Standard Model wins again

The measured multiplicity (black points) of additional muons in events that pass the W boson selection, compared to the expected Standard Model sources shown as stacked colored histograms. This search for new physics concentrates on the bins with three or more additional leptons where the SM background is very small.

In recent years, many alternatives have been proposed to complete the Standard Model. These models have interesting (though sometimes uninformative) names such as the Next-to Minimal Supersymmetric Model, the little Higgs models and R-parity violating models. Predictions of these models include production and decays of new particles. For example, if some of the new particles were noticeably lighter than the W and Z bosons, they would produce soft, or low-momentum, decay products, possibly including multiple soft muons and electrons, depending on the new particle couplings.

In looking for new physics, CDF scientists searched for multiple electrons and muons in electroweak events that contained an identified W or Z boson. Of particular interest are processes that produce lepton jets, which are a large number of low-energy leptons, often close together. These lepton jets could have been missed in traditional searches for larger energy, isolated leptons, and they could be a signature of certain exotic Higgs decays.

By searching within these W and Z events, we ensure that the search starts from a well-understood sample. New algorithms were developed to look for low-energy, non-isolated leptons. The numbers of these leptons were compared to the number expected from known processes. These predicted numbers decrease very rapidly at high lepton multiplicity, which is the region where the new physics would likely appear.

We compared the electron and muon results with the Standard Model predictions. Once again, the Standard Model provides a good description of the observed data. However, the possibility still remains that new light and weakly coupled particles beyond the Standard Model Higgs are mixed in with the Tevatron and LHC data. There is more searching still to be done.

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—Edited by Andy Beretvas

These physicists were responsible for this analysis. From left: Scott Wilbur, Henry Frisch, Carla Grosso-Pilcher, Dan Krop (not pictured); all from the University of Chicago.