Dibosons the hard way

This is the result of the fit to data in events where CDF scientists identified b-jets using the b-tagger.

This is the result of the fit to data in events where CDF scientists identified b-jets using the b-tagger.

Physicists at CDF are working hard to continue to improve their experiment’s sensitivity to the Higgs boson. One group is now examining a physics process similar to that of the Standard Model Higgs but more prevalent in order to improve their analysis tools. Specifically, they are conducting a search for a diboson signal comprised of one W and one Z particle that requires the Z to decay into a pair of b jets. This diboson physics analysis is almost identical to the search for the Standard Model Higgs particle but the Higgs decay is replaced by the Z boson.

This analysis builds on a previous observation of the combined production of WW, WZ and ZZ diboson pairs in events with neutrinos and jets. After employing a variety of methods to reduce and model the large amounts of background events, CDF scientists searched for a bump in the distribution of the mass of jet pairs near the W and Z masses in two samples: jets that are identified as coming from b quarks, called b jets, and those that are not.

To pick out b jets from other kinds of jets, CDF scientists have created a new b-jet tagger. This is a computer algorithm that looks at an event and tries to identify jets that do not come from the primary vertex – meaning b quarks that travel some distance before decaying into a jet of particles. This new tagger is unique in that it takes a more holistic approach; using as much information available in a given event as it can rather than the current strict cut-based approach where only certain high quality tracks are considered in order to spot signs that the jet in question came from the production and decay of a B hadron.

The new tagging algorithm performs very well compared to existing b taggers and, for this analysis, has slightly improved its ability to identify b-quarks. It has also made a large improvement in reducing the rate of mistags. Using approximately half of the available data set, CDF measured the cross section for production of WZ + ZZ particles by proton-anti proton collisions to be equal to 5.8 +3.6/-3.0 picobarns, which is in good agreement with the Standard Model prediction of 5.1 picobarns.

Efforts are currently underway to incorporate elements of this tagger into existing Higgs searchs as CDF prepares for its final Higgs results this winter.

Learn more

—Edited by Andy Beretvas

These CDF physicists contributed to this data analysis. Clockwise from top left: Stephen Poprocki, Cornell University; Sasha Pronko, Fermilab; Wes Ketchum, University of Chicago; Vadium Rusu, Fermilab; John Freeman, Fermilab; and Peter Wittich, Cornell University.

These CDF physicists contributed to this data analysis. Clockwise from top left: Stephen Poprocki, Cornell University; Sasha Pronko, Fermilab; Wes Ketchum, University of Chicago; Vadium Rusu, Fermilab; John Freeman, Fermilab; and Peter Wittich, Cornell University.