Pythia LHE and HEP output inconsistent

[Markus Ebert]

Hi,

I noticed that the LHE and HEP output from pythia are inconsistent.
As a test, I generated the process "p p > e+ e-" using madGraph 5 (with standard settings) and ran it through Pythia6.
MadGraph reports a cross section of 949.6pb, but when inspecting the pythia output files with madAnalysis5, only the HEP file contains the right cross section. The LHE file gives a value of 9.4959e+11pb.
(Plots, e.g. of PT(j) are almost identical after normalization to one, except that HEP contains many events at pt=0)

I know LHE files should not be used, but still it would be good to know what's going wrong, especially as I need the cross section for later PGS runs.

I'm not sure if its maybe a MadAnalysis problem, how could I check this?

Cheers,
Markus

[Rikkert Frederix (frederix)]

Dear Markus,

I'm not able to reproduce this bug, so I would like to have some more information. In particular, which of the lhe files has the wrong cross section? The LHE file after running pythia or before running pythia?

Best regards,
Rikkert

[Markus Ebert (markus-ebert)]

Hi Rikkert,

the wrong cross section is only in the lhe-file after running pythia, i.e. x_pythia_events.lhe.gz, whereas the MadGraph-output events.lhe.gz resp. unweighted_events.lhe.gz are correct.
I've seen this behaviour in several calculations I did, not only in the test case described above. However, as I noted, there might be a problem with the MadAnalysis-Readout. I checked ma5-version 1.1.7 and 1.1.8, they both show this problem.

Cheers,
Markus

[Rikkert Frederix (frederix)]

Dear Markus,

It looks like a MadAnalysis5 problem. I've subscribed Benjamin Fuks to this bug, as he is one of the main authors of that code.

Best,
Rik

[Benjamin Fuks (fuks)]

Dear Markus,

The normalization of the LHE output constructed by MadGraph once pythia has run has changed from the initial release of the code. But MadAnalysis 5 has not been updated accordingly. This will be fixed in v1.1.9. In the meantime, it is sufficient to replace the file
  tools/SampleAnalyzer/DataFormat/MCSampleFormat.h by the one below.

Cheers,

Benjamin