Simplified Likelihoods for the Re-interpretation of LHC Searches for BSM physics

This twikipage documents the ongoing studies and developments in Simplified Likelihoods for re-interpretation of existing searches for new physics at the LHC


Please contact Nick Wardle ( and Matthew Citron ( for more details.


The simplified likelihood framework adopted at CMS is described in this note. The note describes details on how to construct the simplified likelihood using a minimal set of inputs from the experimental collaboration.

This method follows similar considerations discussed in this paper and this paper.

The idea behind the simplified likelihood is to allow for re-interpretation of physics analyses using predicted/measured backgrounds, often provided by the collaborations, and the correlations between different bins. This allows therefore to combine these different bins in a sensible way and properly account for the uncertainty model. CMS is regularly providing the necessary information for the simplified likelihood mostly in the SUSY and EXO groups, for example as in the SUSY analysis CMS-SUS-PAS-16-036.


A simple, python based code for producing the simplified likelihood is available on GitHub here, including some basic instructions.

List of development topics.

  • Work in Progress
    • Beyond linear approximations for correlation structure/gaussian approximation for prior pdf. It is known that the Gaussian approximation has several drawbacks, including allowing for -ve signal rates. There are ongoing studies to avoid this by extending the SL beyond the linear (gaussian) case: See the workshop at the IIOP in Natal, Brazil for details link

  • Other extensions
    • Inclusion of Signal uncertainties including their covariance.
    • Breakdown of correlation matrices into source components (eg theory vs experimental).
    • Provision of MC datasets sampled from experimental priors to construct non-parametric versions of the constraints for the SL

-- NicholasWardle - 2018-03-15

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