Measurement of exclusive pion pair production in proton–proton collisions at √s=7 TeV with the ATLAS detector

The exclusive production of pion pairs in the process pp→ ppπ+π- has been measured at s=7TeV with the ATLAS detector at the LHC, using 80μb-1 of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forward ATLAS ALFA detector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. A cross-section measurement 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

Measurement of the total and differential Higgs boson production cross-sections at √s = 13 TeV with the ATLAS detector by combining the H → ZZ * → 4ℓ and H → γγ decay channels

The total and differential Higgs boson production cross-sections are measured through a combined statistical analysis of the H → ZZ * → 4ℓ and H → γγ decay channels. The results are based on a dataset of 139 fb −1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider. The measured total Higgs boson production cross-section is 55.5−3.8+4.0 pb, consistent with the Standard Model prediction of 55.6 ± 2.5 pb. All results from the two decay channels are compatible with each other, and their combination agrees with the Standard Model predictions. A combined statistical interpretation of the measured fiducial cross-sections as a function of the Higgs boson transverse momentum is performed in order to probe the Yukawa couplings to the bottom and charm quarks. A similar interpretation is performed by including also the constraints from the measurements of Higgs boson production in association with a W or Z boson in the H → bb¯ and cc¯ decay channels. [Figure not available: see fulltext.

Measurement of the Z boson invisible width at s=13 TeV with the ATLAS detector

A measurement of the invisible width of the Z boson using events with jets and missing transverse momentum is presented using 37 fb−1 of 13 TeV proton–proton data collected by the ATLAS detector in 2015 and 2016. The ratio of Z→inv to Z→ll events, where inv refers to non-detected particles and l is either an electron or a muon, is measured and corrected for detector effects. Events with at least one energetic central jet with pT≥110 GeV are selected for both the Z→inv and Z→ll final states to obtain a similar phase space in the ratio. The invisible width is measured to be 506±2(stat.)±12(syst.) MeV and is the single most precise recoil-based measurement. The result is in agreement with the most precise determination from LEP and the Standard Model prediction based on three neutrino generations

Measurement of the polarisation of W bosons produced in top-quark decays using dilepton events at root s=13 TeV with the ATLAS experiment

A measurement of the polarisation of $W$ bosons produced in top-quark decays is presented, using proton-proton collision data at a centre-of-mass energy of $\sqrt{s} = 13$ TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139 fb$^{-1}$. The measurement is performed selecting $t\bar{t}$ events decaying into final states with two charged leptons (electrons or muons) and at least two $b$-tagged jets. The polarisation is extracted from the differential cross-section distribution of the $\cos{\theta^{*}}$ variable, where $\theta^{*}$ is the angle between the momentum direction of the charged lepton from the $W$ boson decay and the reversed momentum direction of the $b$-quark from the top-quark decay, both calculated in the $W$ boson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the $\cos{\theta^{*}}$ angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be $f_{0} = 0.684 \pm 0.005\,\mathrm{(stat.)} \pm 0.014\,\mathrm{(syst.)}$, $f_{\mathrm{L}} = 0.318 \pm 0.003\,\mathrm{(stat.)} \pm 0.008\,\mathrm{(syst.)}$ and $f_{\mathrm{R}} = -0.002 \pm 0.002\,\mathrm{(stat.)} \pm 0.014\,\mathrm{(syst.)}$, in agreement with the Standard Model prediction

Search for pair production of squarks or gluinos decaying via sleptons or weak bosons in final states with two same-sign or three leptons with the ATLAS detector

A search for pair production of squarks or gluinos decaying via sleptons or weak bosons is reported. The search targets a final state with exactly two leptons with same-sign electric charge or at least three leptons without any charge requirement. The analysed data set corresponds to an integrated luminosity of 139fb−1 of proton-proton collisions collected at a centre-of-mass energy of 13TeV with the ATLAS detector at the LHC. Multiple signal regions are defined, targeting several SUSY simplified models yielding the desired f inal states. A single control region is used to constrain the normalisation of the WZ +jets background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of several supersymmetric models featuring R-parity conservation or R-parity violation, yielding exclusion limits surpassing those from previous searches. In models considering gluino (squark) pair production, gluino (squark) masses up to 2.2 (1.7) TeV are excluded at 95% confidence leve

Determination of the Relative Sign of the Higgs Boson Couplings to W and Z Bosons Using WH Production via Vector-Boson Fusion with the ATLAS Detector

The associated production of Higgs and W bosons via vector-boson fusion is highly sensitive to the relative sign of the Higgs boson couplings to W and Z bosons. In this Letter, two searches for this process are presented, using 140 fb^{-1} of proton-proton collision data at sqrt[s]=13 TeV recorded by the ATLAS detector at the LHC. The first search targets scenarios with opposite-sign couplings of the W and Z bosons to the Higgs boson, while the second targets standard model-like scenarios with same-sign couplings. Both analyses consider Higgs boson decays into a pair of b quarks and W boson decays with an electron or muon. The data exclude the opposite-sign coupling hypothesis with a significance beyond 5σ, and the observed (expected) upper limit set on the cross section for vector-boson fusion WH production is 9.0 (8.7) times the standard model value at 95% confidence level

Accuracy versus precision in boosted top tagging with the ATLAS detector

The identification of top quark decays where the top quark has a large momentum transverse to the beam axis, known as top tagging, is a crucial component in many measurements of Standard Model processes and searches for beyond the Standard Model physics at the Large Hadron Collider. Machine learning techniques have improved the performance of top tagging algorithms, but the size of the systematic uncertainties for all proposed algorithms has not been systematically studied. This paper presents the performance of several machine learning based top tagging algorithms on a dataset constructed from simulated proton-proton collision events measured with the ATLAS detector at √s = 13 TeV. The systematic uncertainties associated with these algorithms are estimated through an approximate procedure that is not meant to be used in a physics analysis, but is appropriate for the level of precision required for this study. The most performant algorithms are found to have the largest uncertainties, motivating the development of methods to reduce these uncertainties without compromising performance. To enable such efforts in the wider scientific community, the datasets used in this paper are made publicly available