Carlomat_4.0, a new version of the general purpose Monte Carlo program

Abstract

A new version of the general purpose Monte Carlo program carlomat is presented that substantially improves efficiency of the phase space integration by automatic inclusion of parameterizations which map away the t-channel poles and peaks due to soft and collinear photon or gluon emission. The quadruple precision versions of the routines for computation of the helicity amplitudes and phase-space parameterizations have been written and calls to them implemented in the code generation part of the program. This allows to better control numerical stability of the Monte Carlo programs generated, in particular for reactions with a virtual photon or gluon exchange in the t-channel. A new option of generating s-channel kinematics has been added which takes into account peaks due to Feynman propagators of intermediate bosons which decay into on shell final state particles, or of the top quark. It speeds up both the compilation and execution time of the Monte Carlo programs with respect to the kinematics based on topologies of the Feynman diagrams used in versions 2 and 3 of carlomat. To further speed up the execution time, the main routine of the Monte Carlo program has been supplemented with the Message Passing Interface which allows to run the program parallelly on several processors. Program summaryProgram Title:carlomat, version 4.0CPC Library link to program files:https://doi.org/10.17632/9rx84cpvwc.1Developer's repository link:https://www.kk.us.edu.pl/carlomat.htmlLicensing provisions: GPLv3Programming language: Fortran 90/95Supplementary material: ‘instructions.pdf’Journal reference of previous version: Comput. Phys. Commun. 196 (2015) 563Does the new version supersede the previous version?: YesReasons for the new version: Efficient integration over a multi particle phase space can be a challenging task. Phase space parameterizations allowing to smoothen t-channel poles, or peaks due to soft and collinear photon or gluon emission, which were not taken into account in former versions of carlomat, are now included in the multichannel Monte Carlo integration routine automatically generated by the program. This allows to obtain more reliable predictions for quite a number of reactions that are of interest, either as signal or potential sources of background, both in high or low energy particle colliders.Summary of revisions: Phase space integration routines, originally developed in Phys. Rev. D43 (1991) 3619 and Comput. Phys. Commun. 159 (2004) 106, have been brought to the form suitable for implementation in the general purpose Monte Carlo (MC) program. The code generation part of carlomat has been appropriately modified in order to include new phase space parameterizations in the automatically generated multichannel MC integration routine. Quadruple precision versions of the routines for computation of the helicity amplitudes and phase-space parameterizations have been written and calls to them implemented in the code generation part of carlomat. A number of new options have been introduced in the program which allow to control generation of the multichannel integration routine and the process of MC integration. Among them a new option of generating s-channel kinematics has been added which takes into account peaks due to Feynman propagators of intermediate bosons which decay into on shell final state particles, or of the top quark. It speeds up both the compilation and execution time of the Monte Carlo programs with respect to the kinematics based on topologies of the Feynman diagrams used in versions 2 and 3 of the program. To further speed up the execution time, the main routine of the Monte Carlo program has been supplemented with the Message Passing Interface in order to allow for a parallel run of the program on several processors.Nature of problem: Automatic generation of the MC codes for a wide class of reactions that can be measured in current and future accelerators.Solution method: The Fortran 90/95 MC programs are generated with a meta program that is also written in Fortran 90/95. All amenities of the former versions of carlomat are kept in the current version, too.