  The relationship between jet production in the central region and the
underlying-event activity in a pseudorapidity-separated region is studied in
4.0 pb$^{-1}$ of $\sqrt{s} = 2.76$ TeV $pp$ collision data recorded with the
ATLAS detector at the LHC. The underlying event is characterised through
measurements of the average value of the sum of the transverse energy at large
pseudorapidity downstream of one of the protons, which are reported here as a
function of hard-scattering kinematic variables. The hard scattering is
characterised by the average transverse momentum and pseudorapidity of the two
highest transverse momentum jets in the event. The dijet kinematics are used to
estimate, on an event-by-event basis, the scaled longitudinal momenta of the
hard-scattered partons in the target and projectile beam-protons moving toward
and away from the region measuring transverse energy, respectively. Transverse
energy production at large pseudorapidity is observed to decrease with a linear
dependence on the longitudinal momentum fraction in the target proton and to
depend only weakly on that in the projectile proton. The results are compared
to the predictions of various Monte Carlo event generators, which qualitatively
reproduce the trends observed in data but generally underpredict the overall
level of transverse energy at forward pseudorapidity.

High Energy Physics - Experiment;Nuclear Experiment