A non-trivial question is to know which parameter values to use. The
default values stored in the program are based on comparisons with
LEP
data at around 91 GeV [LEP90], using a
parton-shower picture followed by string fragmentation. Some examples
of more recent parameter sets are found in [Kno96]. If
fragmentation is indeed a universal phenomenon, as
we would like to think, then the same parameters should also apply at
other energies and in other processes. The former aspect, at least,
seems to be borne out by comparisons with lower-energy PETRA/PEP
data and higher-energy LEP2 data. Note, however, that the choice of
parameters is intertwined with
the choice of perturbative QCD description. If instead matrix elements
are used, a best fit to 30 GeV data would require the values
`PARJ(21) = 0.40`, `PARJ(41) = 1.0` and `PARJ(42) = 0.7`.
With matrix elements one does not expect an energy
independence of the parameters, since the effective minimum invariant
mass cut-off is then energy dependent, i.e. so is the amount of soft
gluon emission effects lumped together with the fragmentation
parameters. This is indeed confirmed by the LEP data.
A mismatch in the perturbative QCD treatment could also
lead to small differences between different processes.

It is often said that the string fragmentation model contains a wealth
of parameters. This is certainly true, but it must be remembered that
most of these deal with flavour properties, and to a large extent
factorize from the treatment of the general event shape. In a fit to
the latter it is therefore usually enough to consider the parameters of
the perturbative QCD treatment, like in
and a
shower cut-off (or
itself and
, if matrix
elements are used), the and parameter of the Lund symmetric
fragmentation function (`PARJ(41)` and `PARJ(42)`) and the
width of the transverse momentum distribution
(`PARJ(21)`). In addition, the and parameters
are very strongly correlated by the requirement of having the correct
average multiplicity, such that in a typical plot, the
allowed region corresponds to a very narrow but very long valley,
stretched diagonally from small (,) pairs to large ones.
As to the flavour parameters, these are certainly many more, but most
of them are understood qualitatively within one single framework, that
of tunnelling pair production of flavours.

Since the use of independent fragmentation has fallen in disrespect, it should be pointed out that the default parameters here are not particularly well tuned to the data. This especially applies if one, in addition to asking for independent fragmentation, also asks for another setup of fragmentation functions, i.e. other than the standard Lund symmetric one. In particular, note that most fits to the popular Peterson/SLAC heavy-flavour fragmentation function are based on the actual observed spectrum. In a Monte Carlo simulation, one must then start out with something harder, to compensate for the energy lost by initial-state photon radiation and gluon bremsstrahlung. Since independent fragmentation is not collinear safe (i.e, the emission of a collinear gluon changes the properties of the final event), the tuning is strongly dependent on the perturbative QCD treatment chosen. All the parameters needed for a tuning of independent fragmentation are available, however.