Olga Bessidskaia Bylund

Top quark pair production in association with a Z-boson or a photon at the LHC directly probes neutral top-quark couplings. We present predictions for these two processes in the Standard Model (SM) Effective Field Theory (EFT) at next-to-leading order (NLO) in QCD. We include the full set of CP-even dimension-six operators that enter the top-quark interactions with the SM gauge bosons. For comparison, we also present predictions in the SMEFT for top loop-induced HZ production at the LHC and for $$ t\overline{t} $$ production at the ILC at NLO in QCD. Results for total cross sections and differential distributions are obtained and uncertainties coming from missing higher orders in the strong coupling and in the EFT expansions are discussed. NLO results matched to the parton shower are available, allowing for event generation to be directly employed in an experimental analyses. Our framework provides a solid basis for the interpretation of current and future measurements in the SMEFT, with improved accuracy and precision.

The ATLAS Collaboration has measured the inclusive production of $Z$ bosons via their decays into electron and muon pairs in $p+$Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV at the Large Hadron Collider. The measurements are made using data corresponding to integrated luminosities of 29.4 nb$^{-1}$ and 28.1 nb$^{-1}$ for $Z \rightarrow ee$ and $Z \rightarrow \mu\mu$, respectively. The results from the two channels are consistent and combined to obtain a cross section times the $Z \rightarrow \ell\ell$ branching ratio, integrated over the rapidity region $|y^{*}_{Z}|<3.5$, of 139.8 $\pm$ 4.8 (stat.) $\pm$ 6.2 (syst.) $\pm$ 3.8 (lumi.) nb. Differential cross sections are presented as functions of the $Z$ boson rapidity and transverse momentum, and compared with models based on parton distributions both with and without nuclear corrections. The centrality dependence of $Z$ boson production in $p+$Pb collisions is measured and analyzed within the framework of a standard Glauber model and the model's extension for fluctuations of the underlying nucleon-nucleon scattering cross section.

This Technical Design Report describes the project to upgrade the ATLAS Tile Calorimeter for the operation at the High Luminosity LHC. The High Luminosity LHC is planned to begin operation in 2026 ...

Generalized parton distributions describe the correlations between the longitudinal momentum and the transverse position of quarks and gluons in a nucleon. They can be constrained by measuring photon leptoproduction observables, arising from the interference between Bethe-Heitler and deeply virtual Compton scattering (DVCS) processes. At leading-twist/leading-order, the amplitude of the latter is parametrized by complex integrals of the GPDs ${H,E,\stackrel{\texttildelow{}}{H},\stackrel{\texttildelow{}}{E}}$. As data collected on an unpolarized or longitudinally polarized target constrains $H$ and $\stackrel{\texttildelow{}}{H}$, $E$ is poorly known as it requires data collected with a transversely polarized target, which is very challenging to implement in fixed-target experiments. The only alternative considered so far has been DVCS on a neutron with a deuterium target, while assuming isospin symmetry and absence of final-state interactions. Today, we introduce the polarization of the recoil proton as a new DVCS observable, highly sensitive to $E$, which appears feasible for an experimental study at a high-luminosity facility such as Jefferson Lab.

Top quark pair production in association with a $Z$-boson or a photon at the LHC directly probes neutral top-quark couplings. We present predictions for these two processes in the Standard Model (SM) Effective Field Theory (EFT) at next-to-leading (NLO) order in QCD. We include the full set of CP-even dimension-six operators that enter the top-quark interactions with the SM gauge bosons. For comparison, we also present predictions in the SMEFT for top loop-induced $HZ$ production at the LHC and for $t\bar{t}$ production at the ILC at NLO in QCD. Results for total cross sections and differential distributions are obtained and uncertainties coming from missing higher orders in the strong coupling and in the EFT expansions are discussed. NLO results matched to the parton shower are available, allowing for event generation to be directly employed in an experimental analyses. Our framework provides a solid basis for the interpretation of current and future measurements in the SMEFT, with improved accuracy and precision.

The generation of single top production in the $Wt$ channel at NLO gives rise to doubly-resonant diagrams that overlap with $t \bar t$, and similarly for associated production with a $Z$ or Higgs boson. Several methods exist to remove this overlap, thus avoiding double counting and separating the two processes. In Diagram Removal 1, the amplitudes of doubly-resonant diagrams are set to zero, which also removes the interference of $Wt$ with $t \bar t$. If a measurement is performed in a region where this interference is not negligible, it would be of interest to take it into account. Another method, Diagram Subtraction, preserves the interference. However, it relies upon momentum reshuffling, which introduces uncertainties to the prediction. An alternative method, Diagram Removal 2, does not involve reshuffling and preserves the interference by subtracting the amplitude squared of the doubly-resonant diagrams from the total. The predictions of Diagram Removal 2 from MG5_aMC@NLO are compared with other predictions for $Wt$ and $tWZ$ production in the context of analyses at the ATLAS experiment. Implications of using Diagram Removal 2 are discussed, both for measurements of single top processes and when $Wt$ or $tWZ$ are treated as a background. Differential distributions are shown, investigating in what regions the predictions differ the most.

Measuring the total cross sections of top-quark pair production and single top processes at high precision tests the predictions of the Standard Model and can bring better understanding of properties such as the top-quark mass, electroweak couplings, lepton universality and proton parton distribution functions. Some of the recent measurements by the ATLAS and CMS experiments at the Large Hadron Collider are outlined in this document. Experimental methods are given particular attention, particularly the determination of lepton isolation efficiencies in the ATLAS dilepton measurement of top-quark pair production.

Measurement of the t tbar Z and t tbar W production cross sections in ATLAS at sqrt(s) = 8 TeV and The Electronic Noise in the Tile Calorimeter

On behalf of the ATLAS CollaborationThe large integrated luminosity provided by the LHC enables the production of significant numbers of top quark pairs in association with additional jets or additional gauge bosons.The production of top quark pairs in association with W and Z bosons is presented.The measurement uses events with multiple leptons and in particular probes the coupling between the top quark and the Z boson.A significance of five standard deviations is found for the production of a top quark pair with a W boson.The cross section measurement of photons produced in association with top quark pairs is presented.The measurement uses a data driven technique to extract the signal from the background and achieves a significance of greater than 5 standard deviations.In addition, the production cross section of top quark pairs in association with jets is presented.This process allows better understanding of QCD effects and is an important background to many searches for physics beyond the Standard Model.

The production of t tZ serves as an important probe of the coupling of top quarks to Z bosons.Modifications to the t tZ vertex by physics beyond the Standard Model would have an effect on the t tZ production rates.The first measurements of t tZ production were performed in 8 TeV data by the ATLAS and CMS experiments at the LHC.At 13 TeV collision energy, the production rate of t tZ increases by almost a factor of four and the effects of new physics, if present, should become more prominent.The first measurement by ATLAS of t tZ production at 13 TeV is presented here, with a focus placed on the selection region that requires three charged leptons.This signature constitutes a compromise between a higher branching ratio for hadronic top quark decay chains and the better separation from background of the leptonic top quark decays.Moreover, the t tW process is measured, using final states that contain either three charged leptons or two muons of the same charge.The first observation of t tW was performed by ATLAS using 8 TeV data.Measuring this process can probe several different models for physics beyond the Standard Model.

The Standard Model (SM) of particle physics describes the elementary particles that constitute matter and their interactions. The predictions of the SM have been confirmed by numerous experimental ...