The Underlying Event in Hard Interactions at the Tevatron pbarp Collider

TL;DR

Measured the ambient momentum from spectator interactions in $\bar{p}p$ jet events at $\sqrt{s}=1800$ GeV and 630 GeV by summing charged tracks in two cones at $\eta=\eta^{(1)}$, $\phi=\phi^{(1)}\pm 90^\circ$ with $R=0.7$, and defined $P_T^{90,max}$ and $P_T^{90,min}$. The minimum cone momentum $P_T^{90,min}$ is roughly independent of the leading jet $E_T^{(1)}$ and similar to minimum-bias values, while $P_T^{90,max}$ grows with $E_T^{(1)}$, indicating more radiation in the leading-jet direction. Comparisons to HERWIG and PYTHIA show both models capture the qualitative trends, with PYTHIA requiring tuning (PDF choice, MPI parameters, and $P_T0$) to better match the data; at 1800 GeV the tuned PYTHIA aligns better at low $E_T^{(1)}$ but remains high at large $E_T^{(1)}$, and at 630 GeV both MCs describe the data well after tuning. A Swiss cheese analysis suggests additional contributions beyond the soft underlying event, such as hadronization splash, double-parton scattering, and higher-order radiation, especially when subtracting only the two leading jets; the 3-jet subtraction level approaches the minimum-bias baseline. The underlying-event momentum scales with energy, being about 45% lower at $\sqrt{s}=630$ GeV than at 1800 GeV, with important implications for jet cross-section measurements and MC tuning for current and future colliders.

Abstract

For comparison of inclusive jet cross sections measured at hadron-hadron colliders to next-to-leading order (NLO) parton-level calculations, the energy deposited in the jet cone by spectator parton interactions must first be subtracted. The assumption made at the Tevatron is that the spectator parton interaction energy is similar to the ambient level measured in minimum bias events. In this paper, we test this assumption by measuring the ambient charged track momentum in events containing large transverse energy jets at $\sqrt{s}=1800$ GeV and $\sqrt{s}=630$ GeV and comparing this ambient momentum with that observed both in minimum bias events and with that predicted by two Monte Carlo models. Two cones in $η$--$φ$ space are defined, at the same pseudo-rapidity, $η$, as the jet with the highest transverse energy ($E_T^{(1)}$), and at $\pm 90^o$ in the azimuthal direction, $φ$. The total charged track momentum inside each of the two cones is measured. The minimum momentum in the two cones is almost independent of $E_T^{(1)}$ and is similar to the momentum observed in minimum bias events, whereas the maximum momentum increases roughly linearly with the jet $E_T^{(1)}$ over most of the measured range. This study will help improve the precision of comparisons of jet cross section data and NLO perturbative QCD predictions. %this is new The distribution of the sum of the track momenta in the two cones is also examined for five different $E_T^{(1)}$ bins. The HERWIG and PYTHIA Monte Carlos are reasonably successful in describing the data, but neither can describe completely all of the event properties.

Figures (10)

• Figure 1: An example of a two jet event in the detector region under study. The cones used for the determination of the underlying event contribution are at $\eta = \eta^{(1)}$ and $\phi = \phi^{(1)} \pm 90^{\circ}$ where ($\eta^{(1)},\phi^{(1)}$) is the centroid of the highest $E_T$ jet in the event.
• Figure 2: $P_T^{90,max}$, $P_T^{90,min}$ and their difference $\Delta P_T^{90}$ as a function of the $E_T$ of the highest energy jet at $\sqrt{s}=1800$ GeV.
• Figure 3: $P_T^{90,max}$, $P_T^{90,min}$ and their difference $\Delta P_T^{90}$ as a function of the $E_T$ of the highest energy jet at $\sqrt{s}=1800$ GeV. PYTHIA has been tuned to reproduce the data.
• Figure 4: The distributions for the total $p_T$ in the sum of the $max$ and $min$ cones is plotted for five different bins of the $E_T$ of the highest energy jet. Data, HERWIG and PYTHIA distributions are shown at $\sqrt{s}=1800$ GeV.
• Figure 5: Number of tracks in the $max$ and $min$ cone as a function of the $E_T$ of the leading jet. Data, HERWIG and PYTHIA distributions are plotted at $\sqrt{s}=1800$ GeV.