The ITk detector, the new ATLAS silicon tracking system for the High Luminosity LHC (HL-LHC), will be equipped with 3D pixel sensor modules in the innermost layer (L0). The pixel cell dimensions will be 25 × 100 µ m 2 in the barrel and 50 × 50 µ m 2 in the end-caps, with one readout electrode at the centre of each pixel and four bias electrodes at the corners. Sensors from pre-production wafers (50 × 50 µ m 2 ) produced by FBK have been bump-bonded to ITkPixV1.1 chips at IZM. Bare modules have been assembled in Genoa on Single Chip Cards (SCCs) and characterized in laboratory measurements and in test beam campaigns. Some of these modules have been irradiated in Bonn and at the CERN IRRAD facility. Preliminary results of their characterization after irradiation

Yusong Tian,a,∗ Giovanni Calderini, Imogen Camp, Thibaud Idriss Carcone, Paul Mickael Chabrillat, Artur Cordeiro Oudot Choi, Francesco Crescioli, Jörn Große-Knetter, Šejla Hadžić, Shunsuke Iizaka, Christopher Krause, Lingxin Meng, f Koji Nakamura, Arnulf Quadt, Stefano Terzo, Ana Sofia Torrento Coello and Hua Ye II. Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, DE 37077 Göttingen, Germany LPNHE, Sorbonne Université, Université Paris Cité, CNRS/IN2P3, 4 place Jussieu, FR 75005 Paris, France Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Föhringer Ring 6, DE 80805 München, Germany Division of Physics and Tomonaga Center for the History of the Universe, Faculty of Pure and Applied Sciences, University of Tsukuba, 1 Chome-1-1 Tennodai, Tsukuba, Japan Fakultät Physik, Technische Universität Dortmund, Otto-Hahn-Straße 4, DE 44227 Dortmund, Germany f Physics Department, Lancaster University, Bailrigg, Lancaster LA1 4YW, United Kingdom KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Japan Institut de Física d’Altes Energies (IFAE), Barcelona Institute of Science and Technology, UAB Campus, Edifici CN, ES 08193 Barcelona, Spain Detectors and Instrumentation Department, IJCLab – Laboratoire de Physique des 2 Infinis Irène Joliot-Curie, UMR 9012 – CNRS / Université Paris-Saclay / Université Paris Cité, 15 rue Georges Clémenceau, FR 91405 Orsay, France

Measurements of the suppression and correlations of dijets is performed using 3 $\mu$b$^{-1}$ of Xe+Xe data at $\sqrt{s_{\mathrm{NN}}} = 5.44$ TeV collected with the ATLAS detector at the LHC. Dijets with jets reconstructed using the $R=0.4$ anti-$k_t$ algorithm are measured differentially in jet $p_{\text{T}}$ over the range of 32 GeV to 398 GeV and the centrality of the collisions. Significant dijet momentum imbalance is found in the most central Xe+Xe collisions, which decreases in more peripheral collisions. Results from the measurement of per-pair normalized and absolutely normalized dijet $p_{\text{T}}$ balance are compared with previous Pb+Pb measurements at $\sqrt{s_{\mathrm{NN}}} =5.02$ TeV. The differences between the dijet suppression in Xe+Xe and Pb+Pb are further quantified by the ratio of pair nuclear-modification factors. The results are found to be consistent with those measured in Pb+Pb data when compared in classes of the same event activity and when taking into account the difference between the center-of-mass energies of the initial parton scattering process in Xe+Xe and Pb+Pb collisions. These results should provide input for a better understanding of the role of energy density, system size, path length, and fluctuations in the parton energy loss.

This Letter reports the observation of single top quarks produced together with a photon, which directly probes the electroweak coupling of the top quark. The analysis uses 139  fb^{-1} of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be 688±23(stat) _{-71}^{+75}(syst)  fb, to be compared with the standard model prediction of 515_{-42}^{+36}  fb at next-to-leading order in QCD.

The production of a $W$ boson in association with a single charm quark is studied using 140 $\mathrm{fb}^{-1}$ of $\sqrt{s} = 13\,\mathrm{TeV}$ proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The charm quark is tagged by a charmed hadron, reconstructed with a secondary-vertex fit. The $W$ boson is reconstructed from an electron/muon decay and the missing transverse momentum. The mesons reconstructed are $D^{\pm} \to K^\mp \pi^\pm \pi^\pm$ and $D^{*\pm} \to D^{0} \pi^\pm \to (K^\mp \pi^\pm) \pi^\pm$, where $p_{\text{T}}(e, \mu)>30\,\mathrm{GeV}$, $|\eta(e, \mu)|<2.5$, $p_{\text{T}}(D)>8\,\mathrm{GeV}$, and $|\eta(D)|<2.2$. The integrated and normalized differential cross-sections as a function of the pseudorapidity of the lepton from the $W$ boson decay, and of the transverse momentum of the meson, are extracted from the data using a profile likelihood fit. The measured fiducial cross-sections are $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{-}{+}D^{+}) = 50.2\pm0.2\,\mathrm{(stat.)}\,^{+2.4}_{-2.3}\,\mathrm{(syst.)}\,\mathrm{pb}$, $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{+}{+}D^{-}) = 48.5\pm0.2\,\mathrm{(stat.)}\,^{+2.3}_{-2.2}\,\mathrm{(syst.)}\,\mathrm{pb}$, $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{-}{+}D^{*+}) = 51.1\pm0.4\,\mathrm{(stat.)}\,^{+1.9}_{-1.8}\,\mathrm{(syst.)}\,\mathrm{pb}$, and $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{+}{+}D^{*-}) = 50.0\pm0.4\,\mathrm{(stat.)}\,^{+1.9}_{-1.8}\,\mathrm{(syst.)}\,\mathrm{pb}$. Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. The ratio of charm to anti-charm production cross-sections is studied to probe the $s$-$\bar{s}$ quark asymmetry and is found to be $R_c^\pm = 0.971\pm0.006\,\mathrm{(stat.)}\pm0.011\,\mathrm{(syst.)}$.

A search for flavor-changing neutral-current couplings between a top quark, an up or charm quark and a $Z$ boson is presented, using proton-proton collision data at $\sqrt{s} = 13$ TeV collected by the ATLAS detector at the Large Hadron Collider. The analyzed dataset corresponds to an integrated luminosity of 139 fb$^{-1}$. The search targets both single-top-quark events produced as $gq\rightarrow tZ$ (with $q = u, c$) and top-quark-pair events, with one top quark decaying through the $t \rightarrow Zq$ channel. The analysis considers events with three leptons (electrons or muons), a $b$-tagged jet, possible additional jets, and missing transverse momentum. The data are found to be consistent with the background-only hypothesis and 95% confidence-level limits on the $t \rightarrow Zq$ branching ratios are set, assuming only tensor operators of the Standard Model effective field theory framework contribute to the $tZq$ vertices. These are $6.2 \times 10^{-5}$ ($13\times 10^{-5}$) for $t\rightarrow Zu$ ($t\rightarrow Zc$) for a left-handed $tZq$ coupling, and $6.6 \times 10^{-5}$ ($12\times 10^{-5}$) in the case of a right-handed coupling. These results are interpreted as 95% CL upper limits on the strength of corresponding couplings, yielding limits for $|C_{uW}^{(13)*}|$ and $|C_{uB}^{(13)*}|$ ($|C_{uW}^{(31)}|$ and $|C_{uB}^{(31)}|$) of 0.15 (0.16), and limits for $|C_{uW}^{(23)*}|$ and $|C_{uB}^{(23)*}|$ ($|C_{uW}^{(32)}|$ and $|C_{uB}^{(32)}|$) of 0.22 (0.21), assuming a new-physics energy scale $\Lambda_\text{NP}$ of 1 TeV.

This letter presents a measurement of the nuclear modification factor of large-radius jets in sqrt[s_{NN}]=5.02  TeV Pb+Pb collisions by the ATLAS experiment. The measurement is performed using 1.72  nb^{-1} and 257  pb^{-1} of Pb+Pb and pp data, respectively. The large-radius jets are reconstructed with the anti-k_{t} algorithm using a radius parameter of R=1.0, by reclustering anti-k_{t} R=0.2 jets, and are measured over the transverse momentum (p_{T}) kinematic range of 158<p_{T}<1000  GeV and absolute pseudorapidity |y|<2.0. The large-radius jet constituents are further reclustered using the k_{t} algorithm in order to obtain the splitting parameters, sqrt[d_{12}] and ΔR_{12}, which characterize the transverse momentum scale and angular separation for the hardest splitting in the jet, respectively. The nuclear modification factor, R_{AA}, obtained by comparing the Pb+Pb jet yields to those in pp collisions, is measured as a function of jet transverse momentum (p_{T}) and sqrt[d_{12}] or ΔR_{12}. A significant difference in the quenching of large-radius jets having single subjet and those with more complex substructure is observed. Systematic comparison of jet suppression in terms of R_{AA} for different jet definitions is also provided. Presented results support the hypothesis that jets with hard internal splittings lose more energy through quenching and provide a new perspective for understanding the role of jet structure in jet suppression.