===== Extending the Les Houches Event file format =====
=== => in order to handle reweighting information === 




==== The aim ====
After discussions with both ATLAS, CMS, aMC@NLO and POWHEG people, it has been found out that it is valuable to include in the LHE format some extra information about event reweighting. And thus comes the problematics of standardizing the way to pass this information. This would allow to have, in one single LHE file, unweighted events corresponding to a given MC setup (for instance, central scales, central PDF, etc…), together with weights related to the variation of one or several of the MC parameters. 

The proposal below starts from the first try in this direction in the LH proceedings of 2009 ([[http://arxiv.org/abs/1003.1643|arXiv:1003.1643]]), and include
  * more information in the header,
  * a reorganization of the weights, collecting them into categories.

The current version of the proposal has been implemented in MadGraph5_aMC@NLO, as well as in MadAnalysis 5. 


==== The proposal ====

===First part: the header of the event file===

The header contains the explanation about what these weights:

<code><header>
 ...

 <initrwgt>
  <weight id='1'> This is the original event weight </weight>
  <weightgroup name='scale_variation' combine='envelope'>
     <weight id='2'> muR=2.0 </weight>
     <weight id='3'> muR=0.5 </weight>
  </weightgroup>
   <weightgroup name="mrst2008e40" combine="hessian">
     <weight id='4'> set01 </weight>
     <weight id='5'> set02 </weight> 
     ...
   </weightgroup>
  <weightgroup name='Qmatch_variation' combine='envelope'>
     <weight id='44'> Qmatch=20 </weight>
     <weight id='45'> Qmatch=40 </weight>
  </weightgroup>
  <weight id='46'> BSM benchmark point number 42B, see arXiv XXXX.XXXX </weight>
 </initrwgt>
...
</header></code>

This information in the header should be human-readable and explain what the weights with the corresponding identifiers mean. It can simply contain all the parameters that were used in to generate this weight; or only the ones that were changed compared to the original run; or simply a sentence explaining what this number means. It's up to the user that is doing the analysis to make sure that this information is correctly used (and up to the authors of the codes to make sure that the user has enough information to understand what the weights correspond to).

The weightgroup tag allows to group several weights together (to have the information about how to combine weights to obtain, //e.g.//, scale variation or pdf uncertainties. The attributes ''combine'' is optional. It indicates how to combine the uncertainties. Possible arguments are ''none'', ''hessian'', ''envelope'' or ''gaussian''. If not specified, the default choice is ''combine='none''', all the curves associated with each weights being kept independent.  For ''combine='hessian''', the first weight is the central value and the next weights correspond respectively to the positive and negative variations along a specific direction of the parameter space.

===Second part: within each event===
We  start with an example:
<code>
<event>
7 100  0.10000000E+01  0.20000000E+00  0.00000000E+00  0.00000000E+00
 -2 -1  0  0 0 0  0.12699952E+01  0.55429630E+01  0.57634577E+02  0.57914435E+02  0.00000000E+00 0. 0.
  2 -1  0  0 0 0 -0.91353745E+00  0.13160013E+01 -0.34965448E+02  0.35002128E+02  0.00000000E+00 0. 0.
 23  2  1  1 0 0  0.35645919E+00  0.68589662E+01  0.22669189E+02  0.92916566E+02  0.89846668E+02 0. 0.
-13  2  3  3 0 0  0.51612833E+01  0.21143065E+02  0.53960893E+02  0.58184682E+02  0.10566000E+00 0. 0.
 13  2  3  3 0 0 -0.48048241E+01 -0.14284099E+02 -0.31291705E+02  0.34731884E+02  0.10566000E+00 0. 0.
-13  1  0  0 0 0  0.51612833E+01  0.21143065E+02  0.53960893E+02  0.58184682E+02  0.10566000E+00 0. 0.
 13  1  0  0 0 0 -0.48048241E+01 -0.14284099E+02 -0.31291705E+02  0.34731884E+02  0.10566000E+00 0. 0.
 <rwgt>
  <wgt id='1'> 1.000e+00 </wgt>
  <wgt id='2'> 0.204e+00 </wgt>
  <wgt id='3'> 1.564e+00 </wgt>
  <wgt id='4'> 2.248e+00 </wgt>
  <wgt id='5'> 1.486e+00 </wgt>
  ...
  <wgt id='46'> -0.899e+00 </wgt>
 </rwgt>
</event></code>

The numbers should be normalized in the same way as the original weight of the event, //i.e.// if the weights sum-up to the total cross section, also the new ''<nw>'' weights should sum up to the total cross section (which is in general slightly different because different parameters were used).