Analysis Public
Web Page

Jun 24, 2009

Authors: Yuji Takeuchi, Shinhong
Kim
(U. of Tsukuba) for the CDF collaboration

mailto: yuji_AT_fnal.gov

mailto: yuji_AT_fnal.gov

Abstract |

In this page, we report on a measurement of the spin spin correlation coefficient of top and anit-top in the off-diagonal basis at ttbar production in ppbar collisions at √s=1.96 TeV using ∫Ldt=2.8 fb

What are we going to
measure? |

In this analysis, we
measure κ at the ttbar decay in the dilepton channel, where ttbar
is supposed to decay with the following differential cross-section and
decay rate:

,

where θ_{+} (θ_{-})
denotes angle of a
decay product flight direction
in top (anti-top) rest frame with respect to a choosen quantization
axis for top (anti-top). We suppose that ttbar is produced with
standard
model spin
correlations, but we don't assume the top and anti-top
polarizations are kept until their decays, i.e. we consider κ as a
free parameter that is what we are going to measure.

If we observe non-zero correlation coefficient κ in ttbar production and decay, that consequently indicates the direct evidence that top and anti-top are produced with their spins correlated and decay before losing their spin polarizations as bare quarks.

How do we measure
κ? |

Due to P-violating weak decay of the top (anti-top) quark, the flight direction of decay products has some portion of information on the spin polarization of the parent top quark. Hence the spin correlation between top and anti-top manifests itself on the angular distributions of the decay products. We use the distributions of ( cos θ

In this analysis, we used 195 ttbar candidates in dilepton channel observed in 2.8 fb

Signal Templates Upper: The expected distributions of ( cos θ _{l+}, cos
θ_{l-})
and (cos θ_{b}, cos θ_{bbar}) reconstructed with
CDF-II
detector by
the
Monte Carlo simulation for dilepton channel
candidates in ttbar Monte Carlo (κ=-1.0
~1.0).Lower: Fits of the distributions to a 2 dimensional polynomial function. |

Histogram and Fit: animated gif, gif files (κ = -1, 0, 1), eps files (κ = -1, 0, 1) Histogram only: animated gif, gif files (κ = -1, 0, 1), eps (κ = -1, 0, 1) Fit only: animated gif, gif files (κ = -1, 0, 1), eps (κ = -1, 0, 1) |

Background Templates Upper: The expected distributions of ( cos θ _{l+}, cos
θ_{l-})
and (cos θ_{b}, cos θ_{bbar}) reconstructed with
CDF-II
detector by
the
Monte Carlo simulation for Background (WW/WZ/ZZ,
Drell-Yan(Z/γ*)->ττ, ee, μμ,
W+QCD fake).Middle: 1 σ uncertainty of each bin of the distributions Lower: Fits of the distributions to a 2 dimensional polynomial function. |

eps
file eps file |

Reconstructed
cosθ_{l+}, cosθ_{b} DistributionCross: Dilepton candidates in data of 2.8 fb ^{-1} integrated luminosity.Histogram: Expected signal+background distirbution with 1 σ uncertainty |

eps file(To view with GhostView, you have to unset
Antialias.) eps file(To view with GhostView, you have to unset Antialias.) |

Statistic and Systematic Uncertainties on
measured κ By performing pseudo experiments, we obtain statistical uncertainty on measured κ as a fucntion of assumed κ. As for systematic uncertainty, we consider uncertainties from Monte Carlo sample statistics, shape of signal/background templates, jet energy scale, initial/final state radiation, parton distribution function, gluon fusion fraction and effect of next leading order. |

eps file |

Observed (cosθ_{l+}, cosθ_{l-}) and (cosθ_{b} , cosθ_{bbar}) Distributions in 2.8 fb^{-1} DataLower: -2 Δ Log(Likelihood) as a function of assumed κ. We observed κ = 0.309 which gives maximum likelihood for observed distributions. |

eps file eps file |

Feldman-Cousins Interval for κ Feldman-Cousisns Intervals based on the statistical and systematic uncertainty for measured κ. From this intervals and observed κ = 0.309, we set 68% confidence interval for κ on the assumed Mt=175 GeV/c ^{2}. |

eps file |

Top Mass Dependence of Confidence Interval
for κ We measure 68% confidence interval for κ on assumptions of different top quark mass (167.5, 170.0, 172.5, and 177.5 GeV/c ^{2}). |

eps file |

Results -0.455<κ<0.865 (68% C.L.) or κ = 0.320 ^{+0.545}_{-0.775}for Mt = 175 GeV/c ^{2} |

Last modified: Wed Jun 24 04:50:26 JST 2009 by Yuji Takeuchi