Skip to content

SARAH
SARAH

SPheno_and_Monte Carlo_tools

Last edited by Martin Gabelmann
Page history

SPheno and Monte-Carlo tools

Model files for MC tools

SARAHwrites all necessary files to implement a new model in different MC tools. We show how these model files together with the parameter values obtained by SPheno can be used in a straight forward form. For the output of the models files check

Interplay SARAH–SPheno–MC-Tool

The tool chains SARAH–SPheno–MC-Tools have one very appealing feature: the implementation of a model in the spectrum generator (SPheno) as well as in a MC tool is based on just one single implementation of the model in SARAH. Thus, the user does not need to worry that the codes might use different conventions to define the model. In addition, SPhenoalso provides all widths for the particles so that this information can be used by the MC-Tool to save time.

CalcHep

CalcHep is able to read the numerical values of the masses and mixing matrices from a SLHA spectrum file based on the SLHA+ functionality. SARAH makes use of this functionality to generate model files by default in a way that they automatically expect to find the input values in spectrum files written by a SARAH-generated SPheno version. However, other choices are possible: the parameters can be given via the vars file or tree-level expressions can be calculated internally by CalcHep.

MadGraph

The spectrum file written by SPheno can be directly used as parameter card in MadGraph.

WHIZARD/O’Mega

Since the SLHA reader of WHIZARD is at the moment restricted to the MSSM and the NMSSM, SPheno versions generated by SARAH can write all information about the spectrum and parameters in an additional file in the WHIZARD specific format. This file can then be read by WHIZARD. Currently, the handling of general Lorentz structures in WHIZARD and the support of the UFO format are under development. This will provide the possibility to use WHIZARD with the calculated diphoton and digluon vertices as explained in the following.

Effective diphoton and digluon vertices

The effective diphoton and digluon vertices calculated by SPheno are directly available in the UFO model files and the CalcHep model files: SARAH includes the effective vertices for all neutral scalars to two photons and two gluons, and the numerical values for these vertices are read from the spectrum file generated with SPheno. For this purpose, a new block EFFHIGGSCOUPLINGS is included in these files, which contains the values for the effective couplings including all corrections outlined in . It is important to mention that these effective couplings correspond to the decay of the scalar; if we use CalcHep or MadGraph to compute the decay \Phi \to gg then the value matches (as closely as possible) the NNNLO value, which includes real emission processes such as \Phi \to ggg. Therefore, the corrections at NLO and beyond for \Phi \to gg are not the same as pp\to \Phi via gluon fusion ; the full NNNLO production cross-section includes all processes gg\to \Phi + \text{ jet} and is therefore described by a different k-factor to the decay. This k-factor can for instance be obtained via

k = c_{\Phi gg} \cdot \frac{\sigma_{\rm SM}(pp \to H(M_{\Phi})+ \mathrm{jet})}{\sigma_{\rm MC}(pp \to \Phi)}

where c_{\Phi gg} is the ratio squared of the effective coupling between \Phi and two gluons at LO in the considered model and the SM. These values can for instance be read off by the block HiggsBoundsInputHiggsCouplingsBosons in the SPhenospectrum file. \sigma_{\rm SM}(pp \to H(M_{\Phi})) is the cross section for a SM-like Higgs with mass M_{\Phi}. This value can be calculated for instance with Higlu or Sushi for the considered center-of-mass energy. SPheno also provides values for c_{\Phi gg} \cdot \sigma_{\rm SM}(pp \to H(M_{\Phi})) for the most common energies in the blocks HiggsLHCX (X=7,8,13,14) and HiggsFCC, see also SPheno_Higgs_production.

On the other hand, this approach is not entirely appropriate for more refined collider analyses where the user would like to actually include, for example, a hard jet in the final state (without the full loop corrections to the effective vertex this is not an infra-red safe quantity).