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.
word
Vincia:tuneFile
(default = tunes/nikhef1.cmnd
)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 2: 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Note 3: the entries in the tune file will be superseded by any user modifications made in the main command file given to the VINCIA constructor.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.
Starting from VINCIA version 1.022, dedicated tunes of VINCIA have been included with the standard distribution.
The list of currently available tunes is as follows:
"tunes/nikhef1.cmnd"
(Mar 2013)
First tune to use the 3-jet NLO corrections (no CR).
Uses 2-loop running of alphaS, with the CMW scheme.
Data : LEP-1 event shapes, jet rates, multiplicities, inclusive x spectrum.
Also included proton and B meson spectra,
and the D and D* multiplicities.
"tunes/s2.cmnd"
(May 2012)
Second 2-loop tune, with a smaller alphaS (0.126 instead of 0.128) and a lower IR cutoff (though still fairly high in absolute terms; 1.75 GeV instead of
1.9 GeV, constrained to be above LambdaQCD when including CMW and alphaS scale factors).
This improves the observables in the soft region somewhat, at the cost of making the D parameter slightly soft. Also reduced the number of D vector mesons, to agree better with LEP charm multiplicities, and suppressed leading-baryon formation slightly, to soften the z->1 end of baryon spectra.
Note: the one-loop tunes (Jeppsson) still appear to give a better overall description of shapes and jets. By P. Skands.
"tunes/s1.cmnd"
(May 2012)
First tune intended for use with the new 2-loop running option. Based on Jeppsson4, but has a higher IR cutoff (due to the faster 2-loop running and the new default CMW scale choice) and consequently a somewhat softer shape of the non-perturbative fragmentation function. Does not achieve a very good description in the non-perturbative region of jet rates and event shapes. Not yet competitive with the one-loop tunes.
By P. Skands.
"tunes/jeppsson5.cmnd"
(Dec 2012) A slight
modification of the Jeppssson 4 tune, re-optimized for use with the
default helicity-dependent shower implementation in Vincia 1.0.29.
"tunes/jeppsson4.cmnd"
(Sept 2011) A slight
modification of the Jeppssson 3 tune. Increased Lund a and b
parameters, partly inspired by work done by B. Bravi as a CERN summer
student, and partly to reduce the soft peak of the Kaon spectrum /
reduce the tail of very hard Lambda baryons. Both aspects are still
not completely satisfactory, however. Increased amount of vector meson
production, relative to pseudoscalar. Slightly increased eta and eta'
production. This tune is also recommended for the sector shower. By
P. Skands. "tunes/jeppsson3.cmnd"
(Mar 2011) A slight modification of the
Jeppssson 2 tune, with very slightly increased baryon production, to
improve the K/Lambda ratio. By P. Skands."tunes/jeppsson2.cmnd"
(Apr 2010) A slight modification of the
original Jeppssson tune of VINCIA+PYTHIA to
LEP data, by P. Skands."tunes/jeppsson.cmnd"
(Apr 2010) A first tune of VINCIA+PYTHIA to
LEP data, by M. H. Jeppsson, April 2010.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. In order to make 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.
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 the tune 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.
The value of the strong coupling used in the shower is also considered a tuning parameter. In this case the interpretation is that extracting alphaS from, e.g., event shapes, can be done equally well using a shower code as with more analytical approaches. The difference is that the alphaS value extracted with the shower code is in an a priori unknown scheme, defined by the shower algorithm. An alternative strategy to extracting alphaS directly using the shower would therefore be to take its value from an analytical fit, e.g., using the MSbar scheme, and translating to a scheme more appropriate in the shower context, such as the so-called "CMW" scheme.