If you make use of these tunings please give credit to the appropriate experiment.
Updated 16th May 2000
For further information please contact: Richard Hemingway RYH@physics.carleton.ca for a copy of OPAL Technical Note TN652.
Recently a second tuning has become available, which includes more identified particle production rates. Further details can be found this submission to the Vancover ICHEP98 Conference. The numbers given below are for the colour reconnection model turned off (PRECO=0).
Updated 13th December 1998.
For further information please contact: Gerald Rudolph email@example.com
Updated 3rd April 1998.
Updated 26th June 1997
For further information please contact: Richard Hemingway RYH@physics.carleton.ca
|Parameter||Default||ALEPH I||ALEPH II||DELPHI||L3||OPAL||SLD|
This is Lambda_QCD; it sets the scale for the running strong coupling. At high momentum fractions (x or z) it can be identified with the fundamental QCD scale Lambda-MSbar (5 flavours). However, this relation does not necessarily hold in other regions of phase space, since higher order corrections are not treated precisely enough to remove renormalization scheme ambiguities.
See S. Catani, G. Marchesini and B.R.Webber, Nucl. Phys. B349 (1991) 635.
This is the effective gluon mass used during hadronization. To allow for non-perturbative gluon splitting RMASS(13) must be greater than 2*RMASS(1) or 2*RMASS(2).
The light quark and gluon masses acts as lower cut-offs on the parton shower evolution. They can be simulataneously set to zero provided that the shower cut-off parameters VQCUT and VGCUT are large enough. The condition to avoid divergences in the shower is:
where Q(i)=RMASS(i)+VQCUT or RMASS(i)+VGCUT for i=q or g and QCDL3 is Lambda_QCD(3-flavours) as derived from QCDLAM.
Is the shower cut-off for photons, its default value is SQRT(S), which implies no photon radiation.
After experimental cuts results show little sensitivity to VPCUT in the range 0.1 to 1.0 GeV.
CLMAX and CLPOW
These two parameters determine the maximum allowed cluster mass before forced cluster splitting occurs, according to:
Smaller values of CLPOW will increase the yield of heavier clusters (and hence of baryons) for heavy quarks, without affecting light quarks much.
This determines the mass distribution in the cluster splitting CL1 --> CL2 + CL3 when CL1 is above the maximum allowed mass. The masses of CL2 and CL3 are generated uniformly in Mass**PSPLT.
PSPLT is more influential for heavy flavour clusters. Accordingly, in version 6.1 this parameter was converted into a two-dimensional array, with PSPLT(1) controlling light (d,u,s,c) quark clusters, PSPLT(2) those that contain a b-quark.
During cluster splitting in the CLDIR=1 (default) option, a hadron which contains a quark which came from the perturbative shower (as opposed to the non-perturbative gluon splitting) retains knowledge of this quark's direction. CLSMR controls a Gaussian smearing of the hadron direction about the original constituent quark direction. Strictly the distribution is exponential in 1-COS(THETA), with mean CLSMR; increasing CLSMR increases the decorrelation.
These parameters are particularly important for b-hadrons. Accordingly, in version 6.1 they were converted into two-dimensional arrays, with CLDIR(1) and CLSMR(1) controlling light (d,u,s,c) quark clusters, CLDIR(2) and CLSMR(2) those that contain a b-quark.
PWT(3) and PWT(7)
These are a priori weights for choosing a strange quark or a diquark when splitting a cluster to form hadrons.
SNGWT and DECWT
These are a priori weights for the relative production rates of singlet (Lambda-like) and decuplet baryons comparred to octet baryons.
Analogous weights, REPWT, exist for same flavour meson of different internal S & L quantum numbers.
30 May 2000