On strategies for determination and characterization of the underlying event

We discuss the problem of the separation and description of the underlying event (UE) within two existing approaches to UE measurement: the "traditional" method, widely used at Tevatron, and a recently proposed jet-area/median method. A simple toy model of UE is developed in order to understand how these approaches perform. We find that both methods are comparably good for measuring average properties of the UE but the jet-area/median approach is favorable for determining fluctuations. We also use the latter method to study the UE from several existing Monte Carlo generator tunes. We investigate which characteristics of the underlying event might be useful to measure in order to improve understanding of its properties and to simulate it well. These include transverse momentum density per area, intra and inter-event fluctuations and correlations.

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