Measurements of fiducial cross-sections for [Formula: see text] production with one or two additional b-jets in pp collisions at [Formula: see text]=8 TeV using the ATLAS detector

Abstract

Fiducial cross-sections for [Formula: see text] production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb[Formula: see text] 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 [Formula: see text] events with at least one additional b-jet is measured to be 950 [Formula: see text] 70 (stat.) [Formula: see text] (syst.) fb in the lepton-plus-jets channel and 50 [Formula: see text] 10 (stat.) [Formula: see text] (syst.) fb in the [Formula: see text] channel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 [Formula: see text] 3.5 (stat.) [Formula: see text] 5.7 (syst.) fb in the dilepton channel ([Formula: see text], [Formula: see text], and ee) using a method based on tight selection criteria, and 13.5 [Formula: see text] 3.3 (stat.) [Formula: see text] 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 [Formula: see text] 0.33 (stat.) [Formula: see text] 0.28 (syst.)% for the ratio of [Formula: see text] production with two additional b-jets to [Formula: see text] production with any two additional jets. All measurements are in good agreement with recent theory predictions.

Fig. 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

Fig. 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, $e\mathit{\mu }$ or $\mathit{\mu }\mathit{\mu }$). 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

Fig. 3

Distribution of the MV1c discriminant for the jet with the third highest MV1c weight in the lepton-plus-jets (left) and ttb $e\mathit{\mu }$ (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

Fig. 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

Fig. 5

The MV1c distribution of jets with the third highest MV1c weight in the lepton-plus-jets analysis (top) and ttb $e\mathit{\mu }$ 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’

Fig. 6

Effect of the uncertainty on the fitted value of ${\mathit{\mu }}_{ttb}$ ($\mathrm{\Delta }{\widehat{\mathit{\mu }}}_{ttb}$) and pull of the dominant nuisance parameters in the lepton-plus-jets (left) and ttb $e\mathit{\mu }$ analyses (right). The shaded and hashed areas refer to the top axis: the shaded bands show the initial impact of that source of uncertainty on the precision of ${\mathit{\mu }}_{ttb}$; the hatched areas show the impact on the measurement of that source of uncertainty, after the profile likelihood fit at the $\pm 1\mathit{\sigma }$ level. The points and associated error bars show the fitted value of the nuisance parameters and their errors and refer to the bottom axis; a mean of zero and a width of 1 would imply no constraint due to the profile likelihood fit. Dashed lines are shown at 0 and $\pm$1 for reference. Only the ten highest ranked uncertainties on ${\mathit{\mu }}_{ttb}$ are shown. The index on the b-tagging uncertainties refers to the fixed, but arbitrary position in the list of eigenvectors associated with each jet flavour

Fig. 7

The MV1c distribution of jets with the third and fourth highest MV1c weight in the dilepton channel for all signal and background components. 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 data (left) are compared to the nominal predictions (Pre-fit), and (right) to the output of the fit (Post-fit). The points include the statistical uncertainty on the data. The hashed area shows the total uncertainties. 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’

Fig. 8

Comparison of the measured cross-sections in the three fiducial phase-space regions with theoretical predictions obtained from a variety of different generators. The measurements are shown with the contributions from $t\overline{t}V$ and $t\overline{t}H$ removed to allow direct comparison to the predictions containing only the pure QCD matrix elements. The coloured bands indicate the statistical and total uncertainties of the measurements. The errors on the theoretical prediction are obtained by simultaneously varying the renormalisation and factorisation scales by a factor of two. These variations have not been calculated for the LO Pythia 8 samples or for the Powheg+Pythia 6 sample

Fig. 9

Comparison of the measured ratio of the ttbb and ttjj cross-sections in the fiducial phase-space region of the ttbb fit-based analysis with theoretical predictions obtained from a variety of different generators. The measurements are shown with the contributions from $t\overline{t}V$ and $t\overline{t}H$ removed to allow direct comparison to the pure QCD generators. The coloured bands indicate the statistical and total uncertainties of the measurement. The error on the MadGraph+Pythia prediction is obtained by simultaneously varying the renormalisation and factorisation scales by a factor of two. These variations have not been calculated for the LO Pythia 8 samples or for the Powheg+Pythia 6 sample