Measurements of spin correlation in top-antitop quark events from proton-proton collisions ats=7  TeVusing the ATLAS detector

Abstract

Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb −1 . Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leadingorder accuracy.

Measurement of Spin Correlation in Top-Antitop Quark Events and Search for Top Squark Pair Production inppCollisions ats=8 TeVUsing the ATLAS Detect

“…As in Ref. [42], the size of the variation is compatible with the recent measurements of additional jet activity in tt events [103]. The Wt normalization is varied within the theoretical uncertainties of the cross-section calculation [86], and the sensitivity to the interference between Wt production and tt production at NLO is studied by comparing the predictions of POWHEG-BOX with the …”

“…As in Ref. [42], the uncertainty due to the top quark mass is evaluated but not included in the systematic uncertainties, since it would have no significant impact on the results.…”

“…The orientations of the top and antitop quark spins are transferred to the decay products and can be measured directly via their angular distributions [3,17,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. The strength of their correlation has been studied previously by the CDF and D0 collaborations in proton-antiproton scattering at and by the ATLAS and CMS collaborations in proton-proton scattering at 7 TeV [41][42][43].…”

“…[42] with the following exceptions: since this analysis employs b tagging, the associated uncertainty is estimated by varying the relative normalizations of simulated b-jet, c-jet, and light-jet samples. The uncertainty due the choice of generator is determined by comparing the default to an alternative tt sample generated with the POWHEG-BOX generator interfaced with PYTHIA.…”

Opening Up the Compressed Region of Top Squark Searches at 13 TeV LHC

“…In the region that m χ ≪ m~t ≈ m t , spin correlations of the top quarks can help to distinguish the signal from the background [15][16][17]. Such analysis has been done by the CDF, D0, ATLAS, and CMS collaborations [18][19][20][21][22][23][24][25]. However, with larger m~t this method does not work well due to a smaller production rate.…”