# Systematic Errors and Pythia Matching

Hello,

I am looking to compute an estimate of the total uncertainty on a Pythia8 Matched cross-section and would be incredibly grateful for any advice.

I have set use_syst = True in the run card.

Madgraph then outputs:

#

# original cross-section: 4.29231243503e-05

# scale variation: +9.53% -7.25%

# emission scale variation: +9.53% -16.8%

# central scheme variation: +5.33% - 0%

# PDF variation: +27.3% -27.3%

#

# dynamical scheme # 1 : 4.52103e-05 +0.0207% -0.95% # \sum ET

# dynamical scheme # 2 : 4.52103e-05 +0.0207% -0.95% # \sum\sqrt{m^2+pt^2}

# dynamical scheme # 3 : 4.50192e-05 +0.469% -2.45% # 0.5 \sum\sqrt{m^2+pt^2}

# dynamical scheme # 4 : 4.51387e-05 +0.579% -1.64% # \sqrt{\hat s}

In addition to :

=== Results Summary for run: run_01 tag: tag_1 ===

Cross-section : 4.294e-05 +- 9.316e-08 pb

Nb of events : 10000

Pythia8 merged cross-sections are:

> Merging scale = 55 : 1.8171e-05 +/- 3.9e-09 [pb]

> Merging scale = 82.5 : 1.8261e-05 +/- 3.9e-09 [pb]

> Merging scale = 110 : 1.8059e-05 +/- 3.9e-09 [pb]

Notice that because Systematics computation is turned on, the merging did not veto events but modified their weights instead.

The resulting hepmc/stdhep file should therefore be use with those weights.

As far as I can see, the stated uncertainty on the pythia merged x-section is the statistical uncertainty, and the relative uncertainties calculated systematic studies are relevant to the unmatched cross-section.

I have the following questions:

1) Can I assume that the relative systematic error on the matched cross-section is the same as that on the unmatched? If not, how can I find/estimate the systematic uncertainty on the matched cross-section?

2) To get the total systematic uncertainty on the unmatched x-section , do I add (in quadrature) the scale variation, emission scale variation, central scheme variation and pdf variation. Do I need to do anything with the dynamical scales? Is it reasonable to presume that the scale and pdf variations are uncorrelated?

3) I have not changed any of the parameters in the run card related to the systematics calculation - so I guess that they are running on default. Should I have stated any of these explicitly?

I would be extremely grateful for any help with this.

Thanks very much

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