Measurement of the top-quark mass using a leptonic invariant mass in pp collisions at s√ = 13 TeV with the ATLAS detector

A measurement of the top-quark mass (mt) in the tt¯ → lepton + jets channel is presented, with an experimental technique which exploits semileptonic decays of b-hadrons produced in the top-quark decay chain. The distribution of the invariant mass mℓμ of the lepton, ℓ (with ℓ = e, μ), from the W-boson decay and the muon, μ, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract mt. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb−1 of s√ = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. The measured value of the top-quark mass is mt = 174.41 ± 0.39 (stat.) ± 0.66 (syst.) ± 0.25 (recoil) GeV, where the third uncertainty arises from changing the PYTHIA8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup

Evidence for the direct two-photon transition from ψ(3686)\psi(3686) to J/ψJ/\psi

The two-photon transition $\psi(3686)→\gamma\gamma J/\psi$ is studied in a sample of $1.06×10^8$ $\psi(3686)$ decays collected by the BESIII detector. The branching fraction is measured to be $(3.1±0.6(\text{stat})_{−1.0}^{+0.8}(\text{syst}))×10^{−4}$ using $J/\psi→e^+e^−$ and $J/\psi→μ^+μ^−$ decays, and its upper limit is estimated to be $4.5×10^-4$ at the 90% confidence level. This work represents the first measurement of a two-photon transition among charmonium states. The orientation of the ψ(3686) decay plane and the J/ψ polarization in this decay are also studied. In addition, the product branching fractions of sequential $E$1 transitions $\psi(3686)$→$\gamma\chi_{cJ}$ and $\chi_{cJ}$→$\gamma J/\psi(J=0,1,2)$ are reported