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

TL;DR

This study measures the forward energy density ratio in proton-proton collisions at different energies using CMS data, revealing discrepancies between models and observed energy dependence, especially across multiple energies.

Contribution

It introduces a novel observable, the forward energy density ratio, and analyzes its energy dependence in pp collisions at three energies, providing new insights into the underlying event modeling.

Findings

Models reproduce the ratio's dependence on jet pt at each energy.

All models fail to simultaneously describe the energy dependence of the forward energy density.

The forward energy density increases with sqrt(s) in both inclusive and jet events.

Abstract

The underlying event activity in proton-proton collisions at forward pseudorapidity (-6.6 < eta < -5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/d(eta), for events with a charged-particle jet produced at central pseudorapidity (abs(eta[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, pt, at three different pp centre-of-mass energies (sqrt(s) = 0.9, 2.76, and 7 TeV). In addition, the sqrt(s) 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 pt at each sqrt(s) separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with sqrt(s) in both inclusive events and in events with a central jet.