Jet and hadron production in e + e − annihilations

This note covers three topics in the study of identified hadrons and jet production, coming from recent analyses by the DELPHI experiment at LEP. A description of DELPHI can be found in [1]; its performance is discussed in [2]. The selection of qq̄ events at the Z peak (LEP 1) is easy: being the production of hadronic final states enhanced by three orders of magnitude over the continuum, simple cuts against leptonantilepton and two-photon events allow easily the definition of samples with contaminations at the per mil level. The situation is different at LEP 2 (above the Z). The hadronic cross-section is at the present energies of the order of 100 pb, and it is dominated by the radiative return to the Z peak. The cross-section corresponding to effective c.m. energies above 0.85 √ s is presently of the order of 20 pb, with a sizeable contamination from WW events; thus, even with large luminosities, we are left with a few hundreds of events per energy point (see the table below). As a consequence, the cuts against contaminations cannot be too severe (reaching purities of about 0.8, with efficiencies of about 0.8). √ s (GeV) Lumi/exp. Events with (pb−1) √ s′ > 0.85 √ s 133 12 900 161 10 350 172 10 290 183 55 1,300 189 175 3,800