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We investigate the possibility to constrain the intrinsic charm probability \\[w_{\\mathrm{c} {\\bar{{\\mathrm{c}}}= P_{{\\mathrm{c}{\\bar{\\mathrm{c}}} /\\mathrm{p}}\\] using first ATLAS data on the associated production of prompt photons and charm-quark jets in pp collisions at \\[\\sqrt{s} = 8\\] TeV. The upper limit \\[w_{\\mathrm{c} {\\bar{{\\mathrm{c}}}< 1.93\\] % is obtained at the 68 % confidence level. This constraint is primarily determined from the theoretical scale and systematical experimental uncertainties. Suggestions for reducing these uncertainties are discussed. The implications of intrinsic heavy quarks in the proton for future studies at the LHC are also discussed.

A bstract The hard-scatter processes in hadronic collisions are often largely contaminated with soft background coming from pileup in proton-proton collisions, or underlying event in heavy-ion collisions. There are multiple methods to remove the effect of pileup for jets. Two such methods, Area Subtraction and Constituent Subtraction, use the pileup density as the main ingredient to estimate the magnitude of pileup contribution on an event-by-event basis. The state-of-the-art approaches to estimating pileup density are sensitive to the number of hard-scatter jets in the event. This paper presents a new pileup-density estimation method that minimizes the sensitivity on the presence of hard-scatter jets in the event. Using a detector-level simulation, we provide a comparison of the new method with the state-of-the-art estimation methods. We observe a significantly lower bias for the estimated pileup density when using the new method. We conclude that the new method has the potential to significantly improve pileup mitigation in proton-proton collisions or the underlying event subtraction in heavy-ion collisions.

The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with pp collisions at a centre-of-mass energy s = 13  TeV. The absolute luminosity scale is determined using van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosity for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2 pp data sample corresponds to an integrated luminosity of 140.1 ± 1.2   fb - 1 , i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 2017–2018 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of 338.1 ± 3.1   pb - 1 .

A technique is presented to measure the efficiency with which c-jets are mistagged as b-jets (mistagging efficiency) using tt¯ events, where one of the W bosons decays into an electron or muon and a neutrino and the other decays into a quark–antiquark pair. The measurement utilises the relatively large and known W→cs branching ratio, which allows a measurement to be made in an inclusive c-jet sample. The data sample used was collected by the ATLAS detector at s=13 TeV and corresponds to an integrated luminosity of 139 fb-1. Events are reconstructed using a kinematic likelihood technique which selects the mapping between jets and tt¯ decay products that yields the highest likelihood value. The distribution of the b-tagging discriminant for jets from the hadronic W decays in data is compared with that in simulation to extract the mistagging efficiency as a function of jet transverse momentum. The total uncertainties are in the range 3–17%. The measurements generally agree with those in simulation but there are some differences in the region corresponding to the most stringent b-jet tagging requirement.

Measurements of the Standard Model Higgs boson decaying into a bb¯ pair and produced in association with a W or Z boson decaying into leptons, using proton–proton collision data collected between 2015 and 2018 by the ATLAS detector, are presented. The measurements use collisions produced by the Large Hadron Collider at a centre-of-mass energy of s=13Te, corresponding to an integrated luminosity of 139fb-1. The production of a Higgs boson in association with a W or Z boson is established with observed (expected) significances of 4.0 (4.1) and 5.3 (5.1) standard deviations, respectively. Cross-sections of associated production of a Higgs boson decaying into bottom quark pairs with an electroweak gauge boson, W or Z, decaying into leptons are measured as a function of the gauge boson transverse momentum in kinematic fiducial volumes. The cross-section measurements are all consistent with the Standard Model expectations, and the total uncertainties vary from 30% in the high gauge boson transverse momentum regions to 85% in the low regions. Limits are subsequently set on the parameters of an effective Lagrangian sensitive to modifications of the WH and ZH processes as well as the Higgs boson decay into bb¯.

A search is presented for four-top-quark production using an integrated luminosity of 139 fb - 1 of proton–proton collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be 24 - 6 + 7  fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.

A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic t t ¯ plus missing transverse momentum final state is presented. The analysis of 139  fb - 1 of s = 13  TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via t ~ → t ( ∗ ) χ ~ 1 0 , with t ( ∗ ) denoting an on-shell (off-shell) top quark and χ ~ 1 0 the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where m t ~ > m t + m χ ~ 1 0 , top squark masses are excluded in the range 400–1250 GeV for χ ~ 1 0 masses below 200 GeV at 95% confidence level. In the situation where m t ~ ∼ m t + m χ ~ 1 0 , top squark masses in the range 300–630 GeV are excluded, while in the case where m t ~ < m W + m b + m χ ~ 1 0 (with m t ~ - m χ ~ 1 0 ≥ 5  GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.

A bstract Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs ( t t ¯ ) are measured using the full LHC Run 2 data sample collected with the ATLAS detector. The data were collected at a pp collision energy of s = 13 TeV and correspond to an integrated luminosity of 140 fb − 1 . The measurements use events containing an oppositely charged eμ pair and b -tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton p T distributions is obtained by reweighting the t t ¯ sample so as to reproduce the top-quark p T distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to be σ t t ¯ = 829 ± 1 stat ± 13 syst ± 8 lumi ± 2 beam pb , where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.

A bstract A search for the electroweak production of pairs of charged sleptons or charginos decaying into two-lepton final states with missing transverse momentum is presented. Two simplified models of R -parity-conserving supersymmetry are considered: direct pair-production of sleptons ℓ ~ ℓ ~ , with each decaying into a charged lepton and a χ ~ 1 0 neutralino, and direct pair-production of the lightest charginos χ ~ 1 ± χ ~ 1 ∓ , with each decaying into a W -boson and a χ ~ 1 0 . The lightest neutralino χ ~ 1 0 is assumed to be the lightest supersymmetric particle (LSP). The analyses target the experimentally challenging mass regions where m ℓ ~ − m χ ~ 1 0 and m χ ~ 1 ± − m χ ~ 1 0 are close to the W -boson mass (‘moderately compressed’ regions). The search uses 139 fb − 1 of s = 13 TeV proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider. No significant excesses over the expected background are observed. Exclusion limits on the simplified models under study are reported in the ℓ ~ χ ~ 1 0 and χ ~ 1 ± χ ~ 1 0 mass planes at 95% confidence level (CL). Sleptons with masses up to 150 GeV are excluded at 95% CL for the case of a mass-splitting between sleptons and the LSP of 50 GeV. Chargino masses up to 140 GeV are excluded at 95% CL for the case of a mass-splitting between the chargino and the LSP down to about 100 GeV.

Single- and double-differential cross-section measurements are presented for the production of top-quark pairs, in the lepton + jets channel at particle and parton level. Two topologies, resolved and boosted, are considered and the results are presented as a function of several kinematic variables characterising the top and tt¯ system and jet multiplicities. The study was performed using data from pp collisions at centre-of-mass energy of 13 TeV collected in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 36fb-1. Due to the large tt¯ cross-section at the LHC, such measurements allow a detailed study of the properties of top-quark production and decay, enabling precision tests of several Monte Carlo generators and fixed-order Standard Model predictions. Overall, there is good agreement between the theoretical predictions and the data.