A New Twist on Top Quark Spin Correlations

TL;DR

This paper introduces a robust method using azimuthal decay angles to measure top quark spin correlations at the LHC, enabling detailed tests of the Standard Model and searches for new physics like anomalous dipole moments.

Contribution

It presents a novel approach to analyze top quark spin correlations through azimuthal angles, sensitive to QCD effects and new physics, with demonstrated robustness and potential for current LHC data.

Findings

Spin correlations show sinusoidal modulations in azimuthal angles.

Method is robust under full kinematic reconstruction.

Potential to set limits on top quark anomalous dipole moments.

Abstract

Top-antitop pairs produced at hadron colliders are largely unpolarized, but their spins are highly correlated. The structure of these correlations varies significantly over top production phase space, allowing very detailed tests of the Standard Model. Here, we explore top quark spin correlation measurement from a general perspective, highlighting the role of azimuthal decay angles. By taking differences and sums of these angles about the top-antitop production axis, the presence of spin correlations can be seen as sinusoidal modulations resulting from the interference of different helicity channels. At the LHC, these modulations exhibit nontrivial evolution from near-threshold production into the boosted regime, where they become sensitive to almost the entire QCD correlation effect for centrally produced tops. We demonstrate that this form of spin correlation measurement is very robust under full kinematic reconstruction, and should already be observable with high significance using the current LHC data set. We also illustrate some novel ways that new physics can alter the azimuthal distributions. In particular, we estimate the power of our proposed measurements in probing for anomalous color-dipole operators, as well as for broad resonances with parity-violating couplings. Using these methods, the 2012 run of the LHC may be capable of setting simultaneous limits on the top quark's anomalous chromomagnetic and chromoelectric dipole moments at the level of 3*10^{-18}cm (0.03/m_t).