings of the 14th International Workshop on High Energy Physics and Quantum Field Theory (QFTHEP 99)', B.B. Levchenko and V.I. Savrin eds. (SINP MSU Moscow), pg.303.