Efficient evolution of unpolarized and polarized parton distributions with QCD- Pegasus

Abstract

The Fortran package QCD- Pegasus is presented. This program provides fast, flexible and accurate solutions of the evolution equations for unpolarized and polarized parton distributions of hadrons in perturbative QCD. The evolution is performed using the symbolic moment-space solutions on a one-fits-all Mellin inversion contour. User options include the order of the evolution including the next-to-next-to-leading order in the unpolarized case, the type of the evolution including an emulation of brute-force solutions, the evolution with a fixed number n f of flavors or in the variable- n f scheme, and the evolution with a renormalization scale unequal to the factorization scale. The initial distributions are needed in a form facilitating the computation of the complex Mellin moments. Program summary Title of program: QCD- Pegasus Version: 1.0 Catalogue identifier: ADVN Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADVN Program obtainable from: CPC Program Library Queen's University of Belfast, N. Ireland License: GNU Public License Computers: all Operating systems: all Program language: Fortran 77 (using the common compiler extension of procedure names with more than six characters) Memory required to execute: negligible ( < 1 MB ) Other programs called: none External files needed: none Number of lines in distributed program, including test data, etc.: 8157 Number of bytes in distributed program, including test data, etc.: 240 578 Distribution format: tar.gz Nature of the physical problem: Solution of the evolution equations for the unpolarized and polarized parton distributions of hadrons at leading order (LO), next-to-leading order and next-to-next-to-leading order of perturbative QCD. Evolution performed either with a fixed number n f of effectively massless quark flavors or in the variable- n f scheme. The calculation of observables from the parton distributions is not part of the present package. Method of solution: Analytic solution in Mellin space (beyond LO in general by power-expansion around the lowest-order expansion) followed by a fast Mellin inversion to x-space using a fixed one-fits-all contour. Restrictions on complexity of the problem: The initial distributions for the evolution are required in a form facilitating an efficient calculation of their complex Mellin moments. The ratio of the renormalization and factorization scales μ r / μ has to be a fixed number. Typical running time: One to ten seconds, on a PC with a 2.0 GHz Pentium-IV processor, for performing the evolution of 200 initial distributions to 500 ( x , μ ) points each. For more details see Section 6.