Programs in Physics & Physical Chemistry
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|Manuscript Title: Two-photon physics with GALUGA 2.0.|
|Authors: G.A. Schuler|
|Program title: GALUGA|
|Catalogue identifier: ADHE_v1_0|
Distribution format: tar.gz
|Journal reference: Comput. Phys. Commun. 108(1998)279|
|Programming language: Fortran.|
|Computer: IBM RS/6000.|
|Operating system: UNIX.|
|Keywords: Particle physics, Elementary, Event simulation, Monte Carlo, Two-photon, e+e Azimuthal dependence.|
Nature of problem:
Hadronic two-photon reactions in a new energy domain are becoming accessible with LEP2. Unlike purely electroweak processes, hadronic processes contain dominant non-perturbative components parametrized by suitable structure functions, which are functions of the two-photon invariant mass W and the photon virtualities Q1 and Q2. It is hence advantageous to have a Monte Carlo program that can generate events with the possibility to keep W and, optionally, Qi at fixed, user-defined values. Moreover, at least one program with an exact treatment of both the kinematics and the dynamics over the whole range m**2 >> m**2(W/sqrt(s))**4 ~< s (m is the electron mass and sqrt(s) the e+e- c.m. energy) is needed, (i) to check the various approximations used in other programs, and (ii) to be able to explore additional information on the hadronic physics, e.g. coded in azimuthal dependences.
The differential cross section for e+e- -> e+e-X at given two-photon invariant mass W is rewritten in terms of four invariants with the photon virtualities Qi as the two outermost integration variables (next to W), in order to simultaneously cope with antitagged and tagged electron modes. Due care is taken of numerically stable expressions while keeping all electron-mass and Qi dependences. Special attention is devoted to the azimuthal dependences of the cross section. Cuts on the scattered electrons are to a large extent incorporated analytically and suitable mappings introduced to deal with the peaking structure of the differential cross section. The event generation yields either weighted events or unweighted ones (i.e. equally weighted events with weight 1), the latter based on the hit-or-miss technique. Optionally, VEGAS  can be invoked to (i) obtain an accurate estimate of the integrated cross section and (ii) improve the event generation efficiency through additional variable mappings provided by the grid information of VEGAS. The program is set up so that additional hadronic (or leptonic) reactions can easily be added.
Other subprograms used: VEGAS, RANLUX , HBOOK  and DATIME .
Summary of revisions:
Differences with earlier version 
The integration time depends on the required cross section accuracy and the applied cuts. For instance, 13 seconds on an IBM RS/6000 yields an accuracy of the VEGAS integration of about 0.1 per cent for the antitag mode or of about 0.2 per cent for a typical single-tag mode; within the same time the error of the simple Monte Carlo integration is about 0.5 per cent for either mode. Event generation with or without VEGAS improvement and for either tag mode takes about 4x10**-4 (2x10**-3) seconds per event for weighted (unweighted) events.
|||G.P. Lepage, J. Comp. Phys. 27 (1978) 192.|
|||F. James, "RANLUX: a Fortran implementation of the high-quality pseudorandom number generator of Luscher", CERN Program Library V115, Comput. Phys. Commun. 79 (1994) 111; M. Luscher, Comput. Phys. Commun. 79 (1994) 100.|
|||R. Brun and D. Lienart, "HBOOK User Guide -- Version 4", CERN Program Library Y250, 1988.|
|||J. Harms et al., "DATIME: Job Time and Date", CERN Program Library Z007, 1991.|
|||G.A. Schuler, preprint CERN-TH/96-313 (1996).|
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