Measurement of Helicity Fractions and Spin Correlation in Top Quark Pairs Using Reconstructed Lepton+Jets Events

This analysis has been documented in public CDF note 10211 by the University of Michigan authors Dave Mietlicki, Alexei Varganov, and Dan Amidei. It is an update of analysis which used 4.3 inverse femtobarns

Abstract

In the standard model, top quarks decay weakly before any hadronization processes take effect, enabling top spin information to be transmitted to the top quark decay products. Standard model top pair production produces a characteristic spin correlation which can be modified by new production mechanisms such as Z' bosons or Kaluza-Klein gluons. We report on the observation and measurement of the top pair helicity fractions and spin correlation in 5.3 inverse femtobarns of reconstructed lepton+jet data. In the helicity basis, we find the opposite helicity fraction

FOH = 0.74 ± 0.24stat ± 0.11syst

and a spin correlation coefficient

κhelicity = 0.48 ± 0.48stat ± 0.22 syst

In the beamline basis, we find the opposite spin fraction

FOS = 0.86 ± 0.32stat ± 0.13syst

and a spin correlation coefficient

κbeam = 0.72 ± 0.64stat ± 0.26 syst

Introduction

In this measurement we analyze a 5.3/fb dataset in the lepton+jets channel, consisting of a total of 725 events. The event selection requires one central lepton with large transverse momentum, missing transverse energy of at least 20 GeV, and 4 or more tight jets, at least one of which must be tagged as a b jet. Additionally, a chi-squared based kinematic reconstruction is used to reconstruction the original top quarks (we constrain the top quark mass to 172.5 GeV), and we require that the chi-squared value for this reconstruction be lses than 9. The background is calculated using a combination of Monte Carlo samples and data samples, and predicts a total of 110 ± 19 background events.

Our analysis revolves around the decay angles of the lepton, the down quark, and the bottom quark which comes from the hadronically decaying top, which carry information about the spin of the parent top quark. In the helicity basis, this angle is defined as the angle between the decay product momentum (in the top rest frame) and the top quark momentum (in the top quark pair rest frame), while in the beamline basis it is defined to be the angle between the decay product momentum (in the top rest frame) and the direction of the beamline (in the top quark pair rest frame).

In order to validate our event selection and background model, we look at the cosines of the lepton, down quark, and bottom quark decay angles in both the helicity basis and the beamline basis in the figures below.

In these figures, our selected data sample is compared to the sum of our background model and a top pair signal sample created using PYTHIA, which does not contain a spin correlation effect. These figures also contain results for a Kolmogorov-Smirnov (KS) test comparing the data to the prediction, showing good agreement.

Signal Templates

In top quark decays, the angular distributions of the top decay products determined by the helicity of the parent top quark via the equation
f[cos(θ)] = 1/2⋅[1 ± Ai⋅cos(θi)]
where the positive sign is for right-handed top quarks and the negative sign refers to left-handed top quarks (the signs are reversed for antitop decays). The correlation coefficient Ai varies for each decay product, being equal to +1.0 for the charged lepton or down quark, -0.41 for the bottom quark, and -0.31 for the neutrino or up quark.

Helicity Basis Templates

We created our helicity basis templates by modifying the HERWIG source code to implement this angular distribution for the charged lepton or down type quark, and then allowing the internal herwig machinery to propagate the appropriate angular distributions to the other decay products. Using this modified HERWIG, we then created four different simulated Monte Carlo samples, corresponding to the four possible top pair helicity states, top-right antitop-left (RL) , top-left antitop-right (LR), top-right antitop-right (RR), top-left antitop-left (LL).
cos(θ) for lepton or down type quark variable
in top quark decays at truth level for
the four different top pair helicity states
cos(θ) for lepton or down type quark variable
in anti top quark decays at truth level for
the four different top pair helicity states
cos(θ) for bottom quark variable in top quark
decays at truth level for the four
different top pair helicity states
cos(θ) for bottom quark variable in anti top
decays at truth level for the four
different top pair helicity states
cos(θ) for neutrino or up type quark variable
in top quark decays at truth level for
the four different top pair helicity states
cos(θ) for neutrino or up type quark variable
in anti top quark decays at truth level for
the four different top pair helicity states

With the simulated samples prepared for the four different top pair helicity states, the same and opposite helicity templates were created by combining the LR and RL samples to form an opposite helicity sample (OH) and combining the LL and RR samples to form a same helicity sample (SH). To show the effect of the top quark pair helicity states on the distributions of interest in this analysis, the figure below shows the variable cos(θlep+)cos(θlep-), comparing the distribution at truth level in HERWIG without spin correlations to the same and opposite helicity templates respectively.

Distribution of cosine product variable using
truth momenta for unpolarized HERWIG
sample and the helicity basis templates.

Down Quark Identification

The figure above assumes that the down quark can be identified 100% efficienctly, but one of the difficulties of this analysis is that this is not the case. In order to choose the down quark, we use the special precription according to which the jet closest to the b jet in the W rest frame will be the d jet approximately 60% of the time. The figure below again shows cos(θlep)cos(θdown) at truth level, comparing HERWIG without spin correlations to our same and opposite helicity templates, but in this figure the down quark is chosen using this prescription. This probabilistic choice reduces the difference between our templates and uncorrelated herwig, but a significant effect is still present.
Distribution of cosine product variable using
truth momenta for unpolarized HERWIG
sample and the helicity basis templates with probabilistic choice of down quark.

Beam Basis Templates

To create our beamline basis templates, rather than using a modified version of HERWIG, we reweight a sample created using the event generator PYTHIA. By reweighting according to the decay product helicity angle, we perform the same algorithm for template creation that was used in our helicity basis templates, but this algorithm is imposed after event generation rather than before. The resulting beamline basis templates are are shown below for the distribution cos(θlep)cos(θdown), with the left figure using the true down quark while the right figure chooses the down using the algorithm described above.
Distribution of cosine product variable using
truth momenta for unpolarized HERWIG
sample and OH template.
Distribution of cosine product variable using
truth momenta for unpolarized HERWIG
sample and SH template.

Measurement Method

With the same and opposite helicity templates established, we can use them in performing our fit. Our fitting method is a binned likelihood fit to the data, using three separate templates - the same helicity template, the opposite helicity template, and the background template.

Figures below show the composition and validation of the background templates for both the helicity basis (top) and the beam basis (bottom). The distributions on the left show the various components that go into the background template, and their relative sizes, for the variable cos(θlep)cos(θdown). The largest component of our background model consists of W + heavy flavor jet events. We also validate our background model by considering the anti-tag sample, to verify that our our background shape is properly modeled. The figures on the right side shows the distribution for the product cos(θlep)cos(θdown) in the anti-tag sample, compared to our background model summed with the small expected contribution from top pair signal events.

Helicity basis distribution of cos(θlep)cos(θdown) variable
for the various components of our background
template
Helicity basis distribution of cos(θlep)cos(θdown) variable
in anti-tag data sample compared to the sum
of our background model and a signal model
Beamline basis distribution of cos(θlep)cos(θdown) variable
for the various components of our background
template
Beamline basis distribution of cos(θlep)cos(θdown) variable
in anti-tag data sample compared to the sum
of our background model and a signal model

We consider two separate helicity angle bilinears in our fit, cos(θlep)cos(θdown) and cos(θlep)cos(θbot). Two 1-dimensional likelihood fits could be performed using these two variables, but pseudoexperiments show that there is a significant gain in sensitivity when the two variables are combined into a single 2-dimensional fit, so this is the chosen method for our measurement. When performing the fit, the background normalization is Gaussian-constrained to the predicted value, but the same helicity fraction FSH and opposite helicity fraction FOH are allow to float freely. We do not require that FSH and FOH be constrained to physical values between 0 and 1, but we do require FSH + FOH = 1. The fitting method has been checked for bias using pseudoexperiemnts with various input falues for FOH and no bias was found. In the helicity basis, we expecte a statistical uncertainty of 0.22, while in the beamline basis, we expect a statistical uncertainty of 0.31.

Systematic Uncertainties

There are a number of systematic effects that contribute to our uncertainty which need to be taken into account. These include uncertainties in the background size and shape, uncertainties in the exact detector response, and uncertainties in the underlying structure of the colliding particles. Each of these uncertainties is handled its own unique way, but all follow the same general procedure. We start with a template consisting of a nominal background and signal model, and then replace either the background or signal model with a model where the appropriate systematic effect has been varied. Our fit is then performed using this new template, and the result compared to the nominal result in order to determine the systematic uncertainty. We calculate these systematic uncertainties separately in each basis. The systematic uncertainties are summarized in the tables below for the helicity basis (top) and beamline basis (bottom). The largest uncertainty in both bases is the "MC Generator" uncertainty, which results from the fact that when performing our fit in some simulated samples with known opposite spin fractions, there is a small deviation of our measurement from the expected result. Although this is our largest systematic uncertainty, it is still small compared to our statistical uncertainty.
Summary of helicity basis systematic uncertainties
Summary of beamline basis systematic uncertainties

Result

With our fitting procedure established and all systematics uncertainties calculated, the final result of our 2-dimensional fit of cos(θlep)cos(θdown) vs. cos(θlep)cos(θbot) in data corresponding to an integrated luminosity of 5.3/fb in the helicity basis returns an oppsite helicity fraction of

FOH = 0.74 ± 0.24stat ± 0.11syst

Converting this to the spin correlation coefficient using κ = 2⋅fo-1 yields

κhelicity = 0.48 ± 0.48stat ± 0.22 syst

In the beamline basis, we find the opposite spin fraction

FOS = 0.86 ± 0.32stat ± 0.13syst

and a spin correlation coefficient

κbeam = 0.72 ± 0.64stat ± 0.26 syst

The top figures below show the 1-dimensional distributions for cos(θlep)cos(θdown) (left) and cos(θlep)cos(θbot) (right) in the helicity basis, where our data is compared to the sum of the background model, same helicity model, and opposite helicity model, with the normalizations determined by the result of our fit. The bottom figures below show the same distributions in the beamline basis.

Helicity basis distribution of the cos(θlep)cos(θdown)
variable in data compared to the sum of our
background model, the SH template, and the OH
template, where the OH fraction in signal is 0.74
Helicity basis distribution of the cos(θlep)cos(θbot)
variable in data compared to the sum of our
background model, the SH template, and the OH
template, where the OH fraction in signal is 0.74
Beamline basis distribution of the cos(θlep)cos(θdown) variable
in data compared to the sum of our background
model, the same spin template, and the opposite spin template,
where the opposite spin fraction in signal is 0.86
Helicity basis distribution of the cos(θlep)cos(θbot) variable
in data compared to the sum of our background
model, the same spin template, and the opposite spin template,
where the opposite spin fraction in signal is 0.86


These measurements became public CDF result on July 1, 2010.