Ratios of top-quark pair to $Z$-boson cross sections measured from proton--proton collisions at the LHC centre-of-mass energies of $\sqrt s=13$TeV, 8TeV, and 7TeV are presented by the ATLAS Collaboration. Single ratios, at a given $\sqrt s$ for the two processes and at different $\sqrt s$ for each process, as well as double ratios of the two processes at different $\sqrt s$, are evaluated. The ratios are constructed using previously published ATLAS measurements of the $t\overline{t}$ and $Z$-boson production cross sections, corrected to a common phase space where required, and a new analysis of $Z \rightarrow \ell^+ \ell^-$ where $\ell=e,\mu$ at $\sqrt s=13$TeV performed with data collected in 2015 with an integrated luminosity of $3.2$fb$^{-1}$. Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the $Z\rightarrow e^+e^-$ and the $Z\rightarrow \mu^+ \mu^-$ channels for each $\sqrt s$ value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-$x$ values near 0.1 and the light-quark sea for $x<0.02$.
Measured fiducial cross section times leptonic branching ratio for Z/gamma* production in the Z/gamma* -> e+e- final state at 13TeV.
Measured fiducial cross section times leptonic branching ratio for Z/gamma* production in the Z/gamma* -> mu+ mu- final state at 13TeV.
Breakdown of systematic uncertainties in percent for the measured fiducial cross section times leptonic branching ratio for Z/gamma* production in the Z/gamma* -> e+e- final state at 13TeV.
Measurements of $K_S^0$ and $\Lambda^0$ production in $t\bar{t}$ final states have been performed. They are based on a data sample with integrated luminosity of 4.6 $\mathrm{fb}^{-1}$ from proton-proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the ATLAS detector at the Large Hadron Collider. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a $b$-tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several Monte Carlo simulations using different hadronisation and fragmentation schemes, colour reconnection models and different tunes for the underlying event has been made. The production of neutral strange particles in $t\bar{t}$ dileptonic events is found to be well described by current Monte Carlo models for $K_S^0$ and $\Lambda^0$ production within jets, but not for those produced outside jets.
The transverse momentum ($p_{T}$) distribution for $K^{0}_{S}$ production inside $b$-jets for unfolded data to particle level, normalised to the total number of top pair dileptonic events and scaled to the bin width. The systematic uncertainties are, in order, due to; the MC modelling, the tracking inefficiencies, the jet energy scale (JES), the jet energy resolution (JER), out-of-fiducial events and the unfolding non-closure.
The energy fraction ($x_{K}$) distribution for $K^{0}_{S}$ production inside $b$-jets for unfolded data to particle level, normalised to the total number of top pair dileptonic events and scaled to the bin width. The systematic uncertainties are, in order, due to; the MC modelling, the tracking ineficiencies, the jet energy scale (JES), the jet energy resolution (JER), out-of-fiducial events and the unfolding non-closure.
The energy distribution for $K^{0}_{S}$ production inside $b$-jets for unfolded data to particle level, normalised to the total number of top pair dileptonic events and scaled to the bin width. The systematic uncertainties are, in order, due to; the MC modelling, the tracking ineficiencies, the jet energy scale (JES), the jet energy resolution (JER), out-of-fiducial events and the unfolding non-closure.
The Standard Model of particle physics encapsulates our current best understanding of physics at the smallest scales. A fundamental axiom of this theory is the universality of the couplings of the different generations of leptons to the electroweak gauge bosons. The measurement of the ratio of the rate of decay of $W$ bosons to $\tau$-leptons and muons, $R(\tau/\mu) = B(W \to \tau \nu_\tau)/B(W \to \mu \nu_\mu)$, constitutes an important test of this axiom. A measurement of this quantity with a novel technique using di-leptonic $t\bar{t}$ events is presented based on 139 fb${}^{-1}$ of data recorded with the ATLAS detector in proton--proton collisions at $\sqrt{s}=13$ TeV. Muons originating from $W$ bosons and those originating from an intermediate $\tau$-lepton are distinguished using the lifetime of the $\tau$-lepton, through the muon transverse impact parameter, and differences in the muon transverse momentum spectra. The value of $R(\tau/\mu)$ is found to be $0.992 \pm 0.013 [\pm 0.007 (stat) \pm 0.011 (syst)]$ and is in agreement with the hypothesis of universal lepton couplings as postulated in the Standard Model. This is the most precise measurement of this ratio, and the only such measurement from the Large Hadron Collider, to date.
The number of data and fitted simulated events in each bin of the $|d_{0}^{\mu}|$ distribution in the $5<p_{\textrm{T}}^{\mu}<10$ GeV selection in the $e-\mu$ channel.
The number of data and fitted simulated events in each bin of the $|d_{0}^{\mu}|$ distribution in the $5<p_{\textrm{T}}^{\mu}<10$ GeV selection in the $\mu-\mu$ channel.
The number of data and fitted simulated events in each bin of the $|d_{0}^{\mu}|$ distribution in the $10<p_{\textrm{T}}^{\mu}<20$ GeV selection in the $e-\mu$ channel.
The ratio of branching ratios of the $W$ boson to muons and electrons, $R^{\mu/e}_W=B(W\rightarrow \mu\nu)/B(W\rightarrow e\nu)$, has been measured using 140 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the $t\bar{t}$ production cross-section in the $ee$, $e\mu$ and $\mu\mu$ dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio $R^{\mu\mu/ee}_Z$ for the $Z$ boson measured in inclusive $Z\rightarrow ee$ and $Z\rightarrow\mu\mu$ events. By using the precise value of $R^{\mu\mu/ee}_Z$ determined from $e^+e^-$ colliders, the ratio $R^{\mu/e}_W$ is determined to be $R^{\mu/e}_W = 0.9995 \pm 0.0022 \pm 0.0036 \pm 0.0014$. The three uncertainties correspond to data statistics, experimental systematics and the external measurement of $R^{\mu\mu/ee}_Z$, giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in $W$-boson decays at the 0.5% level.
The measurement of the ratio of the rate of decay of W bosons to muons and electrons, $R(\mu/e)=B(W\rightarrow\mu\nu)/B(W\rightarrow e\nu)$.
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