Studying the underlying event in Drell-Yan and high transverse momentum jet production at the Tevatron

T. Aaltonen et al. (CDF Collaboration)

Phys. Rev. D 82, 034001 – Published 2 August 2010

Abstract

We study the underlying event in proton-antiproton collisions by examining the behavior of charged particles produced in association with a large transverse momentum jet ( $\sim 2.2 {fb}^{- 1}$ ) or with a Drell-Yan lepton pair ( $\sim 2.7 {fb}^{- 1}$ ) in the $Z$ -boson mass region [ $70 < M (pair) < 110 GeV / c^{2}$ ] as measured by CDF at 1.96 TeV center-of-mass energy. We use the direction of the lepton pair or the leading jet in each event to define regions of $η − ϕ$ space that are sensitive to the modeling of the underlying event. The data are corrected to the particle level to remove detector effects and are then compared with several QCD Monte Carlo models.

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Received 17 March 2010

DOI:https://doi.org/10.1103/PhysRevD.82.034001

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Vol. 82, Iss. 3 — 1 August 2010

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Figure 1
Illustration of the way QCD Monte Carlo models simulate a proton-antiproton collision in which a hard two-to-two parton scattering with transverse momentum, $p_{T} (hard)$ , has occurred. The hard-scattering component of the event consists of particles that result from the hadronization of the two outgoing partons (i.e. the initial two jets) plus the particles that arise from initial and final-state radiation (i.e. multijets). The underlying event consists of particles that arise from the beam-beam remnants and from multiple-parton interactions.
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Figure 2
Illustration of the way QCD Monte Carlo models simulate Drell-Yan lepton-pair production. The hard-scattering component of the event consists of the two outgoing leptons plus particles that result from initial-state radiation. As in the hard two-to-two parton scattering of Fig. 1, the underlying event consists of particles that arise from the beam-beam remnants and from multiple-parton interactions.
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Figure 3
Illustration of correlations in azimuthal angle $Δ ϕ$ relative to (left) the direction of the leading jet (highest $p_{T}$ jet) in the event, jet#1, in high- $p_{T}$ jet production or (right) the direction of the lepton pair in Drell-Yan production. The angle $Δ ϕ = ϕ - ϕ_{jet # 1}$ ( $Δ ϕ = ϕ - ϕ_{pair}$ ) is the relative azimuthal angle between charged particles and the direction of jet#1 (lepton pair). The toward region is defined by $| Δ ϕ | < 60 °$ and $| η | < 1$ , while the away region is $| Δ ϕ | > 120 °$ and $| η | < 1$ . The two transverse regions $60 ° < - Δ ϕ < 120 °$ , $| η | < 1$ and $60 ° < Δ ϕ < 120 °$ , $| η | < 1$ are referred to as transverse 1 and transverse 2, respectively. Each of the two transverse regions have an area in $η − Δ ϕ$ space of $Δ η Δ ϕ = 4 π / 6$ . The overall transverse region corresponds to combining the transverse-1 and transverse-2 regions. The transMAX (transMIN) refer to the transverse region (transverse-1 or transverse-2) containing the largest (smallest) number of charged particles or to the region containing the largest (smallest) scalar $p_{T}$ sum of charged particles.
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Figure 4
(a) The response factors $F_{res} = CDFSIM / GEN$ for PYTHIA Tune A (pyA) with tight and loose track cuts for leading-jet events in the transverse region. (b) The same as (a) for the toward region. (c) Compares the response factors for PYTHIA Tune A (pyA) and HERWIG without MPI (HW) for tight track cuts for leading-jet events in the transverse region. (d) The same as (c) for the toward region. The correction factor is the reciprocal of the response factor ( $F_{cor} = 1 / F_{res}$ ).
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Figure 5
The response factors $F_{res} = CDFSIM / GEN$ for the charged-particle density $d N / d η d ϕ$ in the transverse region for leading-jet events and for Drell-Yan events. The plots show the response factors for PYTHIA Tune A (pyA) with tight track cuts (leading-jet) and for PYTHIA Tune AW (pyAW) with tight track cuts (Drell-Yan). The correction factor is the reciprocal of the response factor ( $F_{cor} = 1 / F_{res}$ ).
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Figure 6
(a) CDF data at 1.96 TeV on the density of charged particles in the toward, away, and transverse regions for leading-jet events compared with PYTHIA Tune A (pyA) at the particle level. (b) The same as (a) but for the scalar ${PT}_{sum}$ density of charged particles. (c) CDF data at 1.96 TeV on the density of charged particles in the toward, away, and transverse regions for Drell-Yan events compared with PYTHIA Tune AW (pyAW) at the particle level. (d) The same as (c) but for the scalar ${PT}_{sum}$ density of charged particles.
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Figure 7
(a) CDF data at 1.96 TeV on the density of charged particles in the transverse region for leading-jet events compared with PYTHIA Tune A (pyA) and HERWIG without MPI (HW). (b) The same as (a) but for the scalar ${PT}_{sum}$ density of charged particles. (c) CDF data at 1.96 TeV on the density of charged particles in the transverse region for Drell-Yan events compared with PYTHIA Tune AW (pyAW) and HERWIG without MPI (HW). (d) The same as (c) but for the scalar ${PT}_{sum}$ density of charged particles. (e) Compares (a) with (c) without the HERWIG curves. (f) Compares (b) with (d) without the HERWIG curves.
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Figure 8
(a) CDF data at 1.96 TeV on the average charged-particle transverse momentum in the transverse region for leading-jet events compared with PYTHIA Tune A (pyA) and HERWIG without MPI (HW). (b) The same as (a) but for the average maximum charge particle $p_{T}$ . (c) CDF data at 1.96 TeV on the average charged-particle transverse momentum in the transverse region for Drell-Yan events compared with PYTHIA Tune AW (pyAW) and HERWIG without MPI (HW). (d) The same as (c) but for the average maximum charge particle $p_{T}$ . (e) Compares (a) with (c) without the HERWIG curves. (f) Compares (b) with (d) without the HERWIG curves.
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Figure 9
(a) CDF data at 1.96 TeV on the transMAX and transMIN density of charged particles for leading-jet events compared with PYTHIA Tune A (pyA) and HERWIG without MPI (HW). (b) The same as (a) but for the scalar ${PT}_{sum}$ density of charged particles. (c) CDF data at 1.96 TeV on the transMAX and transMIN density of charged particles for Drell-Yan events compared with PYTHIA Tune AW (pyAW) and HERWIG without MPI (HW). (d) The same as (c) but for the scalar ${PT}_{sum}$ density of charged particles. (e) Compares the transMIN regions from (a) with (c) without the HERWIG curves. (f) Compares the transMIN regions from (b) with (d) without the HERWIG curves.
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Figure 10
(a) CDF data at 1.96 TeV on the transDIF density of charged particles for leading-jet and Drell-Yan events ( $transDIF = transMAX - transMIN$ ). The Drell-Yan data are compared with PYTHIA Tune AW and the leading-jet data are compared with PYTHIA Tune A. (b) The same as (a) for density of charged particles in the away region. (c) The same as (a) for the transDIF scalar ${PT}_{sum}$ density. (d) The same as (b) for the scalar ${PT}_{sum}$ density.
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Figure 11
(a) Compares the CDF data at 1.96 TeV on the charged-particle density in the toward and transverse regions for Drell-Yan events. Also shows the predictions of PYTHIA Tune AW. (b) Compares the CDF data on the charged-particle density in the toward and transMIN regions for Drell-Yan events and also shows the predictions of PYTHIA Tune AW. (c) The same as (a) for the scalar ${PT}_{sum}$ density. (d) The same as (a) for the average charged-particle $p_{T}$ of charged particles.
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Figure 12
(a) CDF data at 1.96 TeV on the density of charged particles in the toward region for Drell-Yan events. The data are compared with HERWIG without MPI (HW), HERWIG with JIMMY MPI (JIM), and three PYTHIA Tunes (pyAW, pyDW, and ATLAS). (b) The same as (a) but for the scalar ${PT}_{sum}$ density of charged particles. (c) The same as (a) for the toward average charge particle $p_{T}$ . (d) The same as (a) for the toward average maximum charge particle $p_{T}$ . (e) The same as (a) for the transMIN charged particle density. (f) The same as (a) for the transMIN charged ${PT}_{sum}$ density.
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Figure 13
(a) CDF data on the density of charged particles for Drell-Yan events in the toward region compared with Tune DW (pyDW R2). Also shows the predictions of Tune DW at 7 (LHC7) and 14 TeV (LHC14). (b) CDF data at the density of charged particles for leading-jet events in the transverse region compared with Tune DW (R2). Also shows the predictions of Tune DW at 7 (LHC7) and 14 TeV (LHC14). (c) Predictions of Tune DW and HERWIG without MPI (HW) for the density of charged particles for Drell-Yan events in the toward region at 1.96 TeV (R2) and at 14 TeV (LHC14). Also shows the prediction of Tune DWT at 14 TeV. (d) CDF data at on the average $p_{T}$ of charged particles for Drell-Yan events in the toward region compared with Tune DW (pyDW R2) and HERWIG without MPI (HW R2). Also shows the predictions of Tune DW at 7 (LHC7) and 14 TeV (LHC14).
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Figure 14
(a) CDF min-bias data on the average $p_{T}$ of charged particles versus the multiplicity for charged particles with $p_{T} > 0.4 GeV / c$ and $| η | < 1$ from Ref. [20]. The data are compared with PYTHIA Tune A (pyA), the PYTHIA ATLAS tune, and PYTHIA Tune A without MPI (pyAnoMPI). (b) CDF data on the average $p_{T}$ of charged particles versus the multiplicity for charged particles with $p_{T} > 0.5 GeV / c$ and $| η | < 1$ for Drell-Yan events. The data are compared with PYTHIA Tune AW, the PYTHIA ATLAS tune, HERWIG without MPI (HW), and HERWIG with JIMMY MPI (JIM). (c) The same as (b) for the average $p_{T}$ of the lepton pair versus the multiplicity for charged particles.
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Figure 15
(a) CDF data at 1.96 TeV on the average $p_{T}$ of charged particles versus the multiplicity for charged particles for Drell-Yan events in which $p_{T} (lepton-pair) < 10 GeV / c$ . The data are compared with PYTHIA Tune AW (pyAW), the PYTHIA ATLAS tune, HERWIG without MPI (HW), and HERWIG with JIMMY MPI (JIM). (b) Comparison of the average $p_{T}$ of charged particles versus the charged multiplicity for min-bias events from Ref. [20] with the Drell-Yan events with $p_{T} (lepton-pair) < 10 GeV / c$ from this analysis. The min-bias data require $p_{T} > 0.4 GeV / c$ and are compared with PYTHIA Tune A (pyA), while the Drell-Yan data require $p_{T} > 0.5 GeV / c$ and are compared with PYTHIA Tune AW (pyAW).
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