The Underlying Event in Hadron-Hadron Collisions
  We measured the ambient energy in jet events using data of the CDF experiment at center-of-mass energies of 1800 and 630 GeV by examining the transverse momentum of charged particles in a region of the detector far away from the two most energetic jets in the event. The energy is compared to that found in minimum bias events. CDF data are compared to Monte Carlo predictions: neither Herwig nor Pythia can reproduce the data in detail. 
Valeria Tano