PHOJET, some important parameters for tuning


Program version 1.05

Most of the program parameters and switches are stored in the common block /MODELS/. After the initialization call to PHOINP, the parameters can be changed.

   CHARACTER*8 MDLNA
   COMMON /MODELS/ MDLNA(50),ISWMDL(50),PARMDL(200),IPAMDL(200)

PHOJET switches

Multiple interactions

ISWMDL(15)
multiple soft and hard interaction in non-diffractive events (default 3)
ISWMDL(16)
multiple soft and hard interaction in diffraction dissociation (treated as pomeron-particle scattering, default 4)

Diffraction dissociation

ISWMDL(21)
decay of diffractively produced resonances according to transversally polarized photons (default 2)
ISWMDL(22)
mass distribution of the diffractively produced system (see manual, default 1)
ISWMDL(23)
simulation of low-mass diffraction (default 1)

PHOJET parameters

Soft interaction

The x distributions of low-pt quarks are regularized by a minimum chain mass cutoff. This cutoff is stored in CUTMU.
   COMMON /CUTOFF/ PTCUT(4),CUTMU(4),FPS(4),FPH(4),PSOMIN,XSOMIN
For each interaction type, a separate cutoff can be set (default: 1 GeV).

Hard interaction

By default, the scales Lambda for alpha_s calculations in hard scattering are taken from the parton distribution functions. These scales can be modified changing the the following factors The variables PTCUT(1-4) store the transverse momentum cutoff for the to distinguish between hard and soft interactions.
   COMMON /CUTOFF/ PTCUT(4),CUTMU(4),FPS(4),FPH(4),PSOMIN,XSOMIN
By default, an energy-dependent cutoff is calculated according to the c.m. energy of the collision. The values stored in PTCUT(1-4) should not be changed by the user. Instead, the cutoff values can be set using The virtuality cutoff for initial state radiation (ISR) is stored in The default value for the ISR cutoff is the (squared) transverse momentum cutoff.

Diffraction dissociation

The lowest mass sampled in inelastic diffractive processes is given by the maximum of both the minimum chain mass cut and the particle mass+PARMDL(46). By default, PARMDL(46) = 0.16 GeV. Note that the 1/M**2 mass distribution is suppressed by threshold effects at low masses.

JETSET parameters

There is no tuned parameter set available, the published comparisons with data have been done using the JETSET versions 7.3 and 7.4 with PARJ(42) = 1.0. All other parameters have been kept at their defaults.

Some important JETSET parameters

The defaults given for JETSET Version 7.3
 PARJ( 2) strangeness suppression (0.3)  
 MSTJ(12) popcorn effect (2)                    
 PARJ(19) popcorn (1.0)                  
 PARJ(41) Lund fragmentation function a (0.5)                   
 PARJ(42) Lund fragmentation function b (0.9)                   
 PARJ(21) sigma in soft pt distribution (0.35)

Ralph Engel March 5, 1997