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Manuscript Title: YFS2: the second order Monte Carlo for fermion pair production at LEP/SLC with the initial state radiation of two hard and multiple soft photons.
Authors: S. Jadach, B.F.L. Ward
Program title: YFS2.02
Catalogue identifier: ABLO_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 56(1990)351
Programming language: Fortran.
Computer: IBM 3081/3090/3033.
Operating system: CERNVM, SLACVM, UTKVM1.
RAM: 100K words
Word size: 32
Keywords: Particle physics, Elementary, Event simulation, Radiative corrections, Initial state Bremsstrahlung, Monte carlo simulation, Electrodynamics quantum, Exponentiation, Multiphoton emission, Z0 boson, E+e- annihilation, Electroweak theory.
Classification: 11.2.

Nature of problem:
High statistics data samples will be available soon in LEP and SLC experiments allowing for a precise measurements of the Z0 mass, width and various asymmetrics. Since initial state QED bremsstrahlung distorts the shape of the Z0 resonance very strongly it will not be possible to make any statement about the agreement of these data with the standard electroweak model before one is able to calculate all these effects very precisely. The above QED effects depend usually on the experimental acceptance and selection criteria and it is therefore practically impossible to calculate them analytically. It is already known that in order to reach a sufficient precision level one has to sum up contributions from multiple soft photons and from up to two hard photons. In particular any program of the class presented in CPC 28(1983)185 is not sufficiently precise. A number of analytical exact and approximate calculations exist for the total cross section. They are very instructive but they cannot help in removing detector acceptance from the data and/or provide the integrated cross section in the presence of the realistic, complicated set of cut-offs.

Solution method:
The Monte Carlo event generator is the well known answer to the above problems. Any given experimental acceptance and cut-offs may be introduced easily by rejecting some part of the generated events. The main technical problem in the construction of the Monte Carlo event generator with multiple soft and hard photons is related to a necessity of generating photon momenta within a multibody Lorentz invariant phase space with the simultaneous importance sampling for strong singularities due to the bremsstrahlung matrix element and Z0 resonance. An elegant solution of this problem is presented in this work.

Outgoing fermion f may be any lepton or quark except of f = e-,Nu e. The program is best suited for calculating the total cross section and the beam polarization asymmetry and also for all sorts of detector acceptance studies. Pure electroweak corrections are not included, but we provide a detailed explanation of how to do it. Due to omission of the final state bremsstrahlung and its interference with the initial state bremsstrahlung this program cannot be used for calculation of the final state polarization asymmetry (Tau pair production) and some care is necessary in the case of calculation of the forward backward asymmetry.

Running time:
CPU time for one event depends on center of mass energy and infrared cut-off. Typically, at the Z0 position generating 1000 events costs 9 CPU seconds and off Z0 resonance about 18 CPU seconds on the IBM 3081.