Description of the method

The method consist to use a sample of events (weighted or unweighted events) and associate to those events a new weight corresponding to a new theoretical hypothesis. This weight can be propagate through the all simulation chain in order to avoid to have to perform the full-simulation on a huge number of sample. This methods works only if the original hypothesis and the new one are both significant in the same part of the phase-space. the new weight is given by $$W_{new} = |M_{new}|^{2 /|M_{old}|}2 * W_{old} $$

Technical details

Limitation

1. The two theoretical hypothesis should differ ONLY by difference in the model parameters. So this should be the same process, the same model and the same pdf.
2. The scale choice for the reweighting is done events by events and is given by the value in the LHE file. The same scale is used for both Hypothesis
3. In presence of decay chain the order of the particle in the events file is important. This is important if you want to use this tools with LHE events not produced by MadGraph/MadEvent.

How to get the code

The code is currently (30/06/13) in beta, and is still expect to be modify before the official release (especially on the interface). In order to download the code, you need to have bazaar install on your computer. Then you can download the code via the command

bzr branch lp:~maddevelopers/madgraph5/reweight_module After that in order to update your version. You only need to do: bzr pull

How to use the code.

1. go to the process directory.
2. launch the ./bin/madevent script
3. type reweight RUN_NAME
4. then you will see the following question: {{{Do you want to edit one cards (press enter to bypass editing)? 1 / reweight : reweight_card.dat you can also
   • enter the path to a valid card.

[0, done, 1, reweight, enter path][60s to answer]}}}

If you didn't define the content of this file already. You need to do it. The syntax is explained inside the file, and you can see example below (validation section). The important point, is that the first line should be launch, and then you specify which parameter, you want to modify. This is the exact same syntax has for scripting a scan over parameter.

5. exit the file and you are done.

Input/Output format

1. the output format follows the Leshouches agreement (not yet published): ​http://phystev.in2p3.fr/wiki/2013:groups:tools_lheextension#the_proposal

as an example the header looks like:

<initrwgt>
<weightgroup type='mg_reweighting'>
<weight id='mg_reweight_1'>set param_card dim6 1 100.0
</weight>
<weight id='mg_reweight_2'>set param_card dim6 2 100.0
</weight>
<weight id='mg_reweight_3'>set param_card dim6 3 100.0
</weight>
</weightgroup>
</initrwgt>

and one associated events:

<event>
 8      0 +7.9887000e-06 1.24664300e+02 7.95774700e-02 1.23856500e-01
        1 -1    0    0  501    0 +0.0000000e+00 +0.0000000e+00 +1.3023196e+03 1.30231957e+03 0.00000000e+00 0.0000e+00 -1.0000e+00
       -2 -1    0    0    0  501 +0.0000000e+00 +0.0000000e+00 -1.4499581e+02 1.44995814e+02 0.00000000e+00 0.0000e+00 1.0000e+00
      -24  2    1    2    0    0 -1.2793809e+01 -8.3954553e+01 -1.1792566e+02 1.65987064e+02 8.02071978e+01 0.0000e+00 0.0000e+00
       23  2    1    2    0    0 +1.2793809e+01 +8.3954553e+01 +1.2752494e+03 1.28132832e+03 9.12640692e+01 0.0000e+00 0.0000e+00
       11  1    3    3    0    0 -1.2462673e+01 +1.3647422e+01 -2.6083861e+01 3.19677669e+01 0.00000000e+00 0.0000e+00 -1.0000e+00
      -12  1    3    3    0    0 -3.3113586e-01 -9.7601975e+01 -9.1841804e+01 1.34019297e+02 0.00000000e+00 0.0000e+00 1.0000e+00
        4  1    4    4  502    0 -1.8321803e+01 +9.0929609e+01 +9.3905973e+02 9.43629724e+02 0.00000000e+00 0.0000e+00 -1.0000e+00
       -4  1    4    4    0  502 +3.1115612e+01 -6.9750557e+00 +3.3618969e+02 3.37698598e+02 0.00000000e+00 0.0000e+00 1.0000e+00
<rwgt>
<wgt id='mg_reweight_1'> 4.55278761371e-06 </wgt>
<wgt id='mg_reweight_2'> 2.65941887458e-06 </wgt>
<wgt id='mg_reweight_3'> 8.68203803896e-06 </wgt>
</rwgt>
</event>

The reweight_card in that case was:

launch
   set Dim6 1 100
   set Dim6 2 0
   set Dim6 3 0
   set Dim6 4 0
   set Dim6 5 0
launch
   set Dim6 1 0
   set Dim6 2 100
   set Dim6 3 0
   set Dim6 4 0
   set Dim6 5 0
launch
   set Dim6 1 0
   set Dim6 2 0
   set Dim6 3 100
   set Dim6 4 0
   set Dim6 5 0

Note:

1. The production events were in this case made via the SM where all those coefficient were in fact set to zero. Therefore all the line setting the line to zero were superfluous, and this is the reason why they didn't appear in the banner.
2. You can also specify a path to a param_card in the reweight_card. The content of the header being computed automatically with the difference of the two param_card.

Validation

The validation of this tools is not yet finish. Use this tools with care. In particular in presence of decay chain, if you have identical particles in the final states coming from different decay. (For example p p > w+ z, w+ > e+ ve, z > e+ e-)

p p > e+ e- cross-section