The Underlying Event in Hard Scattering Processes

TL;DR

The paper investigates the underlying event in hard proton-antiproton scattering at 1.8 TeV by leveraging transverse-region measurements of charged particles and comparing them to QCD Monte-Carlo generators (ISAJET, HERWIG, PYTHIA). It introduces transMAX/transMIN and transverse-cone analyses to separate hard-scattering and beam-beam-remnant contributions, revealing that multiple parton interactions are essential for reproducing the observed activity. ISAJET overpredicts soft remnant production and lacks proper shower coherence, while HERWIG and PYTHIA with color-coherence better describe the data; PYTHIA with MPI provides the most accurate overall description but requires careful tuning to PDFs. The work demonstrates that a combination of non-perturbative beam remnants and perturbative MPI processes shapes the underlying event and highlights the need for precise modeling and tuning in collider phenomenology.

Abstract

We study the behavior of the "underlying event" in hard scattering proton-antiproton collisions at 1.8 TeV and compare with the QCD Monte-Carlo models. The "underlying event" is everything except the two outgoing hard scattered "jets" and receives contributions from the "beam-beam remnants" plus initial and final-state radiation. The data indicate that neither ISAJET or HERWIG produce enough charged particles (with PT > 0.5 GeV/c) from the "beam-beam remnant" component and that ISAJET produces too many charged particles from initial-state radiation. PYTHIA which uses multiple parton scattering to enhance the "underlying event" does the best job describing the data.

Figures (29)

• Figure 1: Illustration of the way QCD Monte-Carlo models simulate a proton-antiproton collision in which a "hard" $2$-to-$2$ parton scattering with transverse momentum, $p_T({\rm hard})$, has occurred. The resulting event contains particles that originate from the two outgoing partons ( plus initial and final-state radiation) and particles that come from the breakup of the proton and antiproton ("beam-beam remnants"). The "underlying event" is everything except the two outgoing hard scattered "jets" and consists of the "beam-beam remnants" plus initial and final-state radiation. The "hard scattering" component consists of the outgoing two "jets" plus initial and final-state radiation.
• Figure 2: Illustration of the way PYTHIA models the "underlying event" in proton-antiproton collision by including multiple parton interactions. In adddition to the hard $2$-to-$2$ parton-parton scattering with transverse momentum, $p_T({\rm hard})$, there is a second "semi-hard" $2$-to-$2$ parton-parton scattering that contributes particles to the "underlying event".
• Figure 3: Illustration of correlations in azimuthal angle $\Delta\phi$ relative to the direction of the leading charged jet in the event, chgjet#1. The angle $\Delta\phi=\phi-\phi_{\rm chgjet\#1}$ is the relative azimuthal angle between charged particles and the direction of chgjet#1. The"toward" region is defined by $|\Delta\phi|<60^\circ$ and $|\eta|\!<\!1$, while the "away" region is $|\Delta\phi|>120^\circ$ and $|\eta|\!<\!1$. The "transverse" region is defined by $60^\circ<|\Delta\phi|<120^\circ$ and $|\eta|\!<\!1$. Each region has an area in $\eta$-$\phi$ space of $4\pi/3$.
• Figure 4: Illustration of the topology of an average proton-antiproton collision in which a "hard" $2$-to-$2$ parton scattering has occurred. The "toward" region as defined in FIG. \ref{['snow_fig3']} contains the leading charged particle "jet", while the "away" region, on the average, contains the "away-side" jet. The "transverse" region is perpendicular to the plane of the hard $2$-to-$2$ scattering and is very sensitive to the "underlying event".
• Figure 5: Data on the average number of charged particles ($p_T\!>\!0.5\,{\rm GeV/c}$, $|\eta|\!<\!1$) in the "transverse� region defined in FIG. \ref{['snow_fig3']} as a function of transverse momentum of the leading charged jet compared with the QCD Monte-Carlo predictions of HERWIG 5.9, ISAJET 7.32, and PYTHIA 6.115 with their default parameters and with $p_T\!({\rm hard})>3{\rm\,GeV/c}$. Each point corresponds to the $\langle\! N_{\rm chg}\!\rangle$ in a $1{\rm\,GeV/c}$ bin. The solid (open) points are the Min-Bias (JET20) data. The theory curves are corrected for the track finding efficiency and have an error ( statistical plus systematic) of around $5\%$.