QCD background for the UA1W→tb¯signal

The cc¯X and bb¯X background affecting the W→tb¯ e/μ+2 jets signal reported by the UA1 Collaboration is discussed. Calculations are based on an O(${\alpha }_s$ ${\mathrm{}}^2$)+leading-pole approximation scheme, which includes jet radiation in the initial and final states of the hard binary parton process, and include the heavy-flavor excitation contribution, the stability of which has been checked. The main conclusions are the following. (i) Selection cuts naturally induce a topological event structure on the surviving background mimicking that expected for a W→tb¯ signal. (ii) The background rates for μ+2 jets events, on which we concentrate, are compatible within theoretical uncertainties with the number of events experimentally observed. The importance of purely leptonic decay modes, which survive the lepton-isolation cuts, and the bias induced by lepton-isolation cuts on background event topologies are also clarified.