Evidence for Spin Correlation in $t\overline{t}$ Production

We present a measurement of the ratio of events with correlated $t$ and $\overline{t}$ spins to the total number of $t\overline{t}$ events. This ratio $f$ is evaluated using a matrix-element-based approach in 729 $t\overline{t}$ candidate events with a single lepton $\ell$ (electron or muon) and at least four jets. The analyzed $p\overline{p}$ collisions data correspond to an integrated luminosity of $5.3{\mathrm{fb}}^{-1}$ and were collected with the D0 detector at the Fermilab Tevatron collider operating at a center-of-mass energy $\sqrt{s}=1.96\mathrm{TeV}$. Combining this result with a recent measurement of $f$ in dileptonic final states, we find $f$ in agreement with the standard model. In addition, the combination provides evidence for the presence of spin correlation in $t\overline{t}$ events with a significance of more than 3 standard deviations.

Figure 1

The distribution of the discriminant $R$ for $\ell +\mathrm{\text{jets}}$ events with four jets and an invariant mass of the two light-flavor jets within $\pm 25\mathrm{GeV}$ of the mass of the $W$ boson. The expectation (including background) for complete spin correlation as predicted by the SM ($f=1$) and the case of no spin correlation ($f=0$), as well as the $t\overline{t}$ contribution for $f_{\mathrm{meas}}$, where $f_{\mathrm{meas}}$ was taken from the combined fit in the $\ell +\mathrm{\text{jets}}$ and dilepton final states, are shown. The first and last bins include contributions from $R<0.37$ and $R>0.60$, respectively. The bin width is chosen to minimize the statistical uncertainty.

Figure 2

Bands for 68%, 95% and 99.7% C.L. of $f$ as a function of $f_{\mathrm{meas}}$ for the combined dilepton and $\ell +\mathrm{\text{jets}}$ fit. The thin light-color line indicates the most probable value of $f$ as a function of $f_{\mathrm{meas}}$. The vertical dotted black line shows the measured value of $f_{\mathrm{meas}}=0.85$.