Study of the Underlying Event at Forward Rapidity in Proton-Proton Collisions at the LHC

We study the underlying event activity in pp collisions at forward rapidity ($5.2 < |\eta| < 6.6$) by measuring the ratio of the energy density, $dE/d\eta$, between events with a charged particle jet produced at central rapidity ($|\eta| < 2$) and inclusive events. The energy density ratio is measured as a function of the charged particle jet transverse momentum at three different proton-proton centre-of-mass energies ($\sqrt{s} = 0.9$, 2.76 and 7 TeV). In addition, the relative increase of the energy density in inclusive events and in events with a central charged particle jet as a function of centre-of-mass energy is studied. The results are compared to MC generators for pp collisions and discussed in terms of the underlying event activity.