A measurement of the four-lepton invariant mass spectrum is made with the ATLAS detector, using an integrated luminosity of 36.1 fb −1 of proton-proton collisions at √ s = 13 TeV delivered by the Large Hadron Collider. The differential cross-section is measured for events containing two same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with different mass regions dominated in the Standard Model by single Z boson production, Higgs boson production, and Z boson pair production, and non-negligible interference effects at high invariant masses. The measurement is compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. These calculations are used to interpret the data in terms of gg → Z Z → 4 and Z → 4 subprocesses, and to place constraints on a possible contribution from physics beyond the Standard Model.

Search for flavour-changing neutral current top-quark decays t → qZ in proton-proton collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of √ s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb −1 , are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t → qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t → bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 × 10 −4 (2.4 × 10 −4 ) on the t → uZ branching ratio and 2.4 × 10 −4 (3.2 × 10 −4 ) on the t → cZ branching ratio, constituting the most stringent limits to date.

The results of a search for new heavy bosons decaying to an electron or muon and a neutrino using proton–proton collision data at a centre-of-mass energy of  TeV are presented. The dataset was collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 . As no excess of events above the Standard Model prediction is observed, the results are used to set upper limits on the boson cross-section times branching ratio to an electron or muon and a neutrino as a function of the mass. Assuming a boson with the same couplings as the Standard Model W boson, masses below 5.1 TeV are excluded at the 95% confidence level.

No abstract

Observation of the isospin-violating decay J/ψ→ϕπ^{0}f_{0}(980) Observation of the isospin-violating decay J/ψ → φπ 0 f 0 (980) Using a sample of 1.31 billion J/ψ events collected with the BESIII detector at the BEPCII collider, the decays J/ψ → φπ + π − π 0 and J/ψ → φπ 0 π 0 π 0 are investigated. The isospin violating decay J/ψ → φπ 0 f0(980) with f0(980) → ππ, is observed for the first time. The width of the f0(980) obtained from the dipion mass spectrum is found to be much smaller than the world average value. In the π 0 f0(980) mass spectrum, there is evidence of f1(1285) production. By studying the decay J/ψ → φη ′ , the branching fractions of η ′ → π + π − π 0 and η ′ → π 0 π 0 π 0 , as well as their ratio, are 3 also measured.M