beowulf

Beowulf is a computer program that calculates infrared-safe event shape variables for electron-positron annihilation in next-to-leading order Quantum Chromodynamics (QCD). It demonstrates a certain method of calculation. The style is analogous to that of Beowulf the Geat.

In its latest incarnation (see version 4.0 below) it teams up with Pythia to calculate infrared safe three jet quantities including showers and hadronization.

This program is not, by the way, related in any way to the multiprocessor computer project of the same name. The computer project project employs an army of small processors to attack a big problem. This is the opposite of the style of Beowulf the Geat, who attacked the monster Grendel all by himself.

Style of calculation

QCD calculations are normally performed by doing integrations over the three-momenta of virtual particles analytically, while the integrations over the momenta of particles in the final state are performed numerically. This highly successful method was introduced by Ellis, Ross, and Terrano (Nucl. Phys. B178, 421 (1981)). The idea of beowulf is to perform all of the integrations numerically. The basic method is described in a short paper, Phys. Rev. Lett. 81, 2638 (1998). The preprint version is hep-ph/9804454. Results for the thrust distribution are shown in this paper. A detailed presentation of the most important techniques of the method is given in Phys. Rev. D62, 014009 (2000). The preprint version of this is hep-ph/9910292. The method for sampling in the space of loop momenta in order to perform the numerical integrations is described in Phys. Rev. D64, 034018 (2001). The preprint version of this is hep-ph/0103262. The method can also be adapted to Coulomb gauge, as described in hep-ph/0204113, by M. Kramer and D. E. Soper. The use of Coulomb gauge provides the potential to go beyond a purely order alpha_s^2 calculation by including, for instance, renormalon or parton showering effects. One may expect that the approximations needed to include such effects will be simplest in a gauge in which unphysically plarized gluons do not propagate over long distances.

Also available are a talk (in gzipped postscript) from the LHC Workshop at CERN in February 1998, a talk at the RADCOR 98 conference at Barcelona in September 1998, hep-ph/9812324 , another talk at the DIS 99 conference at Zeuthen in April 1999 (which seems to have some problems with its links), and a talk at the RADCOR 2000 conference at Carmel in September 2000 hep-ph/0102031. Finally, there is a talk (in .pdf format) on doing the loop integrals by numerical integration and another on implementing showers from a workshop Monte Carlo tools for the LHC at CERN in July 2003.

beowulf version 1.1

Version 1.1 is available. Version 1.0 was perfect, of course, but there is always a reason to introduce a new version. In this case, Version 1.0 did not have any bugs that I know of, but Version 1.1 has a more systematic system of sampling points in the integration space, as described in hep-ph/0103262, and is a bit faster than Version 1.0.

There are two program files. The top level routines in beowulf.f can be easily modified by the user to calculate any three-jet-like infrared safe quantity or to change the parameters that the program uses. The subrountines in beowulfsubs.f should not be modified.

This is actually version 1.1.1. I thank Mark Ramtohul of the University of Edinburgh for pointing out some glitches in version 1.1, corrected here, that caused certain fortran compilers to complain.

beowulf version 2.0

Version 2.0, as described in hep-ph/0204113, is available. This version does the calculations in either Feynman gauge or Coulomb gauge, as the user chooses. Even though the results any one particular graph depend on the gauge, the results summed over graphs should be gauge independent. There is no real advantage to using Coulomb gauge as long as we stick to a pure next-to-leading order perturbative expansion, as in this program. However, it is hoped that the Coulomb gauge option will prove useful for extensions that go beyond fixed order perturbation theory.

Version 2.0 has the additional advantage of allowing three modes of operation: 1) Born level, 2) next-to-leading order, and 3) coefficient of the (strong coupling over pi) squared in the perturbative expansion. Previous versions allowed only the third option. The third option does not waste any computer time calculating the Born contribution, which is typically known analytically, but this option may not be convenient for users who want numbers that can be directly compared to experiments.

There are two program files. The top level routines in beowulf.f can be easily modified by the user to calculate any three-jet-like infrared safe quantity or to change the parameters that the program uses. The subrountines in beowulfsubs.f should not be modified.

beowulf version 3.0

Version 3.0, as described in hep-ph/0306222 and hep-ph/0306268 is now available. This version calculates infrared safe three jet observables in electron positron annihilation at next-to-leading order in the strong coupling while adding parton showers to the outgoing partons.

There are two program files. The top level routines in beowulf.v3_0.f90 can be easily modified by the user to calculate any three-jet-like infrared safe quantity or to change the parameters that the program uses. The subrountines in beowulfsubs.v3_0.f90 should not be modified.

The current version number is v3.0 for the top level routines and v3.01 for the subroutines. (The switch "dosoft" did not operate correctly when set to .false. in v3.0).

beowulf version 3.1

Version 3.1 is now available. It differs from version 3.01 in the definition of the scale parameter kappa used to start "secondary showering."

There are two program files. The top level routines in beowulf.v3_1.f90 can be easily modified by the user to calculate any three-jet-like infrared safe quantity or to change the parameters that the program uses. The subrountines in beowulfsubs.v3_1.f90 should not be modified.

beowulf version 4.0

Version 4.0 is now available. Here beowulf is connected to Pythia so that three jet events with full showering and hadronization are produced.

There are five program files. The master program in main.f and the top level routines in beowulf.v4_0.f90 can be easily modified by the user to calculate any three-jet-like infrared safe quantity or to change the parameters that the program uses. The subrountines in beowulfsubs.v4_0.f90, pythiamore.f, and pythialess should not be modified.

Earlier versions

Comments

I would be pleased to hear your comments by email at the address below.

Last updated 4 September 2005

Davison E. Soper
Institute of Theoretical Science
University of Oregon
Eugene OR 97403 USA
soper@physics.uoregon.edu