Basic Input

Basic Model Input

The main input for SModelS is the full model definition, which can be given in the two following forms:

  • a SLHA (SUSY Les Houches Accord) file containing masses, branching ratios and cross sections for the BSM states (see an example file here)
  • a LHE (Les Houches Event) file containing parton level events (see an example file here)

The SLHA format is usually more compact and best suited for supersymmetric models. On the other hand, a LHE file can always be generated for any BSM model (through the use of your favorite MC generator). [*] In this case, however, the precision of the results is limited to the MC statistics used to generate the file. We also point out that all the decays appearing in the LHE input are assumed to be prompt and this input format should no be used if the model contains meta-stable particles.

In the case of SLHA input only, the production cross sections for the BSM states also have to be included in the SLHA file as SLHA blocks, according to the SLHA cross section format (see example file). For the MSSM and some of its extensions, they may be calculated automatically using Pythia and NLLfast, as discussed in cross section calculator.

In the case of LHE input, the total production cross section as well as the center-of-mass energy should be listed in the <init></init> block, according to the standard LHE format (see example file). Moreover, all the Z2-even particles (see definition in final states) should be set as stable, since in SModelS they are effectively considered as final states. When generating the events it is also important to ensure that no mass smearing is applied, so the mass values for a given particle are the same throughout the LHE file.

New Particles

Besides information about the masses and branching ratios, the user must also define which particles are Z2-odd states (Intermediate states) and which are Z2-even (Final states). These definitions must be given in a model file, including the particle’s quantum numbers, as illustrated in the mssm.py file. A path to a user’s own model file can be specified in the parameter file, in the [particles] section.

SLHA Format for Cross Sections

A list of cross section blocks (one for each production process) must be included in the SLHA file for the SLHA-based Decomposition. The SLHA format for each cross section block is shown below (see the Les Houches note):

_images/xsecBlock.png

The above example shows the cross section for \(pp \rightarrow \tilde{\tau}_1^- + \tilde{\nu}_{\tau}\) at a center-of-mass energy of 8 TeV and at leading order. The only information used by SModelS are the center-of-mass energy, the outgoing particle PDGs, the cross section value and the QCD order. If the input file contains two cross sections for the same process but at different QCD orders, only the highest order will be used.

  • Reading of cross sections from an input file is implemented by the getXsecFromSLHAFile method
[*]SModelS can easily be used for non-SUSY models as long as they present a Z2-type symmetry. However, it is the responsibility of the user to make sure that the SMS results in the database actually apply to the model under consideration.