Measurement of the dependence of transverse energy production at large pseudorapidity on the hard scattering kinematics of proton--proton collisions at $\sqrt{s} = 2.76$ TeV with ATLAS

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 $pp$ events recorded with the ATLAS detector at the LHC. The underlying event is characterised through measurements of the average sum of the transverse energy at large pseudorapidity downstream to 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. Further insight is gained by estimating, from the dijet kinematics on an event-by-event basis, the scaled longitudinal momenta of the hard-scattered partons in the target and projectile beam-protons moving to and away from the transverse energy measuring region, respectively. These definitions are chosen by analogy to proton--lead collisions, where, by convention, the proton is considered to be the projectile and the nucleus is considered the target, and the event centrality is characterized by the transverse energy production downstream of the target. Transverse energy production at large pseudorapidity is observed to vary with a steep linear dependence on the longitudinal momentum fraction in the target proton and only weakly with that in the projectile proton.