Comparison of the Z/gamma* + jets to gamma + jets cross sections in pp collisions at sqrt(s) = 8 TeV

TL;DR

The paper provides precise CMS measurements of differential cross sections for Z/γ*+jets and γ+jets at √s = 8 TeV, correcting for detector effects and comparing to LO and NLO QCD predictions. It shows that LO generators (MadGraph+PYTHIA6, Sherpa) generally misestimate rates, while the NLO BlackHat calculation better captures the data shapes, though tails in multi-jet regions remain challenging. The Z/γ cross-section ratio reveals a plateau at high boson pT, with a measured plateau value around a few percent for dileptonic Z and a near-unity total ratio after normalization, informing background modeling for Z→νν vs γ processes. These results validate using γ+jets as a proxy for Z+jets in background-dominated searches, provided the measured Z/γ ratio is applied.

Abstract

A comparison of the differential cross sections for the processes Z/gamma* + jets and photon (gamma) + jets is presented. The measurements are based on data collected with the CMS detector at sqrt(s) = 8 TeV corresponding to an integrated luminosity of 19.7 inverse femtobarns. The differential cross sections and their ratios are presented as functions of pt. The measurements are also shown as functions of the jet multiplicity. Differential cross sections are obtained as functions of the ratio of the Z/gamma* pt to the sum of all jet transverse momenta and of the ratio of the Z/gamma* pt to the leading jet transverse momentum. The data are corrected for detector effects and are compared to simulations based on several QCD calculations.

Figures (8)

• Figure 1: The purity fit on the photon component of the photon isolation $I_\text{ph}^\mathrm{PF}$ in the photon transverse momentum bin between 100 and 111$\,\text{Ge\spaceV}$ in data (left). The photon purity as function of the photon transverse momentum (right). The dots are the data points, the dot-dashed line is the signal template, and the dotted line represents the background component. The solid line represents the fit and the legend shows the resulting purity fraction of 66.7%.
• Figure 2: Top left: Differential cross section for ${Z}$ boson production as a function of $p_{\mathrm{T}}\xspace^{{Z}}\xspace$ for an inclusive ${Z}+\text{jets}$, $n_{\text{jets}}\xspace\geq1$ selection of detector-corrected data in comparison with estimations from MadGraph+pythia6, sherpa, and BlackHat. Top right: Differential cross section for photon production as a function of $p_{\mathrm{T}}\xspace^{\gamma}$ for an inclusive $\gamma+\text{jets}$, $n_{\text{jets}}\xspace\geq1$ selection for central rapidities $\lvert y^{\gamma} \rvert<1.4$ in detector-corrected data is compared with estimations from MadGraph+pythia6 and BlackHat. A detailed explanation is given in Section \ref{['diffcrosssections']}. The bottom plots give the ratio of the various theoretical estimations to the data in the ${Z}+\text{jets}$ case (bottom left) and $\gamma+\text{jets}$ case (bottom right).
• Figure 3: Top left: Differential cross section for ${Z}$ boson production as a function of $p_{\mathrm{T}}\xspace^{{Z}}\xspace$ for an inclusive ${Z}+\text{jets}$, $n_{\text{jets}}\xspace\geq2$ selection of detector-corrected data in comparison with estimations from MadGraph+pythia6, sherpa, and BlackHat. Top right: Differential cross section for photon production as a function of $p_{\mathrm{T}}\xspace^{\gamma}$ for an inclusive $\gamma+\text{jets}\xspace$, $n_{\text{jets}}\xspace\geq2$ selection for central rapidities $\lvert y^{\gamma} \rvert<1.4$ in detector-corrected data is compared with estimations from MadGraph+pythia6 and BlackHat. A detailed explanation is given in Section \ref{['diffcrosssections']}. The bottom plots give the ratio of the various theoretical estimations to the data in the ${Z}+\text{jets}$ case (bottom left) and $\gamma+\text{jets}$ case (bottom right).
• Figure 4: Ratio of the inclusive rates for $n_{\text{jets}}\xspace\geq2$ and $n_{\text{jets}}\xspace\geq1$ versus the transverse momentum of the boson for ${Z}+\text{jets}$ in detector-corrected data compared to estimations from MadGraph+pythia6, sherpa, and BlackHat (top left) and for $\gamma+\text{jets}\xspace$ for central rapidities $\lvert y^{\gamma} \rvert<1.4$ in detector-corrected data compared with estimations from MadGraph+pythia6 and BlackHat (top right). A detailed explanation is given in Section \ref{['diffcrosssections']}. The bottom plots give the ratio of the various theoretical estimations to the data in the ${Z}+\text{jets}$ case (bottom left) and $\gamma+\text{jets}$ case (bottom right).
• Figure 5: The measured distribution of the observable $p_{\mathrm{T}}\xspace^{{Z}}/H_{\mathrm{T}}\xspace$ ratio for $n_{\text{jets}}\xspace\geq2$ (top left) and $n_{\text{jets}}\xspace\geq3$ (top right) for ${Z}+\text{jets}$ in detector-corrected data compared with estimations from MadGraph+pythia6, sherpa, and BlackHat. A detailed explanation is given in Section \ref{['diffcrosssections']}. The bottom plots give the ratio of the various theoretical estimations to the data in the $n_{\text{jets}}\xspace\geq2$ case (bottom left) and $n_{\text{jets}}\xspace\geq3$ case (bottom right).