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Manuscript Title: WPHACT 1.O, a program for WW, Higgs and 4 fermion physics at e+e- colliders.
Authors: E. Accomando, A. Ballestrero
Program title: WPHACT
Catalogue identifier: ADEN_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 99(1997)270
Programming language: Fortran.
Computer: DEC VAX.
Operating system: VMS, OVMS, UNIX.
RAM: 500K words
Word size: 32
Keywords: Particle physics, Elementary, Event simulation, High energy electron Positron collisions, Four-fermion final state, W-pair production, Higgs, Z-pair production, Lep2, Nlc, Corrections qed, Electron structure, Functions, Coulomb corrections, Anomalous couplings.
Classification: 11.2.

Subprograms used:
Cat Id Title Reference
ACTU_v1_0 PYTHIA 5.7 AND JETSET 7.4 CPC 82(1994)74

Nature of problem:
The forthcoming experiments at the high energy electron-positron collider LEP2 will be mainly concerned with WW physics and Higgs search. The production of two W's will allow the direct study of the triple- boson coupling and a precise measurement of the mass of the W. The search for the Higgs is of primary importance for understanding the problem of mass generation in the Standard Model (SM). Small deviations from the SM will be important for discovering possible new physics. Both WW and Higgs production will result in a four-fermion final state. It is therefore mandatory to have accurate predictions for all physical processes with a four fermion final state in order to have full control on signals as well as backgrounds to the processes of interest. The same kind of processes will also play a fundamental role in electron- positron accelerators at higher energy which will be able to extend Higgs search at higher values of the mass and probe triple gauge boson physics and gauge cancellations.

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. [3]-[4]. The number of Feynman diagrams in the various channels varies from 3 to 144. The velocity in computing these amplitudes that the above mentioned method allows, becomes therefore essential to take exactly into account fermion masses and to obtain high precision in a reasonable amount of CPU time. Different integration variables for the phase space are employed in order to dispose of the peak structure of the resonating diagrams for the different processes. The adaptive routine VEGAS [1] is used for the numerical evaluation of the integrals. Distributions can be produced to study the behaviour of any variable of interest. For simulation purposes, the program can also be used as an event generator that provides unweighted events. An interface to standard hadronization packages and specifically to JETSET [5] is provided.

Only for processes with b's in the final state the masses of the fermions are accounted for. Final state radiation is not implemented. Initial state radiation (ISR) is included through Structure Functions and no photons pt is computed. QCD corrections are introduced only in an approximate way.

Unusual features:
REAL*8 and COMPLEX*16 variables and STRUCTURE declarations are used. Compilation on HP/Apollo stations has to be performed with -k option.

Running time:
The running time strongly depends on the process considered and on the precision requested. Some examples are reported in Table 1. For the typical final state mu- nubarmu u dbar with ISR the time per call on AlphaStation 600 5/333 is 6. x 10**-5 sec. The longest time for call is 6. x 10**-4 sec. for b bbar b bbar. At Lep2 energies, 5 M calls (about 5 minutes) are used to obtain for mu- nubarmu u dbar a cross section (with ISR) with a typical estimated relative error of 2 x 10**-4 sec. The same process can be evaluated in about 40 sec. with less than 1 M calls at permill level. All above running times have to be multiplied approximately by a factor 3 for an AlphaServer 2100 4/200 computer.