Hard Color Singlet exchange processes in PYTHIA: HARDCOL

Introduction

Elastic parton–parton scattering by hard color singlet exchange, calculated from the BFKL equation, has been implemented as an add-on to the event generator PYTHIA.

A generator is available, HARDCOL (http://arxiv.org/abs/hep-ph/0111090); the code (Fortran) is public and can be downloaded from the author's site: http://www.isv.uu.se/thep/hardcol/ .

HARDCOL is now integrated in CMSSW, the package is GeneratorInterface/GenExtensions.

HARDCOL has specific features that operate at the parton level. The generation is stopped at this level and the output file is saved in the LHE format. It contains the information of the elastic parton–parton scattering at large momentum transfer by BFKL pomeron exchange and of the remaining partons. The hadronization process can be performed by PYTHIA (or other hadronizers) using the LHEInterface.

PYTHIA version

The HARDCOL refers to PYTHIA v6.421 (Latest modification in August 5th)

Generating events

To generate events using the HardcolGenerator, you can proceed in the following way.

Setup your environment:

> cmsenv (or eval `scramv1 runtime -(c)sh`)

Add the package to your CMSSW project area:

> cvs co GeneratorInterface/GenExtensions

In GeneratorInterface/GenExtensions there is the usual subdirectory structure:

• bin: contains the BuildFile and the subdirectory HARDCOL
  • HARDCOL: contains the code sources: main.cc, all the fortran files and the include files needed to run the generator.
• data: contains the data files, dJYs05rc1.dat, dJYs1rc1.dat and dJYs2rc1.dat.
• test: contains hardcol_par_set.nam, the namelist with the generator parameters which can be set by the user, HardcolLHE_cfg.py, the configuration file to processed the HardcolGenerator output using the LHEInterface package, and HardcolPythiaHAD_cfg.py, the configuration file to processed the HardcolGenerator output using the Pythia6HadronizerFilter package.

Build the package typing:

> scramv1 b 

In the test directory, set the parameters (see below) and run the generator.

> cd test 

> HardcolGenerator 

This will produce a HARDCOL.lhe file to be processed by the Pythia6HadronizerFilter for the hadronization. A LHE file example, with 1000 events, is HARDCOL_LHE.log.

Setting the parameters

In GeneratorInterface/GenExtensions/test/, the file hardcol_par_set.nam is the namelist to set some generator parameters.

• ENERGYOFLHC (=1.0d+4): Energy of the colliding beam in GeV.
• NUMOFEVENTS (=1000): number of Hardcol events to be generated.
• MSTP2 (=1): Select fixed (0) or running alpha_ s (1, 2) in the prefactor.
• CKIN3 (=45.): pThat cutoff.
• MSEL0 (=0): Switch off preselected PYTHIA processes
• MSUB406 (=1): Subprocesses qq-->qq, qqbar-->qqbar using BFKL.
• MSUB407 (=1): Subprocesses qg->qg, qbar g->qbar g using BFKL.
• MSUB408 (=1): Subprocesses gg->gg using BFKL.
• MSTP198 (=1): 1, 2, 3 Choose s_0 = 0.5, 1.0 or 2.0 GeV (s_0: an infrared regulator interpreted as gluon mass squared).
• irandom : the integer number that specifies the random number seed (0 < MRPY(1) < 900000000).

The LHE file is read using the Pythia6HadronizerFilter available in Pythia6Interface package.

Hardcol Production in 31X

1. Creation of ASCII LHE files outside the CMS framework
2. Publication of corresponding articles into MCDB http://mcdb.cern.ch
3. Execution of CMSSW jobs (at CERN) for retrieving the files, translating the ASCII LHE files into EDM LHE files and publishing them to DBS
4. Deployment of the files to the T2s for production.

ff, fg and gg (BFKL color singlet)

• Sample description: Jet-gap-jet in BFKL framework.
• Collider: proton-proton.
• Center of mass energy: 10 TeV.
• p_T hat cutoff: 40. GeV/c
• HARDCOL version: 1.2

Id	Group/Responsible	Sample description	Binned	Total # of evts	Cross section (pb)	Filter eff.	cfg	Time per event (s)	Size per events (MB)	Additional notes
-	Fwd PAG/S.Amaral	BFKL (HARDCOL)	no	0.5M	34640000.	1.0	-	-	-	MCDB article 290

MCDBSource

This module is essentially the same as LHESource, except that the URL of the file is retrieved from the MCDB from the given article ID (which can be looked up on the web interface). The skipEvents parameter can be used to skip through the contents, although the whole file has to be retrieved if the specified start is in the middle of one of the files. The supportedProtocols parameter lists the preferred protocol. Since MCDB resides at CERN, specifying "rfio" only makes sense there, "gsiftp" should be used from outside. Note that it requires a valid Grid proxy to be set up.

process.source = cms.Source("MCDBSource",
                articleID = cms.uint32(290),
                supportedProtocols = cms.untracked.vstring("rfio")
                # same stuff as for LHESource (seekEvent, firstEvent, firstRun, ...)
}

-- SheilaMaraSilvadoAmaral - 02 Jun 2009