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Manuscript Title: DIPSI, a Monte Carlo generator for elastic vector meson production in charged lepton-proton scattering.
Authors: M. Arneodo, L. Lamberti, M. Ryskin
Program title: DIPSI version 2.4
Catalogue identifier: ADEG_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 100(1997)195
Programming language: Fortran.
Computer: DEC Alphastation 200.
Operating system: VMS, Open-VMS, Ultrix, OSF.
RAM: 1M words
Keywords: Particle physics, Elementary, Event simulation, Elastic vector meson, Production, Diffractive scattering, Pomeron, Deep inelastic, Scattering, Perturbative quantum, Chromodynamics (pqcd), Gluon density, Structure functions, Cross section.
Classification: 11.2.

Nature of problem:
The reaction e(mu) + p -> e(mu) + p + V, in which an electron or a muon scatters from a proton and a vector meson is produced, can be described as follows. The virtual photon radiated by the lepton fluctuates into a quark-antiquark pair which then interacts with the proton via pomeron exchange. This reaction may allow the investigation of the properties of the pomeron as well as of the hadronic features of the photon. The electron-proton collider HERA at DESY has opened the possibility of studying this process at centre of mass energies before unattained. Furthermore, recent calculations in which the pomeron is described in terms of a two-gluon system indicate that the cross section for elastic vector meson production is proportional to the square of the gluon momentum density in the proton. This may provide a new and sensitive tool to extract the gluon distribution in the small xbar region, where xbar is the fraction of the proton's momentum carried by the gluon.

Solution method:
A Monte Carlo generator based on a recent QCD leading logarithm calculation [1] has been written. The user can select, by means of control cards, the type and decay mode of the vector meson, the kinematic regime, the generation range for the independent variables and the distribution from which they are sampled. Each event is assigned a weight proportional to its cross section. It is thus possible to generate the independent variables according to distributions different from those expected by the model, thereby enhancing, if desired, the statistical significance of the sample in regions where the cross section is small.

(i) At present the following vector meson decay modes are implemented:
 - rho0 -> pi+pi-,                                                       
 - omega -> pi+pi-pi0 or pi+pi-,                                         
 - phi -> K+K-, KL0 KS0 or pi+pi-pi0,                                    
 - rho(1450) -> pi+pi-, pi+pi-rho0 or pi0pi0rho0,                        
 - rho(1700) -> pi+pi-, pi+pi-rho0 or pi0pi0rho0,                        
 - J/psi -> e+e-, mu+mu- or pi+pi-pi0,                                   
 - psi'(3600) -> e+e-, mu+mu-, pi+pi-J/psi or pi0pi0J/psi,               
 - Upsilon -> e+e-, mu+mu- or pi+pi-pi0.
(ii) Theoretical restrictions.
The model is applicable in the region xbar < 0.1 and qbar**2 > 2GeV**2, where xbar = (Q**2 + mv**2 + pt**2)/W**2 is the fraction of the proton's momentum carried by the two-gluon system and qbar**2 = (Q**2 +mv**2 + pt**2)/4 is the scale at which the gluon momentum density xbar g(xbar, qbar**2) is probed. Here mv is the vector meson mass, Q**2 is the virtuality of the photon and pt is the vector meson transverse momentum with respect to the virtual photon direction. The program can reproduce the experimental results also for qbar**2 < 2 GeV**2 [2,3], if an appropriate parametrisation for xbarg(xbar,qbar**2) is used; in this region however it is not clear whether the function xbarg(xbar,qbar**2) can be interpreted as the gluon momentum density in the target.

Running time:
The running time depends on the event type, in addition to the type of computer used. For instance, for the process e p -> e p J/psi with J/psi -> e+e-, one event takes 0.7 ms on a DEC Alpha station 200 4/233 (with the OSF1 operating system and 96 Mbyte RAM), 3.1 ms on a DEC 5000/240 and 7.1 ms on a DEC 5000/125 station, both with 48 Mbyte RAM and running the Ultrix operating system. On the same CPUs the process e p -> e p omega with omega -> pi+pi-pi0 uses 1.5, 8.2 and 14.6 ms/event, respectively.

[1] M.G. Ryskin, Z. Phys. C 57(1993)89.
[2] M. Derrick et al., ZEUS Collab., Z. Phys. C 69(1995)39.
[3] L. Lamberti, Fotoproduzione esclusiva di mesoni vettori nell' esperimento ZEUS ad HERA, Tesi di Dottorato, University of Torino (1995), unpublished
[in Italian] .