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Manuscript Title: EKHARA: a Monte Carlo generator for e+e-e+e-π0 and e+e-e+e-π+π- processes
Authors: Henryk Czyz, Sergiy Ivashyn
Program title: EKHARA
Catalogue identifier: AEIO_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 182(2011)1338
Programming language: FORTRAN 77 with quadruple precision.
Computer: PC, main frame.
Operating system: Linux, Unix, MS Windows.
RAM: up to 10 Megabytes for operation of the compiled program
Keywords: Monte Carlo generator, Event simulation, Pion transition form factor, Pair production, Two-photon process, e+ e- collision.
PACS: 13.66.-a, 02.70.Uu.
Classification: 11.2, 11.6.

Nature of problem:
The first version of EKHARA [1,2] was developed to simulate a background for the pion form factor measurement at meson factories coming from the process e+e-e+e-π+π-. The newly added channel, e+e-e+e-π0, will help in the pion transition from factor studies at meson factories [3].

Solution method:
Events consisting of the momenta of the outgoing particles are generated by Monte Carlo methods. The generated events are distributed accordingly to the theoretical cross section. For the e+e-e+e-π0 mode the Monte Carlo sampling developed in [4] was adopted.

In order to compile the code, the FORTRAN 77 compiler should support quadruple precision numbers.

Unusual features:
To avoid numerical cancellations in the e+e-e+e-π+π- mode calculations are carried in the quadruple precision

Running time:
It depends on the requested mode and applied kinematic cuts. An example: on Intel Core2 Quad CPU Q6600 @ 2.40GHz, using only one thread,
  • 105 unweighted e+e-π0 events are generated in 78 seconds (no cuts),
  • 105 weighted e+e-π+π- events are generated in 38 seconds (with cuts [2]).

[1] H. Czyz, E. Nowak-Kubat, Radiative return via electron pair production: Monte Carlo simulation of the process e+e- → π+π-e+e-, Acta Phys. Polon., 2005, B36, 3425-3434
[2] H. Czyz, E. Nowak-Kubat, The reaction e+e-e+e-π+π- and the pion form factor measurements via the radiative return method, Phys. Lett., 2006, B634, 493-497
[3] G. Amelino-Camelia and others, Physics with the KLOE-2 experiment at the upgraded DAΦNE, Eur.Phys. J., 2010, C68, 619-681
[4] G.A. Schuler, Two-photon physics with GALUGA 2.0, Comput. Phys. Commun., 1998, 108, 279-303