Les Houches

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====== Staged approach towards constraining the SMEFT at the LHC ======

Interested people: Fabio Maltoni, Kristin Lohwasser...

**Motivation:** Fiducial cross section measurements and STXS in Higgs physics are being developed and will provide non-maximally sensitive  input to global SMEFT fits.  Direct experimental top-down SMEFT interpretations of LHC SM measurements in Higgs and top physics have not yet taken off, due mostly to a difficulty of handling the richness and diversity of the many theoretical approaches together with the intrinsic technical complications. 

**Aim** :  Stimulate the implementation and development of SMEFT measurements at the LHC by devising a multi-staged approach. 

** Stage 1 (proposal)** : Provide the strongest (even MVA based) constrains from (one or more) processes/observables on one operator at the time. 

** Some advantages ** : 
  - A necessary step before achieving a global fit
  - Way easier than a global fit and feasible today 
  - Scales (less than) linearly with the number of operators
  - Basis agnostic (any operator can be added to the list)
  - Provides reference information to fits based on model independent bottom up approaches (fiducial or simplified XS)
  - Educational purposes, i.e. learning about how each operator behaves
  - Provides benchmarking of sensitivities between different analyses and/or different experiments
  - It offers a simple way to quantify the loss of sensitivity of STXS or FXS with respect to a top-down approach
  - Known issues such as interference vs squared dominance, validity studies, and others can be easily studied 
  - It can be automatised at the analysis level (basically one or few types of analysis really necessary)
  - Other? Please add here...

**Drawbacks/Limitations** : 

  - Results are not constraints on the coefficients and therefore should not be used as such.
  - Other? Please add here...

** Les Houches Project** :  

  - Provide comprehensive list of operators (with simple normalisation) and implementation 
  - Provide minimal list of recommendations (sanity checks, int vs square treatments, perturbative orders, EFT validity,…) 
  - Fully Work out a few examples in Higgs, Top and EW physics. 
  - More?

** Examples of LHC analyses following the SMEFT-one-operator-at-the-time approach **

  - [[http://cms-results.web.cern.ch/cms-results/public-results/preliminary-results/TOP-17-005/index.html|CMS SM measurement of ttW and ttZ]] 
  - [[? |Diboson ]]
Les Houches

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