PROGRAM SUMMARY
Title of program:
GaGaRes
Catalogue identifier:
ADPO
Ref. in CPC:
144(2002)82
Distribution format: gzip file
Operating system: UNIX
Number of lines in distributed program, including test data, etc:
6009
Keywords:
Monte Carlo, Two-photon, e+e Resonance production, Elementary
particle physics, Event simulation.
Programming language used: Fortran
Computer:
Enterprise 450 .
Nature of physical 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.
Method of solution:
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.
Typical 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.
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.
References:
[1] G.A. Schuler, F.A. Berends, R. van Gulik, Nucl. Phys. B 523 (1998)
423.