Evidence for spin correlation in ttbar production

TL;DR

This result provides evidence for the presence of spin correlation in t ̅t events with a significance of more than 3 standard deviations and a recent measurement of f in dileptonic final states finds f in agreement with the standard model.

Abstract

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

Figures (2)

• Figure 1: The distribution of the discriminant $R$ for $\ell$+jets events with four jets and an invariant mass of the two light-flavor jets within $\pm25~\mathrm{Ge V}\xspace$ 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 $t\bar{t}$ contribution for $f_{\text{meas}}$, where $f_{\text{meas}}$ was taken from the combined fit in the $\ell$+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: (color online) Bands for 68%, 95% and 99.7% C.L. of $f$ as a function of $f_{\text{meas}}$ for the combined dilepton and $\ell$+jets fit. The thin light-color line indicates the most probable value of $f$ as a function of $f_{\text{meas}}$. The vertical dotted black line shows the measured value of $\hbox{$f_{\text{meas}}$}\xspace=0.85$.