Presented are recent measurements in ep collisions at HERA, which comprise results that are expected to be sensitive to multiple interactions and the underlying event. A measurement of 3- and 4-jet event cross-sections in photoproduction, as well as two measurements which target the activity in the soft underlying event in photoproduction and deep inelastic scattering at low photon virtuality (Q^2), respectively, are reviewed. The data are compared to various QCD models with and without multiple i nteractions simulated. The multijet event cross-sections are in addition compared to a fixed-order O(@a@a_s^2) calculation.

Ringberg Workshop on New Trends in HERA Physics 2008, Ringberg Castle, Germany, 5 Oct 2008 - 10 Oct 2008; Nuclear physics <Amsterdam> / B / Proceedings supplements 191, 141 - 150 (2009). doi:10.1016/j.nuclphysbps.2009.03.121

Published by Elsevier, Amsterdam