SUSY Les Houches Accord

The PYTHIA 8 program does not contain an internal spectrum calculator (a.k.a. RGE package) to provide supersymmetric couplings, mixing angles, masses and branching ratios. Thus the SUSY Les Houches Accord (SLHA) Ska04All08 is the only way of inputting SUSY models, and SUSY processes cannot be run unless such an input has taken place.

Most of the SUSY implementation in PYTHIA 8 is compatible with both the SLHA1 and SLHA2 conventions (with the exception of R-parity violation and the NMSSM extension in the latter case). Internally, PYTHIA 8 uses the SLHA2 conventions and translates SLHA1 input to these when necessary. See the section on SUSY Processes for more information.

When reading LHEF files, Pythia automatically looks for SLHA information between <slha>...</slha> tags in the header of such files. When running Pythia without LHEF input (or if reading an LHEF file that does not contain SLHA information in the header), a separate file containing SLHA information may be specified using SLHA:file (see below).

With the so-called QNUMBERS extension Alw07, the SLHA input format can also be used for more general BSM models, although the implementation of this extension is not yet complete in PYTHIA 8.

Finally, the SLHA input capability can of course also be used to input SLHA-formatted MASS and DECAY tables for other particles, such as the top quark, furnishing a less sophisticated but more universal complement to the standard PYTHIA 8-specific methods for inputting such information (for the latter, see the section on Particle Data and the scheme to modify it). This may at times not be desirable, so a few options can be used to curb the right of SLHA to overwrite particle data.

The reading-in of information from SLHA or LHEF files is handled by the SusyLesHouches class, while the subsequent calculation of derived quantities of direct application to SUSY processes is done in the CoupSUSY class.

SLHA Switches and Parameters

Controls from where SLHA information is read. Name of an SLHA (or LHEF) file containing the SUSY/BSM model definition, spectra, and (optionally) decay tables. Default void signals that no such file has been assigned. Some programs write SLHA output also for SM particles where normally one would not want to have masses and decay modes changed unwittingly. Therefore, by default, known SM particles are ignored in SLHA files. To be more specific, particle data for identity codes in the ranges 1 - 24 and 81 - 999,999 are ignored. Notably this includes Z^0, W^+- and t. The SM Higgs is modified by the SLHA input, as is other codes in the range 25 - 80 and 1,000,000 - . If you switch off this flag then also SM particles are modified by SLHA input. This parameter provides an alternative possibility to ignore SLHA input for all particles with identity codes below 1,000,000 (which mainly means SM particle, but also includes e.g. the Higgses in two-Higgs-doublet scenarios) whose default masses in PYTHIA lie below some threshold value, given by this parameter. The default value of 100.0 allows SLHA input to modify the top quark, but not, e.g., the Z^0 and W^+- bosons. Controls amount of text output written by the SLHA interface, with a value of 0 corresponding to the most quiet mode.

Internal SLHA Variables

The following variables are used internally by PYTHIA as local copies of SLHA information. User changes will generally have no effect, since these variables will be reset by the SLHA reader during initialization. Corresponds to SLHA block MODSEL entry 3.