Measurements of top-quark pair spin correlations in the $$e\mu $$ channel at $$\sqrt{s} = 13$$ TeV using pp collisions in the ATLAS detector

Abstract

A measurement of observables sensitive to spin correlations in $$t\bar{t}$$tt¯ production is presented, using 36.1 $$\hbox {fb}^{-1}$$fb-1 of pp collision data at $$\sqrt{s} = 13$$s=13 TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the $$t\bar{t}$$tt¯ system. The measured differential cross-sections are compared to predictions by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.

NNLO QCD corrections to leptonic observables in top-quark pair production and decay

“…[10]. This is not surprising since the only difference between the CMS and ATLAS [66] measurements is the binning. It is interesting to note that the agreement with CMS closely mirrors the one found in ref.…”

“…We focus on the inclusive selection 2 which has received a lot of attention recently [64][65][66]. We extend our previous calculation [10] and provide predictions for the 6-bin CMS measurement [64] together with expanded differential predictions (as described in section 2.6) for the ATLAS measurement [66]. To better assess the effect of the α s expansion we compute the distribution with two different scale choices: the default dynamic scale eq.…”

Recent measurements of top quark properties in CMS

“…Both ATLAS [3] and CMS [2] show a tension between data and predictions in the Δ distribution. Within the LHCtopWG an ATLAS-CMS comparison, the first at 13 TeV, is being performed using normalized cross sections at parton level.…”

Top Quark Properties from ATLAS

“…( / ) = ( → )/ ( → ). These proceedings cover three recent results at the ATLAS experiment [1], the charge asymmetry in top-quark-pair production [2], the spin correlations between the top quarks [3], and the test of lepton universality in the decay of the -boson [4].…”

“…Figure 1:The normalized fiducial particle-level differential cross-section in Δ compared to predictions from POWHEG, MadGraph5_aMC@NLO and SHERPA (left) and a comparison of the ratio of the unfolded Δ distribution to POWHEG with additional NLO generators and NNLO fixed-order predictions (right). Figures from Ref [3]…”