Event shapes and azimuthal correlations in Z + jets events in pp collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study measures event shapes and azimuthal correlations in Z+jets production at $\sqrt{s}=7$ TeV using $5.0\ \mathrm{fb}^{-1}$ of data, testing perturbative QCD across jet multiplicities. It introduces topological observables such as $\Delta\phi(Z, j_i)$ and the transverse thrust $\ln\tau_T$, and unfolds results to particle level while comparing multiple MC predictions (MadGraph, Sherpa, Powheg, Pythia). The results show LO multiparton matrix-element plus parton-shower generators describe the data well, with improved agreement from NLO Z+1jet (Powheg) in high-multiplicity events, highlighting the importance of parton showers in modeling multi-jet final states. These measurements provide stringent MC validation and enhanced background modeling for SM processes and new-physics searches at the LHC.

Abstract

Measurements of event shapes and azimuthal correlations are presented for events where a Z boson is produced in association with jets in proton-proton collisions. The data collected with the CMS detector at the CERN LHC at sqrt(s) = 7 TeV correspond to an integrated luminosity of 5.0 inverse femtobarns. The analysis provides a test of predictions from perturbative QCD for a process that represents a substantial background to many physics channels. Results are presented as a function of jet multiplicity, for inclusive Z boson production and for Z bosons with transverse momenta greater than 150 GeV, and compared to predictions from Monte Carlo event generators that include leading-order multiparton matrix-element (with up to four hard partons in the final state) and next-to-leading-order simulations of Z + 1-jet events. The experimental results are corrected for detector effects, and can be compared directly with other QCD models.

Figures (7)

• Figure 1: Topology of ${Z}$ + jets events: (a) for $\ln\tau_\mathrm{T}\rightarrow -\infty$ and $\Delta\phi({Z},j_1)\rightarrow \pi$; (b) for $\ln\tau_\mathrm{T}\rightarrow -1$ and $\Delta\phi({Z},j_1) \ll \pi$.
• Figure 2: Distributions for ${Z}$$\rightarrow \mu\mu$ candidate events in data, compared with expectations from simulated signal and background contributions using MadGraph simulations normalized to the integrated luminosity of the data: (a) as a function of associated jet multiplicity $N_\text{jets}\xspace$, and (b) as a function of $p_{\mathrm{T}}$ of the dimuon pair $(p_{\mathrm{T}}\xspace^{\mu\mu})$ for $N_\text{jets}\xspace\geq 1$. The dibosons $\mathrm{W}\xspace\mathrm{W}\xspace$, $\mathrm{W}\xspace{Z}\xspace$, ${Z}\xspace{Z}\xspace$ and $\mathrm{W}$ + jets backgrounds are collectively denoted as EW in the legends. The plots in (c) and (d) show the ratios of the data to predictions from $\textrm{MC}$. The error bars on the data points represent only their statistical uncertainties and do not include systematic effects.
• Figure 3: Distributions for ${Z} \rightarrow \mathrm{e} \mathrm{e}$ candidate events in data, compared with expectations from simulated signal and background contributions using MadGraph simulations normalized to the integrated luminosity of the data: (a) as a function of associated jet multiplicity $N_\text{jets}\xspace$, and (b) as a function of $p_{\mathrm{T}}$ of the dielectron pair $(p_{\mathrm{T}}\xspace^{ee})$ for $N_\text{jets}\xspace\geq 1$. The dibosons $\mathrm{W}\xspace\mathrm{W}\xspace$, $\mathrm{W}\xspace{Z}\xspace$, ${Z}\xspace{Z}\xspace$ and $\mathrm{W}$ + jets backgrounds are collectively denoted as EW in the legends. The plots in (c) and (d) show the ratios of the data to predictions from $\textrm{MC}$. The error bars on the data points represent only their statistical uncertainties and do not include systematic effects.
• Figure 4: Normalized $\Delta\phi({Z},j_1)$ distributions for the leading jet in the inclusive jet-multiplicity bins $N_\text{jets}\xspace\geq 1, \geq2,$ and $\geq3$: (a) all $p_\mathrm{T}^{Z}$ and (b) $p_\mathrm{T}^{Z}\xspace > 150\,\text{Ge\spaceV}\xspace$. Plots in (c) and (d) show the corresponding ratios of the data (solid points), and of other $\textrm{MC}$ predictions, relative to MadGraph. The ratio for pythia$\textrm{MC}$ is not included in these plots. The error bars on the data points represent their statistical uncertainties, the solid yellow shaded band around the points represents the sum of statistical and systematic uncertainties taken in quadrature, while the cross-hatched (cyan) bands reflect the statistical uncertainties on the MadGraph calculations.
• Figure 5: Normalized $\Delta\phi({Z},j_{i})$ distributions for the inclusive $N_\text{jets}\xspace\geq 3$ jet-multiplicity bin: (a) all $p_\mathrm{T}^{Z}$ and (b) $p_\mathrm{T}^{Z}\xspace > 150\,\text{Ge\spaceV}\xspace$. Plots in (c) and (d) show the ratios of the data and other $\textrm{MC}$ predictions, relative to MadGraph, as described in Fig. \ref{['fig:dphiZJ1jetmultbinned']}.