New open release allows theorists to explore LHC data in a new way

ATLAS News: Open Likelihoods on HEPData
Explore ATLAS open likelihoods on the HEPData platform.
(Image: CERN)

What if you could test a new theory against LHC data? Better yet, what if the expert knowledge needed to do this was captured in a convenient format? This tall order is now on its way from the ATLAS collaboration, with the first open release of full analysis likelihoods from an LHC experiment.

“Likelihoods allow you to compute the probability that the data observed in a particular experiment match a specific model or theory,” explains Lukas Heinrich, CERN research fellow working for the ATLAS Experiment. “Effectively, they summarise every aspect of a particular analysis, from the detector settings, event selection, expected signal and background processes, to uncertainties and theoretical models.” Extraordinarily complex and critical to every analysis, likelihoods are one of the most valuable tools produced at the LHC experiments. Their public release will now enable theorists around the world to explore ATLAS data in a whole new way.

The ATLAS open likelihoods are available on HEPData, an open-access repository for experimental particle physics data. The first open likelihoods released were for a search for supersymmetry in proton-proton collision events containing Higgs bosons, numerous jets of b-quarks and missing transverse momentum. “While ATLAS had published likelihood scans focused on the Higgs boson in 2013, those did not expose the full complexity of the measurements,” says Kyle Cranmer, Professor at New York University. “We hope this first release – which provides the full likelihoods in all their glory – will form a new communication bridge between theorists and experimentalists, enriching the discourse between the communities.”

The search for new physics will benefit significantly from open likelihoods. “If you’re a theorist developing a new idea, your first question is likely: ‘Is my model already excluded by experiments at the LHC?'” says Giordon Stark, postdoctoral scholar at SCIPP, UC Santa Cruz. “Until now, there was no easy way to answer this.”

Likelihoods are an essential link between theory and ATLAS data. (Image: K. Cranmer/ATLAS)

“We plan to make the open release of likelihoods a regular part of our publication process, and have already made them available from a search for the direct production of tau slepton pairs,” says Laura Jeanty, ATLAS Supersymmetry working group convenor. “Over the coming months, we aim to collect feedback from theorists outside the collaboration to best understand how they are using this new resource to further refine future releases.”

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