Early LHC Underlying Event Data - Findings and Surprises

Abstract

The CDF PYTHIA 6.2 Tune DW predictions of the LHC underlying event (UE) data at 900 GeV and 7 TeV are examined in detail. The behavior of the UE at the LHC is roughly what we expected. The new LHC PYTHIA 6.4 Tune Z1 does an even better job describing the UE data at 900 GeV and 7 TeV. However, the modeling of "min-bias" at the LHC (i.e. the overall inelastic cross section) is a very different story. No model describes all the features of "min-bias" collisions at 900 GeV and 7 TeV.

Figures (8)

• Figure 1: Illustration of correlations in azimuthal angle$\Delta \varphi$ relative to (left) the direction of the leading charged particle, PTmax, or to (right) the leading charged particle jet, chgjet#1. The relative angle $\Delta \varphi=\varphi-\varphi_{1}$, where $\varphi_{1}$ is the azimuthal angle of PTmax (or chgjet#1) and $\varphi$ is the azimuthal angle of a charged particle. There are two transverse regions $60^{\circ}< \Delta \varphi<120^{\circ},|\eta|<\eta_{\text{cut }}$ and $60^{\circ}<-\Delta \varphi<120^{\circ},|\eta|<\eta_{\text{cut }}$. The overall "transverse" region of $\eta-\varphi$ space is defined by $60^{\circ}< |\Delta \varphi|<120^{\circ}$ and $|\eta|<\eta_{\text{cut }}$. The "transverse" charged particle density is the number of charged particles in the "transverse" region divided by the area in $\eta-\varphi$ space. Similarly, the "transverse" charged PTsum density is the scalar PTsum of charged particles in the "transverse" region divided by the area in $\eta-\varphi$ space.
• Figure 2: (left column) Fake data at 900 GeV on the transverse charged particle density (top left) and the transverse charged PTsum density (bottom left) as defined by the leading charged particle (PTmax) and the leading charged particle jet (chgjet#1) for charged particles with$\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2$. The fake data (from PYTHIA Tune DW) are generated at the particle level (i.e. generator level) assuming 0.5 M min-bias events at 900 GeV ( 361,595 events in the plot). (right column) CMS preliminary data [9] at 900 GeV on the transverse charged particle density (top right) and the transverse charged PTsum density (bottom right) as defined by the leading charged particle (PTmax) and the leading charged particle jet (chgjet#1) for charged particles with $\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2$. The data are uncorrected and compared with PYTHIA Tune DW after detector simulation (216,215 events in the plot).
• Figure 3: (left column) CMS preliminary data [10] at 900 GeV and 7 TeV on the transverse charged particle density (top left) and the transverse charged PTsum density (bottom left) as defined by the leading charged particle jet (chgjet#1) for charged particles with$\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2$. The data are uncorrected and compared with PYTHIA Tune DW after detector simulation. (right column) ATLAS preliminary data [11] at 900 GeV and 7 TeV on the transverse charged particle density (top right) and the transverse charged PTsum density (bottom right) as defined by the leading charged particle (PTmax) for charged particles with $\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2.5$. The data are corrected and compared with PYTHIA Tune DW at the generator level.
• Figure 4: (left column) CMS preliminary data on the ratio of 7 TeV and 900 GeV ( 7 TeV divided by 900 geV ) for the transverse charged particle density (top left) and the transverse charged PTsum density (bottom left) as defined by the leading charged particle jet (chgjet#1) for charged particles with$\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2$. The data are uncorrected and compared with PYTHIA Tune DW after detector simulation. (right column) ATLAS preliminary data on the ratio of 7 TeV and 900 GeV for the transverse charged particle density (top right) and the transverse charged PTsum density (bottom right) as defined by the leading charged particle (PTmax) for charged particles with $\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<$ 2.5. The data are corrected and compared with PYTHIA Tune DW at the generator level.
• Figure 5: (left column) CMS preliminary data [10] at 900 GeV and 7 TeV on the transverse charged particle multiplicity distribution (top left) and the transverse charged PTsum distribution (bottom left) as defined by the leading charged particle jet with PT(chgjet#1)$>3 \mathrm{GeV}$ for charged particles with $\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2$. The data are uncorrected and compared with PYTHIA Tune DW after detector simulation. (right column) CMS preliminary data at 7 TeV on the transverse charged particle multiplicity distribution (top left) and the transverse charged PTsum distribution (bottom left) as defined by the leading charged particle jet with PT(chgjet#1) $>3 \mathrm{GeV}$ and with PT(chgjet#1) $>20 \mathrm{GeV}$ for charged particles with $\mathrm{p}_{\mathrm{T}}>0.5 \mathrm{GeV} / \mathrm{c}$ and $|\eta|<2$. The data are uncorrected and compared with PYTHIA Tune DW after detector simulation.