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Manuscript Title: GaGaRes: a Monte Carlo generator for resonance production in two- photon physics.
Authors: F.A. Berends, R. van Gulik
Program title: GaGaRes
Catalogue identifier: ADPO_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 144(2002)82
Programming language: Fortran.
Computer: Enterprise 450.
Operating system: UNIX.
Keywords: Monte Carlo, Two-photon, e+e Resonance production, Elementary particle physics, Event simulation.
Classification: 11.2.

Nature of problem:
With the advent of LEP2 higher energies for two-photon reactions became available with high luminosities. This makes it possible to search experimentally for heavier resonances created in two-photon collisions and also to determine the dependence of the two-photon cross sections on the virtualities Q1^2 and Q2^2. Moreover, the decay distributions of the resonances can be studied. These experimental possibilities make it desirable to have a program, which can simulate events as expected from our theoretical understanding of resonance production by two photons.

Solution method:
A model based on the hard scattering approach is used to describe the production of the resonances [1]. For an exact description of the decay of the produced resonance the density matrix is required. Weyl-van-der-Waerden spinor calculations are used to obtain these density matrices. Events consisting of the momenta of the resonance and the outgoing electron and positron are generated by Monte Carlo methods and are distributed according to the theortical cross section.

Additional comments:
Programming issue The program will run on any computer that runs under UNIX and can handle quadruple precision numbers (both real and complex). FORTRAN 90 offers the possibility to define own data types. This should be used on systems where complex numbers are not available in quadruple precision.

Running time:
Depends on the requested accuracy and the generated resonance. On an Enterprise450, one can produce 380 1S0 resonances/sec and 17 3P2 resonances/sec, including the corresponding density matrix. Without density matrices, one can generate 236 3P2 resonances/sec. These numbers are for weighted events. For unweighted events, these rates are typically an order of magnitude lower.

[1] G.A. Schuler, F.A. Berends, R. van Gulik, Nucl. Phys. B 523 (1998) 423.