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Manuscript Title: OEDIPUS: onium evolution, dipole interaction and perturbative unitarisation simulation.
Authors: G.P. Salam
Program title: OEDIPUS
Catalogue identifier: ADGK_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 105(1997)62
Programming language: Fortran.
Computer: DEC Alpha 3000.
Operating system: Unix (OSF3.2/ SunOS-4.1.3/ Solaris-2.4).
RAM: 10M words
Word size: 32
Keywords: Particle physics, Elementary, Qcd, Small-x, Bfkl, Pomeron, Onium-onium scattering, Dipoles, Monte carlo, Unitarity.
Classification: 11.5.

Nature of problem:
BFKL evolution [1] of a heavy (quark-)onium, to give full information on the rapidity and transverse positions of gluons carrying a small fraction x of the onium's longitudinal momentum. This informaiton can be used to calculate a variety of onium-onium scattering cross sections, including multiple pomeron contributions, which restore unitarity.

Solution method:
A Monte Carlo simulation of the evolution in rapidity of the gluon structure of the onium, using Mueller's colour dipole formulation of small-x physics [2].

The number of dipoles (gluons) in an onium grows as an exponential of the rapidity, and inversely with the value of a cutoff used to regulate an ultra-violet divergence. The time for evolution of an onium is proportional to the number of dipoles, while that for calculation of the onium-onium scattering amplitude goes as the square of the number of dipoles. Memory restrictions also arise from the need to store large configurations of dipoles (very large fluctuations can occur in the number of dipoles present).

Running time:
On a DEC Alpha 3000 processor, evolution generates about 10000 gluons per second. At a rapidity Y = 16 and an ultra-violet cutoff of 0.01 times the onium size, a day's running will generate about 10**5 onium- onium scatterings.

[1] Ya.Ya. Balitskii and L.N. Lipatov, Sov. Phys. JETP 28 (1978) 822; E.A. Kuraev, L.N. Lipatov and V.S. Fadin, Sov. Phys. JETP 45 (1977) 199; L.N. Lipatov, Sov. Phys. JETP 63 (1986) 904.
[2] A.H. Mueller and B. Patel, Nucl. Phys. B 425 (1994) 471; A.H. Mueller, Nucl. Phys. B 437 (1995) 107.