Measurements of fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets in $pp$ collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector

TL;DR

This ATLAS study presents fiducial cross-sections for $t\bar{t}$ production in association with one or two extra $b$-jets at $\sqrt{s}=8$ TeV using $L=20.3\ \mathrm{fb}^{-1}$. By defining detector-compatible particle-level objects and three fiducial regions, the analysis extracts cross-sections in both the lepton-plus-jets and dilepton channels, employing a template-based approach and a separate cut-based method for the $ttbb$ final state. The measured values show reasonable agreement with NLO+PS predictions and with merged LO+PS calculations, while offering sensitivity to the treatment of $g\rightarrow b\bar{b}$ in parton showers and to the heavy-flavour content of $t\bar{t}$ final states. The results provide important constraints on backgrounds relevant for $t\bar{t}H$ and improve understanding of heavy-flavour production in top-quark events.

Abstract

Fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets are reported, using an integrated luminosity of 20.3 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for $t\bar{t}$ events with at least one additional $b$-jet is measured to be 950 $\pm$ 70 (stat.) $^{+240}_{-190}$ (syst.) fb in the lepton-plus-jets channel and 50 $\pm$ 10 (stat.) $^{+15}_{-10}$ (syst.) fb in the $e μ$ channel. The cross-section times branching ratio for events with at least two additional $b$-jets is measured to be 19.3 $\pm$ 3.5 (stat.) $\pm$ 5.7 (syst.) fb in the dilepton channel ($e μ$,\,$μμ$, and \,$ee$) using a method based on tight selection criteria, and 13.5 $\pm$ 3.3 (stat.) $\pm$ 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 $\pm$ 0.33 (stat.) $\pm$ 0.28 (syst.)\% for the ratio of $t\bar{t}$ production with two additional $b$-jets to $t\bar{t}$ production with any two additional jets. All measurements are in good agreement with recent theory predictions.

Figures (9)

• Figure 1: Jet multiplicity, $b$-tagged jet multiplicity, and transverse momentum $p_{\text{T}}$ of the jet with the third highest MV1c value in the lepton-plus-jets channel. Events are required to have at least five jets, at least two $b$-tagged jets and one lepton. The data are shown as black points with their statistical uncertainty. The stacked distributions are the nominal predictions from Monte Carlo simulation; the hashed area shows the total uncertainty on the prediction. The bottom sub-plot shows the ratio of the data to the prediction. The non-prompt and fake lepton backgrounds are referred to as 'NP & fakes'. The last bin of the distribution includes the overflow.
• Figure 2: Jet multiplicity, $b$-tagged jet multiplicity, and transverse momentum $p_{\text{T}}$ of the jets with the third and fourth highest MV1c values, in the dilepton channel using the $ttbb$ fit-based selection; events are required to have at least four jets, two $b$-tagged jets and two leptons ($ee$, $e\mu$ or $\mu\mu$). The data are shown in black points with their statistical uncertainty. The stacked distributions are the nominal predictions from Monte Carlo simulation; the hashed area shows the total uncertainty on the prediction. The bottom sub-plot shows the ratio of the data to the prediction. The non-prompt and fake lepton backgrounds are referred to as 'NP & fakes'. The last bin of the distribution includes the overflow.
• Figure 3: Distribution of the MV1c discriminant for the jet with the third highest MV1c weight in the lepton-plus-jets (left) and $ttb$$e \mu$ (right) channels. The $ttb$ signal distribution is compared to the distributions for backgrounds with an additional charm jet ($ttc$) and backgrounds with only additional light jets ($ttl$). The bin edges correspond to the $b$-tagging efficiency of the MV1c weight. The plots are normalised such that the sum over the bins is equal to unity. The statistical uncertainty of these distributions is negligible.
• Figure 4: Distributions of the third and fourth highest MV1c weight among jets for $ttbb$ signal, $ttbX$, $ttcX$ and $ttlX$ background. The bins are labelled with the upper edge of the efficiency point of the third highest and fourth highest MV1c scores in the event. The order of the bins does not affect the cross-section measurement, for this figure the bins have been ordered by decreasing MV1c efficiency point of the fourth and third MV1c score. The plots are normalised such that the sum over the bins is equal to unity. The statistical uncertainty of these distributions is negligible.
• Figure 5: The MV1c distribution of jets with the third highest MV1c weight in the lepton-plus-jets analysis (top) and $ttb$$e \mu$ analysis (bottom) for all signal and background components. The data are compared to the nominal predictions (Pre-fit) (left), and to the output of the fit (Post-fit) (right). The points include the statistical uncertainty on the data. The hashed area shows the uncertainty on the total prediction. The non-prompt and fake lepton backgrounds are referred to as 'NP & fakes'.