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 of possible theoretical approaches together with many 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:

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

Drawbacks/Limitations:

1. Results are not constraints on the coefficients and therefore should not be used as such.
2. Other?

Les Houches Project:

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