Measurement of Cross Sections for $b$ Jet Production in Events with a $Z$ Boson in $p\bar{p}$ Collisions at $\sqrt{s}=1.96$ TeV

TL;DR

This paper reports a precision measurement of Z boson production with one or more b jets in p pbar collisions at 1.96 TeV using the CDF II detector, focusing on the ratio sigma(Z+b jet)/sigma(Z) to reduce systematics. It employs a secondary-vertex b-tagging technique, data-driven background estimates, and a template fit to disentangle b, c, and light jets, yielding Nb = 270 ± 43 and an integrated cross-section ratio of (3.32 ± 0.53 (stat) ± 0.42 (syst)) × 10^-3. The authors compare their results to LO generators and NLO calculations (mcfm), observing significant scale sensitivity and encouraging agreement with lower scale choices. The work provides the first differential measurements of Z+b jet production, testing QCD predictions and informing the b-quark content of the proton with practical implications for Higgs and new-physics searches at hadron colliders.

Abstract

A measurement of the $\bjet$ production cross section is presented for events containing a $Z$ boson produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV, using data corresponding to an integrated luminosity of 2 fb$^{-1}$ collected by the CDF II detector at the Tevatron. $Z$ bosons are selected in the electron and muon decay modes. Jets are considered with transverse energy $E_T>20$ GeV and pseudorapidity $|η|<1.5$ and are identified as $\bjets$ using a secondary vertex algorithm. The ratio of the integrated $Z+\bjet$ cross section to the inclusive $Z$ production cross section is measured to be $3.32 \pm 0.53 {\rm (stat.)} \pm 0.42 {\rm (syst.)}\times 10^{-3}$. This ratio is also measured differentially in jet $E_T$, jet $η$, $Z$-boson transverse momentum, number of jets, and number of $\bjets$. The predictions from leading order Monte Carlo generators and next-to-leading-order QCD calculations are found to be consistent with the measurements within experimental and theoretical uncertainties.

Figures (9)

• Figure 1: Leading order Feynman diagrams for $gb \to Zb$ and $q\bar{q} \to Zb\bar{b}$ production.
• Figure 2: Dilepton invariant mass for events with at least one positive secondary vertex tag. The data (points) are shown together with the Drell-Yan Monte Carlo (open histogram) and the sum of the background contributions (filled histogram). The range where the data are selected for this analysis is indicated by arrows.
• Figure 3: Invariant mass of tracks at the secondary vertex for (a) positively and (b) negatively tagged jets. Shown are the data (points) and the fitted contributions of light, $c$ and $b{\,\rm jets}$. Also shown is the background contribution from $Z$+light jets, $Z+c$ jets, and from other processes with $b$ jets. The fitted number of light jets ($N_l$), $c$ jets ($N_c$), $b$ jets ($N_b$), and the number of background events ($N_{bg}$) is also shown.
• Figure 4: Ratio of the $Z+b{\,\rm jet}$ to the inclusive $Z$ boson cross section versus a) $E_T^{b{\,\rm jet}}$ and b) $\eta^{b{\,\rm jet}}$. Shown are the data (points) compared to the predictions from mcfm calculated with the scales $Q^2=m_Z^2+p_{T,Z}^2$ (solid line) and with $Q^2=\langle p_{T,{\rm jet}}^2\rangle$ (dotted line). The inner error bars represent the statistical errors, and the outer error bars represent the total errors.
• Figure 5: Ratio of the $Z+b{\,\rm jet}$ cross section to the $Z$ boson cross section versus a) $N_{\rm jet}$ and b) $N_{b{\,\rm jet}}$ .Shown are the data (points) compared to the predictions from mcfm calculated with the scales $Q^2=m_Z^2+p_{T,Z}^2$ (solid line) and with $Q^2=\langle p_{T,{\rm jet}}^2\rangle$ (dotted line). The inner error bars represent the statistical errors, and the outer error bars represent the total errors.