Right size, wrong shape for Z’s with b’s

Models of strong force interactions must use input from experiments to improve their predictions of the size and shape of important processes.

To estimate a signal or background process at a particle collider, two seemingly simple questions must be answered: “How much of it is there?” and “Where will it appear in the detector?” Knowing the rate and shape of each process is necessary to build an analysis that can reject backgrounds while keeping and measuring the signal. But interactions that involve the strong force are particularly difficult to predict directly from the theory and instead are estimated from models that are constrained by experimental data. Some of these processes, especially those involving weak force bosons and b‘s, or bottom quarks, are important backgrounds to precision studies of the top quark and the Higgs boson. A recent analysis at DZero focuses on testing whether models are correctly estimating the rate and shape of events with a Z boson and a bottom quark.

The complication in this analysis is that quarks are never seen alone. If a quark is produced in a particle collision, strong force interactions quickly begin to generate more quarks and gluons until the initial quark is part of a spray of many particles, called a jet. Jets produced from a bottom quark tend to be separated by a few millimeters from the rest of the particles produced in an event. DZero scientists looked for this signature to separate bottom quark jets from other jets to create an enriched sample of Z bosons with one or more bottom quarks. By comparing the bottom quark-enriched sample to the total production of Z bosons and jets, they were able to reduce the total uncertainty of the measurement.

The new result is the most precise measurement of the ratio of Z-boson-plus-bottom-quark production to all Z-boson-plus-jets production at the Tevatron and agrees well with the rate predicted by models. However, the analyzers also compared the shape of the data to the predicted distributions and found that none of the models tested provided a consistent description with all of the variables examined. This information will be used to constrain future models and lead to better predictions of Z boson-plus-bottom quark events.

Mike Cooke

These physicists made major contributions to this analysis.
Brad Abbott (University of Oklahoma) is the Tevatron’s representative on the Council of the Open Science Grid, which was used to reconstruct particles from raw detector output and is used to generate simulated events that model the signal and background in most DZero analyses.