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Improved Spin Basis for Angular Correlation Studies in Single Top Quark Production at the Tevatron

Gregory Mahlon, Stephen Parke

TL;DR

Problem: angular correlations in single top production encode the top spin and can reveal the Wtb coupling and potential new physics. Approach: construct a spin basis aligned with the direction of the d-type quark and apply it to the W* and Wg fusion channels, deriving exact spin amplitudes and decay correlations. Key findings: the antiproton basis in W* yields 98% spin-up; the spectator-jet basis in Wg fusion yields 96% spin-up; both bases produce angular correlations 45% and 36% larger than the helicity basis, respectively. Significance: this provides a practical path to maximize spin correlation observables at the Tevatron and improve sensitivity to anomalous top couplings.

Abstract

We show in single top quark production that the spin of the top quark is correlated with the direction of the d-type quark in the event. For single top production in the W* channel, the d-type quark comes dominantly from the antiproton at the Tevatron, whereas for the W-gluon fusion channel the spectator jet is the d-type quark the majority of the time at this machine. Our results are that 98% of the top quarks from the W* process have their spins in the antiproton direction, and 96% of the top quarks in the W-gluon fusion process have their spins in the spectator jet direction. We also compare with the more traditional, but less effective, helicity basis. The direction of the top quark spin is reflected in angular correlations in its decay products.

Improved Spin Basis for Angular Correlation Studies in Single Top Quark Production at the Tevatron

TL;DR

Problem: angular correlations in single top production encode the top spin and can reveal the Wtb coupling and potential new physics. Approach: construct a spin basis aligned with the direction of the d-type quark and apply it to the W* and Wg fusion channels, deriving exact spin amplitudes and decay correlations. Key findings: the antiproton basis in W* yields 98% spin-up; the spectator-jet basis in Wg fusion yields 96% spin-up; both bases produce angular correlations 45% and 36% larger than the helicity basis, respectively. Significance: this provides a practical path to maximize spin correlation observables at the Tevatron and improve sensitivity to anomalous top couplings.

Abstract

We show in single top quark production that the spin of the top quark is correlated with the direction of the d-type quark in the event. For single top production in the W* channel, the d-type quark comes dominantly from the antiproton at the Tevatron, whereas for the W-gluon fusion channel the spectator jet is the d-type quark the majority of the time at this machine. Our results are that 98% of the top quarks from the W* process have their spins in the antiproton direction, and 96% of the top quarks in the W-gluon fusion process have their spins in the spectator jet direction. We also compare with the more traditional, but less effective, helicity basis. The direction of the top quark spin is reflected in angular correlations in its decay products.

Paper Structure

This paper contains 4 sections, 19 equations, 4 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Single Top Production Through A $W^{*}$
  3. $W$-Gluon Fusion
  4. Discussion and Conclusions

Figures (4)

  • Figure 1: Feynman diagram for single top production in the $W^*$ process. The labels indicate the momentum flow utilized in the text.
  • Figure 2: Gauge-invariant set of Feynman diagrams for single top production via $Wg$ fusion. The labels indicate the momentum flow utilized in the text.
  • Figure 3: (a) The differential cross sections (total, antiproton basis spin up, helicity basis left) as a function of the top quark transverse momentum for single top production in the $W^*$ channel at the Tevatron at 2.0 TeV. (b) The absolute value of the spin asymmetry (\ref{['Asym']}) plotted as a function of the top quark transverse momentum for the helicity and antiproton bases.
  • Figure 4: (a) The differential cross sections (total, spectator basis spin up, helicity basis left) as a function of the top quark transverse momentum for single top production via $Wg$ fusion at the Tevatron at 2.0 TeV. (b) The absolute value of the spin asymmetry (\ref{['Asym']}) plotted as a function of the top quark transverse momentum for the helicity and spectator bases.