Precision measurements of top-quark properties and searches for new physics are large parts of the physics program at the Fermilab Tevatron. These studies depend largely on the accurate predictions of their underlying standard model (SM) background compositions. In particular, the ongoing efforts to find evidence for a SM Higgs boson are very sensitive to small fluctuations in background rates.
Many signals of new phenomena and the search for the Higgs require identification of heavy flavor content, such as b quarks, in events. Heavy-flavor jet tagging uses unique properties of hadrons containing heavy flavor quarks to distinguish them from hadrons containing only light-flavor quarks (u,d,s). Complicating this picture, evidence for anomalous production of heavy-flavor quarks (t,b,c) in association with W bosons was reported in Run I. Using both secondary vertex tagging and soft-lepton tagging, the CDF study suggested there was an unusually large number of events containing a W boson and two or three jets in which at least one of the jets was simultaneously tagged by two different heavy-flavor jet tagging algorithms.
Secondary vertex tagging relies on displaced vertices formed by the tracks of particles produced in the decays of long-lived particles, such as B- or D-mesons. Soft-lepton tagging keys on low transverse momentum leptons inside jets, arising from the semileptonic decays of heavy-flavor quarks. Lacking tracking detectors with sufficient resolution, the DØ experiment was unable to test this observation in Run I data.
The Run II upgrade of the DØ detector has provided the means of testing this anomalous result. In a selection of leptonic W-boson decays, we have evaluated the heavy-flavor content of associated jets. We compared the predictions of the expected SM processes to data in events with a W boson and at least one jet tagged with both secondary vertex and soft-lepton tagging algorithms. As seen in the figure below, we find no excess of events beyond SM predictions in this selection (164 pb-1 for the electron+jets channel and 145 pb-1 for the muon+jets channel). This increases our confidence that rare backgrounds to particle searches are becoming better understood.
