VINCIA Tunes

VINCIA accepts input of tune presets in the form of a standard PYTHIA 8 command file whose name and location can be specified by the user. In this way, a particular set of user-defined parameters can easily be made into a tune set by simply copying the relevant parts of the user's normal command file (i.e., omitting the process-specific and program control parameters) into a new file that can then be shared and/or submitted to the VINCIA authors for possible inclusion in future distributions.

Although there are obviously parameters that it makes more sense to tune than others, there is no explicit restriction imposed on what parameters are allowed to be present in this file. This implies some responsibility on the part of the user. As a guideline, the main parameters that need to be properly tuned are the non-perturbative hadronization parameters used in PYTHIA's string fragmentation model. Since PYTHIA and VINCIA treat soft radiation somewhat differently, there can be important differences between the two in the soft region that the hadronization model will not re-absorb automatically and which therefore only a retuning can address. Apart from this, the value of the strong coupling used in the shower is also normally considered a tuning parameter.

In versions before 1.022, no systematic attempt had been made to tune VINCIA to describe, e.g., LEP data, beyond setting the default shower alphaS and cutoff parameters to be similar to those used by PYTHIA 8 and then relying on universality of the corrections below the cutoff. Starting from VINCIA version 1.022, however, dedicated tunes of VINCIA itself have been included with the standard distribution.

word  Vincia:tuneFile   (default = tunes/jeppsson3.cmnd)
Name of a command file containing tune presets for VINCIA. Note: the requested file will only be read in when VINCIA is switched on, in order not to interfere with the PYTHIA settings when VINCIA is switched off. Note: a special value for this parameter is "none", in which case no tune file will be used (i.e., PYTHIA's parameters will be used as they are). Note: the entries in the tune file will be superseded by any user modifications made in the main command file given to the VINCIA constructor. This should allow sufficient flexibility to explore user variations away from the tuned values. The list of currently available tunes is as follows:
option none : No tune file will be read.
option tunes/jeppsson.cmnd : A first tune of VINCIA+PYTHIA to LEP data, by M. H. Jeppsson, April 2010.
option tunes/jeppsson2.cmnd : A slight modification of the original Jeppssson tune of VINCIA+PYTHIA to LEP data, by M. H. Jeppsson and P. Skands, April 2010.
option tunes/jeppsson3.cmnd : A slight modification of the Jeppssson 2 tune, with very slightly increased baryon production, to improve the K/Lambda ratio. March 2011.

Note on user tunes: in order to make the tunings more stable against possible changes in the program defaults (be it PYTHIA or VINCIA), it is advisable to include all relevant parameter values explicitly in the tune file, rather than letting parameters that retain their (version-specific) default values be defined implicitly.

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