====== Extension to the SLHA for cross sections ====== 

Interested people: Olivier Mattelaer, Sezen Sekmen, Benjamin Fuks, Stefan Liebler, Michael Spira, Fabio Maltoni... ADD YOUR NAME

Work started in LH 2013.  Already quite complete.  Needs to be polished and written up. Need to get agreement for the SUSY code developers.

==== The details ====

The plans is to add new blocks to a supersymmetric spectrum with information on production cross section of particles included in the spectrum of interest. We propose to add one new block for each process under consideration.

The block starts with
<code>
XSECTION SQRTS PDG_CODE1 PDG_CODE2 NF PDG_CODE3 ....
</code>
where 
  * **SQRTS**: a floating number representing the center-of-mass energy, to be given in GeV.
  * **PDG_CODE1** and **PDG_CODE2**: two integer numbers, representing the pdg codes of the initial state particles
  * **NF**: an integer number, representing the number of final state particles
  * **PDG_CODE3**, **PDG_CODE4**, ...: NF integer numbers, representing the pdg codes of the final state particles

This general information line is then followed by the cross section themselves, given as a series of line under the form
<code>
  SCALE_SCHEME QCD_ORDER EW_ORDER KAPPA_F KAPPA_R PDF_ID1 PDF_ID2 VALUE VALUEUP VALUEDOWN DUMMY 
</code>


where
  * **SCALE_SCHEME**: an integer number, related to the way the central scales are chosen. So far, the choices are 
    * 0: the central scale is the average mass of the produced particles
    * 1: fixed scales
    * 2: the scale is set to the invariant mass of the final state particles at the LO
    * 3: the scale is set to the transverse mass of the final state particles at the LO
    * 4: some other definition to be described as a comment
  * **QCD_ORDER**: an integer number, which contains information on the order in alpha_s included in the computation. In order to deal with resummation I suggest to add a second digit to QCD_ORDER, i.e. 
    * 0=LO, 
    * 1=NLO, 
    * 2=NNLO,
    * 3=N3LO,
    * 11=NLO+NLL,
    * 21=NNLO+NLL, 
    * 22=NNLO+NNLL, 
    *  TYPE of resummation as a COMMENT.
  * **EW_ORDER**: same as above but with respect to electroweak corrections
  * ** KAPPA_F**: a floating point number, related to the variation of the factorization scale with respect to the central scale. For SCALE_SCHEME set to 1, the scale itself has to be provided.
  * ** KAPPA_R**: same as above, for the renormalization scale
  * **PDF_ID**: the lhapdf code of the employed parton densities
  * **VALUE**: a floating point number, representing the value of the cross section in pb.
  * **DUMMY**: integer number to tag possible different options to be described in the comments
  * CODE: as a COMMENT string representing the name of the program which has computed the cross section
  * VERSION: as a COMMENT string, the version number of the proram employed
  * REFERENCE: Citation 
  * FIX the meaning of DEFAULT values...
 
Stefan comments:
  - OPTIONAL: For EW corrections it would be helpful to somehow encode if full factorization is assumed or not. => Implement as a comment what scheme is used to combine QCD and EW corrections.