carlomat_3.0, an automatic tool for the electron-positron annihilation into hadrons at low energies

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• 作者：Karol Kołodziej ^{karol.kolodziej@us.edu.pl" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Automation of calculations ; Monte Carlo programs ; Electron&ndash ; positron annihilation to hadrons at low energies ; Effective models
• 刊名：Computer Physics Communications
• 出版年：2015
• 出版时间：November 2015
• 年：2015
• 卷：196
• 期：Complete
• 页码：563-568
• 全文大小：513 K

文摘

A new version of carlomat that allows to generate automatically the Monte Carlo programs dedicated to the description of the processes $e^{+}{e}^{-}\to \mathrm{hadrons}$ at low center-of-mass energies is presented. The program has been substantially modified in order to incorporate the photon–vector meson mixing terms and to make possible computation of the helicity amplitudes involving the Feynman interaction vertices of new tensor structures, like those predicted by the Resonance Chiral Theory or Hidden Local Symmetry model, and the effective Lagrangian of the electromagnetic interaction of the nucleons. Moreover, a number of new options have been introduced in the program in order to enable a better control over the effective models implemented. In particular, they offer a possibility to determine the dominant production mechanisms of the final state chosen by the user.

Program summary

Program title: carlomat, version 3.0

Catalogue identifier: AEDQ_v3_0

Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEDQ_v3_0.html

Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland

Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html

No. of lines in distributed program, including test data, etc.: 71596

No. of bytes in distributed program, including test data, etc.: 2804246

Distribution format: tar.gz

Programming language: Fortran 90/95.

Computer: All.

Operating system: Linux.

Catalogue identifier of previous version: AEDQ_v2_0

Journal reference of previous version: Comput. Phys. Comm. 185(2014)323

Classification: 4.4, 11.2, 11.6.

Does the new version supersede the previous version?: Yes

Nature of problem:   Predictions for reactions of low energy e⁺e⁻$e^{+}{e}^{-}$-annihilation into final states containing pions, kaons, light vector mesons, one or more photons and light fermion pairs within the Standard Model and effective models inspired by the Resonance Chiral Theory or Hidden Local Symmetry model. Description of the electromagnetic production of nucleon pairs within the effective Lagrangian approach.

Solution method:   As in former versions, a program for the Monte Carlo (MC) simulation of e⁺e⁻→$e^{+}{e}^{-}\to$ hadrons at low energies is generated in a fully automatic way for a user specified process. However, the user is supposed to select a number of options and adjust arbitrary parameters in the main part of the MC computation program in order to obtain possibly the best description of experimental data. To this end, the user can also easily supplement her/his own formulae for s-dependent vector meson widths or running couplings by appropriately modifying corresponding subroutines.

Reasons for new version:   Processes of e⁺e⁻→$e^{+}{e}^{-}\to$ hadrons in the energy range below the J/Ψ$J/\Psi$ threshold cannot be described in the framework of perturbative quantum chromodynamics. The scalar electrodynamics which has been implemented in carlomat 2.0 [1] does not provide a satisfactory description either. The most promising theoretical frameworks in this context are the Resonance Chiral Theory or Hidden Local Symmetry model which, among others, involve the photon–vector meson mixing and a number of vertices of rather complicated Lorentz tensor structure that is not present in the Standard Model or scalar QED. Already at low energies, the hadronic final states may consist of several particles, such as pions, kaons, or nucleons which can be accompanied by one or more photons, or light fermion pairs such as e⁺e⁻$e^{+}{e}^{-}$, or μ⁺μ⁻${\mu }^{+}{\mu }^{-}$. The number of Feynman diagrams of such multiparticle reactions grows substantially with increasing numbers of interaction vertices and mixing terms of the effective models. Therefore, it is highly desirable to automatize the calculations. At the same time, new program options should provide the user with an easy way of implementing her/his own changes in the program in order to better fit the experimental data.

Summary of revisions:   The code generation part of the program has been substantially modified in order to incorporate the photon–vector meson mixing and calls to new subroutines for computation of the helicity amplitudes of the building blocks and complete Feynman diagrams which contain new interaction vertices and mixing terms. The subroutine library of carlomat has been extended to make possible computation of the helicity amplitudes involving the Feynman interaction vertices of new Lorentz tensor structures. Many subroutines have been modified in order to incorporate the q²$q^2$-dependent couplings and vector meson widths. A number of options have been introduced in order to give a better control of the effective model implemented.

Restrictions:   As in previous versions of the program the number of particles is limited to 12 which exceeds typical numbers of particles of the exclusive low energy e⁺e⁻$e^{+}{e}^{-}$-annihilation processes. However, in the presence of photon–vector meson mixing, the Feynman diagrams proliferate, for example, with currently implemented Feynman rules, there are 90672 diagrams of e⁺e⁻→3(π⁺π⁻)$e^{+}{e}^{-}\to 3\left({\pi }^{+}{\pi }^{-}\right)$. Hence, the compilation time of generated code may become very long already for processes with a smaller number of the final state particles. Many couplings of the effective models are not known with good enough precision and must be adjusted in consecutive runs of the program in order to obtain a satisfactory description of the experimental data.

Running time:   Depends on the selected process. Typical running time for the code generation varies from a fraction of a second for, e.g., e⁺e⁻→π⁺π⁻K⁺K⁻$e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{K}^{+}{K}^{-}$ to about 2 min for e⁺e⁻→3(π⁺π⁻)$e^{+}{e}^{-}\to 3\left({\pi }^{+}{\pi }^{-}\right)$. It may become substantially longer for processes with more particles in the final state. The execution time necessary to produce the appended test output files for e⁺e⁻→π⁺π⁻μ⁺μ⁻γ$e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\gamma$ and e⁺e⁻→π⁺π⁻π⁺π⁻γ$e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}\gamma$ was 13s and 4s, respectively. The code generation for both processes took a fraction of a second time for each process.

References:

[1]

K. Kolodziej, Comput. Phys. Commun. 185 (2014) 323.