====== MC variation II ======

** Project aim: **

  * consider 1 case study, pick a selection of 2-3 observables, perform a selection of MC variations
  * for a given setup X=(FO/matching/merging accuracy, PS model, NP model), produce envelope
  * check if the envelope from setup X behaves as expected or not: for instance, it might happen that even in a region that should be dominated by "hard physics", one has a residual dependence upon the NP model, or a too-large dependence upon some shower parameter, if the variations are not done properly, or if there's an inconsistency.
  * check if envelopes from setup X_i and setup X_j overlap at least partially. If there are very large differences, is this expected (given the kinematic region probed and the perturbative/non-perturbative content of the setups)?

** Disclaimer **

  * to some extent, such a study could be considered a continuation of what can be found in chapter V.1 of  https://arxiv.org/abs/1605.04692 . Nowadays we might aim for having higher accuracy in the ME part, and possibly more developed and more efficient frameworks for "PS-reweighting".


** Possible test cases: **

  * pth in gluon-fusion (pros: directly related to STXS bins)
  * Drell-Yan (pros: there's data available)
  * ttbar (pros: atlas and cms have already several studies performed with identical or very similar setups)


** Meeting on Monday **

  * agree first on the utility/need (from the EXP side) of such a study, and then on the best test case
  * ->Check

** Decisions **

  * Agreement reached points towards ttbar as first choice. Probably as a second choice: gg->H


** Next steps **

  * Work out a strategy (slack?)

** Interested participants (please add your name): **
  
  * ttbar  
    * Emanuele Re
    * Helen Brooks (can provide Vincia+POWHEG, maybe Pythia)

** ========= **
  * Carlo Pandini
  * Simone Amoroso
  * Helen Brooks
  * Josh McFayden
  * Philippe Gras
  * Adil Jueid