Pythia Tune A, Herwig, and Jimmy in Run 2 at CDF

Abstract

We study the behavior of the charged particle and energy components of the "underlying event" in hard scattering proton-antiproton collisions at 1.96TeV. The goal is to produce data on the "underlying event" that is corrected to the particle level so that it can be used to tune the QCD Monte-Carlo models without requiring CDF detector simulation. Unlike the previous CDF Run 2 "underlying event" analysis which used JetClu to define "jets" and compared uncorrected data with the QCD Monte-Carlo models after detector simulation (i.e., CDFSIM), this analysis uses the MidPoint jet algorithm and corrects the observables to the particle level. The corrected observables are then compared with the QCD Monde-Carlo models at the particle level (i.e., generator level). The QCD Monte-Carlo models include PYTHIA Tune A, HERWIG, and a tuned version of JIMMY.

Figures (11)

• Figure 1: Illustration of correlations in azimuthal angle $\phi$ relative to the direction of the leading jet (MidPoint, $R=0.7$, $f_{merge}=0.75$) in the event, jet#$1$. The angle $\Delta\phi=\phi-\phi_{\rm jet1}$ is the relative azimuthal angle between charged particles and the direction of jet#$1$. The "transverse" region is defined by $60^\circ<|\Delta\phi|< 120^\circ$ and $|\eta|\!<\!1$. We examine charged particles in the range $p_T\!>\!0.5\,{\rm GeV/c}$ and $|\eta|\!<\!1$ and calorimeter towers with $|\eta|\!<\!1$, but allow the leading jet to be in the region $|\eta({\rm jet}\# 1)|<2$.
• Figure 2: Illustration of correlations in azimuthal angle $\phi$ relative to the direction of the leading jet (highest $P_T$ jet) in the event, jet#$1$. The angle $\Delta\phi=\phi-\phi_{\rm jet\#1}$ is the relative azimuthal angle between charged particles and the direction of jet#$1$. On an event by event basis, we define "transMAX" ("transMIN") to be the maximum (minimum) of the two "transverse" regions, $60^\circ<\Delta\phi<120^\circ$ and $60^\circ<-\Delta\phi<120^\circ$. "transMAX" and "transMIN" each have an area in $\eta$-$\phi$ space of $\Delta\eta\Delta\phi=4\pi/6$. The overall "transverse" region defined in Fig. 1 contains both the "transMAX" and the "transMIN" regions. Events in which there are no restrictions placed on the second and third highest $P_T$ jets (jet#$2$ and jet#$3$) are referred to as "leading jet" events ( left). Events with at least two jets with $P_T>15\,\textrm{GeV/c}$ where the leading two jets are nearly "back-to-back" ($|\Delta\phi|>150^\circ$) with $P_T({\rm jet}\#2)/P_T({\rm jet}\#1)>0.8$ and $P_T({\rm jet}\#3)<15\,\textrm{GeV/c}$ are referred to as "back-to-back" events ( right).
• Figure 3: Data at $1.96\,\textrm{TeV}$ on the density of charged particles, $dN_{chg}/{d\phi}{d\eta}$ with $p_T\!>\!0.5\,{\rm GeV/c}$ and $|\eta|\!<\!1$ in the "transMAX" region ( top) and the "transMIN" region ( bottom) for "leading jet" and "back-to-back" events defined in Fig. 2 as a function of the leading jet $P_T$ compared with PYTHIA Tune A and HERWIG. The data are corrected to the particle level (with errors that include both the statistical error and the systematic uncertainty) and compared with the theory at the particle level (i.e., generator level).
• Figure 4: Data at $1.96\,\textrm{TeV}$ on scalar $PT{\rm sum}$ density of charged particles, $dPT_{sum}/{d\phi}{d\eta}$, with $p_T\!>\!0.5\,{\rm GeV/c}$ and $|\eta|\!<\!1$ in the "transMAX" region ( top) and the "transMIN" region ( bottom) for "leading jet" and "back-to-back" events defined in Fig. 2 as a function of the leading jet $P_T$ compared with PYTHIA Tune A and HERWIG. The data are corrected to the particle level (with errors that include both the statistical error and the systematic uncertainty) and compared with the theory at the particle level (i.e., generator level).
• Figure 5: Data at $1.96\,\textrm{TeV}$ on average transverse momentum, $\langle\!p_T\!\rangle$, of charged particles with $p_T\!>\!0.5\,{\rm GeV/c}$ and $|\eta|\!<\!1$ in the "transverse" region for "leading jet" and "back-to-back" events defined in Fig. 2 as a function of the leading jet $P_T$ compared with PYTHIA Tune A and HERWIG. The data are corrected to the particle level (with errors that include both the statistical error and the systematic uncertainty) and compared with the theory at the particle level (i.e., generator level).