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

Abstract

The underlying event activity in proton-proton collisions at forward pseudorapidity () 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 () to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, , at three different pp centre-of-mass energies ( √s = 0.9, 2.76, and 7 TeV). In addition, the √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 at each √s separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with √s in both inclusive events and in events with a central jet.

Comparisons to PYTHIA and HERWIG predictions (Paper) (click on plot to get .pdf)

Figure
Ratio of the energy deposited in the pseudorapidity range for events with a charged-particle jet with with respect to the energy in inclusive events, as a function of the jet transverse momentum for 0.9 (left), 2.76 (middle), and 7 TeV (right). Corrected results are compared to the PYTHIA and HERWIG++ MC models. Error bars indicate the statistical uncertainty on the data points, while the grey band represents the statistical and systematic uncertainties added in quadrature.

Energy density in the pseudorapidity range in inclusive events (left) and in events with a charged-particle jet in the range (right) as a function of , normalized to the energy density at 2.76 TeV. The threshold used for jets is 10 GeV/c at all centre-of-mass energies. Corrected results are compared to the PYTHIA and HERWIG++ MC models. Statistical uncertainties are smaller than the marker size, while the grey band represents the statistical and systematic uncertainties added in quadrature.

Comparisons to Cosmic Ray, CASCADE and DIPSY model predictions (Paper) (click on plot to get .pdf)

Figure
Ratio of the energy deposited in the pseudorapidity range for events with a charged-particle jet with with respect to the energy in inclusive events, as a function of the jet transverse momentum for 0.9 (left), 2.76 (middle), and 7 TeV (right). Corrected results are compared to MC models used in cosmic ray physics and to CASCADE and DIPSY. Error bars indicate the statistical uncertainty on the data points, while the grey band represents the statistical and systematic uncertainties added in quadrature.

Energy density in the pseudorapidity range in inclusive events (left) and in events with a charged-particle jet in the range (right) as a function of , normalized to the energy density at 2.76 TeV. The threshold used for jets is 10 GeV/c at all centre-of-mass energies. Corrected results are compared to MC models used in cosmic ray physics and to CASCADE and DIPSY. Statistical uncertainties are smaller than the marker size, while the grey band represents the statistical and systematic uncertainties added in quadrature.

Comparisons to PYTHIA and HERWIG predictions (extra public material) (click on plot to get .pdf)

Figure
Ratio of the energy deposited in the pseudorapidity range for events with a charged-particle jet with with respect to the energy in inclusive events, as a function of the jet transverse momentum for 0.9 (left), 2.76 (middle), and 7 TeV (right). Corrected results are compared to the PYTHIA and HERWIG++ MC models. Error bars indicate the statistical uncertainty on the data points, while the grey band represents the statistical and systematic uncertainties added in quadrature.

Energy density in the pseudorapidity range in inclusive events (left) and in events with a charged-particle jet in the range (right) as a function of , normalized to the energy density at 2.76 TeV. The threshold used for jets is 10 GeV/c at all centre-of-mass energies. Corrected results are compared to the PYTHIA and HERWIG++ MC models. Statistical uncertainties are smaller than the marker size, while the grey band represents the statistical and systematic uncertainties added in quadrature.

Detector level cuts versus particle level selection (extra public material) (click on plot to get .pdf)

Figure

Caption

vs. for all generated events (orange) and for generated events selected at the detector level (blue) for 7 TeV. Single diffractive dissociation events have a or value equal to ( at 7 TeV), while double diffractive dissociation and non-diffractive events have and values ranging from 0 to 1. The green lines indicate the particle level cuts. At the particle level, all events to the top and to the right of the green lines are selected.