Study of the underlying event at forward rapidity in pp collisions at sqrt{s}=0.9,2.76 and 7 TeV

The underlying event activity in proton-proton collisions at forward pseudo-rapidity (−6.6 < η < −5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/dη, for events with a charged-particle jet produced at central pseudorapidity (|η jet| < 2) to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, p T, at three different pp centre-of-mass energies ( TeX , 2.76, and 7 TeV). In addition, the TeX evolution of the forward energy density is studied in inclusive events and in events with a central jet. The results are compared to those of Monte Carlo event generators for pp collisions and are discussed in terms of the underlying event. Whereas the dependence of the forward energy density ratio on jet p T at each TeX separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with TeX in both inclusive events and in events with a central jet.