Simultaneous measurement of forward-backward asymmetry and top polarization in dilepton final states from $t\bar{t}$ production at the Tevatron

Abstract

We present a simultaneous measurement of the forward-backward asymmetry and the top-quark polarization in $t\bar{t}$ production in dilepton final states using 9.7 fb${}^{-1}$ of proton-antiproton collisions at $\sqrt{s}=1.96$ TeV with the D0 detector. To reconstruct the distributions of kinematic observables we employ a matrix element technique that calculates the likelihood of the possible $t\bar{t}$ kinematic configurations. After accounting for the presence of background events and for calibration effects, we obtain a forward-backward asymmetry of $A^{t\bar{t}}=(15.0\pm 6.4\text{ (stat)}\pm 4.9\text{ (syst)})\%$ and a top-quark polarization times spin analyzing power in the beam basis of $\kappa P=(7.2\pm 10.5\text{ (stat)}\pm 4.2\text{ (syst)})\%$, with a correlation of $-56\%$ between the measurements. If we constrain the forward-backward asymmetry to its expected standard model value, we obtain a measurement of the top polarization of $\kappa P=(11.3\pm 9.1\text{ (stat)}\pm 1.9\text{ (syst)})\%.$ If we constrain the top polarization to its expected standard model value, we measure a forward-backward asymmetry of $A^{t\bar{t}}=(17.5\pm 5.6\text{ (stat)}\pm 3.1\text{ (syst)})\%.$ A combination with the D0 $A^{t\bar{t}}$ measurement in the lepton+jets final state yields an asymmetry of $A^{t\bar{t}}=(11.8\pm 2.5\text{ (stat)}\pm 1.3\text{ (syst)})\%.$ Within their respective uncertainties, all these results are consistent with the standard model expectations.

16 pages, 16 figures, published version

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