Study of forward Z+jet production in pp collisions at $\sqrt{s} = 7$ TeV

TL;DR

This study measures forward Z boson production in association with jets in proton-proton collisions at 7 TeV using the LHCb detector, exploiting the forward acceptance to probe low-x PDFs. The analysis unfolds the jet-pT spectrum, corrects for detector effects, and reports cross-sections and ratios for two jet-pT thresholds along with six differential distributions. The results agree with next-to-leading-order QCD predictions at O(αs²), with the higher-order calculations providing a better description of the data, especially for pT^Z and Δφ, and offer valuable constraints on PDFs in the forward region. Overall, the work demonstrates the sensitivity of forward Z+jet measurements to QCD dynamics and proton structure, complementing central-detector results from ATLAS and CMS.

Abstract

A measurement of the $Z(\rightarrowμ^+μ^-)$+jet production cross-section in $pp$ collisions at a centre-of-mass energy $\sqrt{s} = 7$ TeV is presented. The analysis is based on an integrated luminosity of $1.0\,\text{fb}^{-1}$ recorded by the LHCb experiment. Results are shown with two jet transverse momentum thresholds, 10 and 20 GeV, for both the overall cross-section within the fiducial volume, and for six differential cross-section measurements. The fiducial volume requires that both the jet and the muons from the Z boson decay are produced in the forward direction ($2.0<η<4.5$). The results show good agreement with theoretical predictions at the second-order expansion in the coupling of the strong interaction.

Figures (9)

• Figure 1: Jet identification efficiency as a function of the true jet $p_\text{T}$. The uncertainties shown are statistical. The zero on the vertical axis is suppressed.
• Figure 2: Comparison between data (black points) and simulation (red line) in the $p_\text{T}^\text{jet} / p_\text{T}^{\mathrm{Z}\xspace}$ distribution for selected $\mathrm{Z}\xspace$+1-jet events where the $\mathrm{Z}\xspace$ boson and the jet are emitted azimuthally opposed. The uncertainties shown are statistical.
• Figure 3: Ratio of the Z+jet cross-section to the inclusive cross-section, for (top) $p_\text{T}^\text{jet}> 20\text{ GeV}$ and (bottom) $p_\text{T}^\text{jet} > 10\text{ GeV}$. The bands show the LHCb measurement (with the inner band showing the statistical uncertainty and the outer band showing the total uncertainty). The points correspond to different theoretical predictions with the error bars indicating their uncertainties as described in the main text. These results are corrected for FSR from the final state muons from the $\mathrm{Z}\xspace$ boson decay.
• Figure 4: Cross-section for Z+jet production, differential in the leading jet $p_\text{T}$, for $p_\text{T}^\text{jet} > 10\text{ GeV}$. The bands show the LHCb measurement (with the inner band showing the statistical uncertainty and the outer band showing the total uncertainty). The points correspond to different theoretical predictions with the error bars indicating their uncertainties as described in the main text. Predictions are displaced horizontally for presentation. These results are not corrected for FSR from the final state muons from the $\mathrm{Z}\xspace$ boson decay.
• Figure 5: Cross-section for Z+jet production, differential in the leading jet pseudorapidity, for (left) $p_\text{T}^\text{jet}> 20\text{ GeV}$ and (right) $p_\text{T}^\text{jet} > 10\text{ GeV}$. The bands show the LHCb measurement (with the inner band showing the statistical uncertainty and the outer band showing the total uncertainty). Superimposed are predictions as described in Fig. \ref{['res1']}.