LHAPDF + Pythia --answered

[Benjamin Fuks]

Hi everybody,

I am interested in generating event files in the MSSM under the following conditions
  - stable neutralinos and charginos
  - I focus on x1- n2 production
  - I want to include matching with the samples containing one extra jet
  - I want to use MSTW pdfs.

Here is what I have done
  - Edit the paramcard to put the n2 et x1 particles with a zero width
  - Generate the following process
     define p = p b b~
     define j = j b b~
     generate p p > x1- n2
     #add process p p > x1- n2 j
     output
  - Fill out the run card as usual (with ckkw = 1 and xqcut = 50)
  - I run madevent

Now the results. For the moment, the piece of the process with the extra jet is commented since the issue is already there with the lowest-order subprocess only. Firstly, If I am using the CTEQ6L1 standard pdfs, everything is fine. Secondly, when I switch to LHAPDF and the set 21000 (mstw2008lo68cl) in the run_card, it fails at the level of pythia. The HEP file is not generated, the plots are not generated, etc... (I did a ./bin/cleanall before generating events)

You can find in attachment
  - the run_card
  - the pythia_card
  - the plot_card
  - the file run_03_tag_1_debug.log

From the debug file, for some reasons, madevent is looking for pgs (I have ever asked to run pgs) and for the hep file which has not been created. I can also find in hep2lhe.log that the hep file was not found. Do you have an idea about what's going on?

If you need anything else, feel free to ask. Thanks in advance!

Cheers,

Benj

[Johan Alwall (johan-alwall)]

Hello Benj,

In fact this has nothing to do with matching, so I will change the title. The problem is simply that Pythia is using the old fortran PDF library with a limited number of PDFs. If you want to use a newer PDF, you need to either
1. link the new LHAPDF also in Pythia, and include the relevant PDF file where Pythia can find it, or
2. Use some other PDF in Pythia by overriding line 172-185 in pythia-pgs/src/ME2pythia.f:

C...Set PDFLIB or LHAPDF pdf number for Pythia

      IF(PDFSUP(1).NE.19070.AND.(PDFSUP(1).NE.0.OR.PDFSUP(2).NE.0))THEN
c Not CTEQ5L, which is standard in Pythia
         CALL PYGIVE('MSTP(52)=2')
c The following works for both PDFLIB and LHAPDF (where PDFGUP(1)=0)
c But note that the MadEvent output uses the LHAPDF numbering scheme
        IF(PDFSUP(1).NE.0)THEN
           MSTP(51)=1000*PDFGUP(1)+PDFSUP(1)
        ELSE
           MSTP(51)=1000*PDFGUP(2)+PDFSUP(2)
        ENDIF
      ENDIF

(e.g., set PDFSUP(1) to CTEQ6l1, i.e. 10042)

All the best,
Johan

[Benjamin Fuks (fuks)]

Hi Johan,

Now it is perfectly working. I have linked pythia to a more recent version of LHAPDF and it works roughly out of the box. Many thanks,

Cheers,

Benjamin

[Johan Alwall (johan-alwall)]

This is something that needs to be taken care of by the user.