EFT Basis Task Force

This is an ad hoc WG2 task force whose goal is to develop a common EFT approach for LHCHXSWG.

Latest Draft (still work in progress!)

Overall Goals

  • We want to converge on one EFT language that could be universally used in LHC Higgs analyses.
  • Our formulation should be simple enough that it can be used by people not acquainted with nuts and bolts of EFTs.
  • Relationship between parameters and observables should be transparent.
  • The formalism should be simple to implement in monte carlo codes.
  • The formalism should be flexible enough, such that it in the future the application scope may be extended beyond the original one.
  • Matching between EFT parameters and concrete UV models should be simple.
  • Connection to the (extended) kappa formalism should be straightforward.
  • Limits of the EFT validity range should be easy to define.
  • Constraints from electroweak precision observables should be easy to impose.

Some of these goals are probably mutually exclusive, so compromises may be necessary.

Basic Details of the Proposal

  • We restrict to EFT with dimension-6 operators in the linear formulation of electroweak symmetry breaking (that is to say, the Higgs boson belongs to the Higgs doublet). Non-linear effects may arise in physically motivated cases, but the formalism is less predictive and would be more difficult to implement in experimental analyses. Note that LHCXSWG2 is simultaneously developing the extended kappa formalism which should provide enough information to constrain more general theories.
  • We formulate the EFT such that the parameters are the couplings of Z and Higgs bosons, in the spirit EFT primaries. Such formulation should be more user friendly and transparent. Simultaneously, we map that description to the usual operator language. Working with operators rather than couplings may be more convenient in practical calculations. We can decide later which language is more convenient for LHC experiments to report their results.
  • We do not impose flavor universality. While generic constraints on flavor violation are strong, it is plausible that there is a large hierarchy between dimension-6 operators; in particular, many models the operators involving the 3rd generation are much larger than the others. Keeping the more general approach will allow us to obtain much more robust conclusions with just a little bit more effort.
  • We discuss constraints from Z and W pole observables. These are the most stringent constraints on EFT parameters, and they have important impact on possible signals in Higgs searchers. Other precision observables, such as WW production or off-shell fermion scattering, are more difficult to handle and numerically less important (moreover, some issues may still be considered controversial).


  • Give the full list of operators and their relations to the Lagrangian couplings.
  • Clearly spell out the assumptions that go in this particular EFT descriptions.
  • Relate LHC Higgs observables to EFT parameters.
  • Identify the parameters that are strongly constrained by precision observables and therefore should not be varied in LHC analyses.
  • Create the Rosetta stone that will allow to decipher the connection of our formalism to other popular descriptions used in the literature.

-- AdamFalkowski - 2014-11-26

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Topic revision: r7 - 2015-01-14 - AdamFalkowski
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