Person:

Tong, Baojia

Same- and opposite-sign charge asymmetries are measured in lepton+jets tt¯ events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb−1 from proton--proton collisions at a centre-of-mass energy of s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b-hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.

Ratios of top-quark pair to Z-boson cross sections measured from proton--proton collisions at the LHC centre-of-mass energies of s√=13TeV, 8TeV, and 7TeV are presented by the ATLAS Collaboration. Single ratios, at a given s√ for the two processes and at different s√ for each process, as well as double ratios of the two processes at different s√, are evaluated. The ratios are constructed using previously published ATLAS measurements of the tt⎯⎯ and Z-boson production cross sections, corrected to a common phase space where required, and a new analysis of Z→ℓ+ℓ− where ℓ=e,μ at s√=13TeV performed with data collected in 2015 with an integrated luminosity of 3.2fb−1. Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the Z→e+e− and the Z→μ+μ− channels for each s√ value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-x values near 0.1 and the light-quark sea for x<0.02.

A measurement of the ZZ production in the ℓ−ℓ+ℓ′−ℓ′+ and ℓ−ℓ+νν¯ channels (ℓ=e,μ) in proton--proton collisions at s√=8 TeV at the Large Hadron Collider at CERN, using data corresponding to an integrated luminosity of 20.3 fb−1 collected by the ATLAS experiment in 2012 is presented. The fiducial cross sections for ZZ→ℓ−ℓ+ℓ′−ℓ′+ and ZZ→ℓ−ℓ+νν¯ are measured in selected phase-space regions. The total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV is measured from the combination of the two channels to be 7.3±0.4(stat)±0.3(syst)±0.2(lumi) pb, which is consistent with the Standard Model prediction of 6.6+0.7−0.6 pb. The differential cross sections in bins of various kinematic variables are presented. The differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral triple gauge boson couplings in ZZ production.