Abstract: A determination of the jet energy scale is presented using proton-proton collision data with a centre-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 140 fb(-1) collected using the ATLAS detector at the LHC. Jets are reconstructed using the ATLAS particle-flow method that combines charged-particle tracks and topo-clusters formed from energy deposits in the calorimeter cells. The anti-kt jet algorithm with radius parameter R = 0.4 is used to define the jet. Novel jet energy scale calibration strategies developed for the LHC Run 2 are reported that lay the foundation for the jet calibration in Run 3. Jets are calibrated with a series of simulation-based corrections, including state-of-the-art techniques in jet calibration such as machine learning methods and novel in situ calibrations to achieve better performance than the baseline calibration derived using up to 81 fb(-1) of Run 2 data. The performance of these new techniques is then examined in the in situ measurements by exploiting the transverse momentum balance between a jet and a reference object. The b-quark jet energy scale using particle flow jets is measured for the first time with around 1% precision using gamma+jet events.

Abstract: A search for pair-produced vector-like quarks using events with exactly one lepton (e or mu), at least four jets including at least one b-tagged jet, and large missing transverse momentum is presented. Data from proton-proton collisions at a centre-of-mass energy of root s =13 TeV, recorded by the ATLAS detector at the LHC from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb(-1), are analysed. Vector-like partners T and B of the top and bottom quarks are considered, as is a vector-like X with charge +5/3, assuming their decay into a W, Z, or Higgs boson and a third-generation quark. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section of T and B quark pairs as a function of their mass are derived for various decay branching ratio scenarios. The strongest lower limits on the masses are 1.59 TeV assuming mass-degenerate vector-like quarks and branching ratios corresponding to the weak-isospin doublet model, and 1.47 TeV (1.46 TeV) for exclusive T -> Zt (B/X -> Wt) decays. In addition, lower limits on the T and B quark masses are derived for all possible branching ratios.

Abstract: A search for the Higgs boson decaying into a pair of charm quarks is presented. The analysis uses proton- proton collisions to target the production of a Higgs boson in association with a leptonically decaying W or Z boson. The dataset delivered by the LHC at a centre-of-mass energy of root s = 13 TeV and recorded by the ATLAS detector corresponds to an integrated luminosity of 139 fb(-1). Flavour-tagging algorithms are used to identify jets originating from the hadronisation of charm quarks. The analysis method is validated with the simultaneous measurement of WW, WZ and ZZ production, with observed (expected) significances of 2.6 (2.2) standard deviations above the background-only prediction for the (W/Z)Z(-> c (c) over bar) process and 3.8 (4.6) standard deviations for the (W/Z)W(-> cq) process. The (WIZ)H(-> c (c) over bar) search yields an observed (expected) upper limit of 26 (31) times the predicted Standard Model crosssection times branching fraction for a Higgs boson with a mass of 125 GeV, corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier vertical bar k(c)vertical bar < 8.5 (12.4), at the 95% confidence level. A combination with the ATLAS (W/Z)H, H -> b<(b)over bar> analysis is performed, allowing the ratio k(c)/k(b) to be constrained to less than 4.5 at the 95% confidence level, smaller than the ratio of the b- and c-quark masses, and therefore determines the Higgs-charm coupling to be weaker than the Higgs-bottom coupling at the 95% confidence level.

Abstract: The flavour-tagging algorithms developed by the AvTLAS Collaboration and used to analyse its dataset of root s = 13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model t (t) over bar events; similarly, at a c-jet identification efficiency of 30%, a light-jet (b-jet) rejection factor of 70 (9) is obtained.

Abstract: The exclusive production of pion pairs in the process pp -> pp pi(+)pi(-) has been measured at root s = 7 TeV with the ATLAS detector at the LHC, using 80 μb(-1) of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forwardATLASALFAdetector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. Across-sectionmeasurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion-pion invariant mass. Cross-section values of 4.8 +/- 1.0 (stat)(-0.2) (+0.3)(syst) μb and 9 +/- 6 (stat)(-2)(+2) (syst) μb are obtained in the two regions; they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type.