Measurement of differential production cross-sections for a $Z$ boson in association with $b$-jets in 7 TeV proton-proton collisions with the ATLAS detector

TL;DR

We present differential cross-sections for a $Z$ boson produced in association with $b$-jets in $pp$ collisions at $\sqrt{s}=7$ TeV using ATLAS data corresponding to $4.6\ \mathrm{fb}^{-1}$. The analysis unfolds detector-level yields to particle-level in a fiducial region for $Z\rightarrow \ell^{+}\ell^{-}$ and jets defined with the anti-$k_t$ algorithm, then compares to LO and NLO pQCD predictions in both 4FNS and 5FNS, with MPI and QED radiation corrections applied. The results show that NLO calculations generally describe the data within uncertainties, with the 5FNS performing well for $Z+\ge 1\,b$-jet and the 4FNS better for $Z+\ge 2\,b$-jets; angular distributions benefit from higher multiplicity matrix elements. These measurements provide stringent tests of heavy-flavour production in QCD and offer valuable constraints for PDFs and $Z$+heavy-flavour backgrounds relevant to Higgs and new-physics searches.

Abstract

Measurements of differential production cross-sections of a $Z$ boson in association with $b$-jets in $pp$ collisions at $\sqrt{s}=7$ TeV are reported. The data analysed correspond to an integrated luminosity of 4.6 fb$^{-1}$ recorded with the ATLAS detector at the Large Hadron Collider. Particle-level cross-sections are determined for events with a $Z$ boson decaying into an electron or muon pair, and containing $b$-jets. For events with at least one $b$-jet, the cross-section is presented as a function of the $Z$ boson transverse momentum and rapidity, together with the inclusive $b$-jet cross-section as a function of $b$-jet transverse momentum, rapidity and angular separations between the $b$-jet and the $Z$ boson. For events with at least two $b$-jets, the cross-section is determined as a function of the invariant mass and angular separation of the two highest transverse momentum $b$-jets, and as a function of the $Z$ boson transverse momentum and rapidity. Results are compared to leading-order and next-to-leading-order perturbative QCD calculations.

Figures (13)

• Figure 1: Leading order Feynman diagrams contributing to $Z$+$b$-jets production. Process \ref{['fig:feynman1']} is only present in a 5FNS calculation, while \ref{['fig:feynman2']} and \ref{['fig:feynman3']} are present in both the 4FNS and 5FNS calculations.
• Figure 2: Comparison of simulated $E_{\mathrm{T}}^{\mathrm{miss}}$ distributions for (a) 1-tag events and (b) 2-tag events after all other signal selection criteria are applied, normalised to the expected yields in the data sample. The shaded distributions are signal Alpgen+ Herwig+ Jimmy events, and the open distributions are selected $t\bar{t}$ events. The vertical line shows the selection applied to the analysis sample to reject $t\bar{t}$ events while keeping signal events.
• Figure 3: Distributions of (a) CombNNc and (b) CombNN for different jet flavours in simulated $Z$+jets events for all selected tagged jets, in events with at least one tagged jet. The $Z\rightarrow ee$ and $Z\rightarrow\mu\mu$ channels are combined and simulated data are normalised such that the predicted number of jets in 4.6 fb$^{-1}$ are shown.
• Figure 4: Example fits to the distribution of (a) CombNNc at jet-level for 1-tag events with $1.2<|y(Z)|<1.6$, and (b) $\sum(\mathrm{CombNNc})$ at event-level for 2-tag events with $3.2<\Delta R(b,b)<5.0$.
• Figure 5: The tagged-jet CombNNc distribution in the $t\bar{t}$ enriched control region described in the text, with the simulation split by jet flavour (top), and the ratio of data to default simulation (filled circles, bottom). The dashed line shows the $b$-jet template reweighting function derived from the difference between data and simulation in this control region.