A New Twist on Top Quark Spin Correlations

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).

Figures (18)

• Figure 1: Construction of a common coordinate system for the top and antitop. The thick gray line is the beamline. Starting from the lab frame, the $t\bar{t}$ CM system is actively boosted to rest, and then the individual tops are actively boosted to rest. The decay products of the tops are measured in this frame. The overlayed coordinate axes on the bottom figure correspond to our helicity basis.
• Figure 2: Total LO spin correlation strength in $q\bar{q} \to t\bar{t}$. Plotted versus top production angle and squared-velocity in the partonic CM frame.
• Figure 3: LO correlation strength in off-diagonal-basis polar angles in $q\bar{q} \to t\bar{t}$, relative to total strength. Plotted versus top production angle and squared-velocity in the partonic CM frame.
• Figure 4: LO correlation strength in off-diagonal-basis $\phi+\bar{\phi}$ in $q\bar{q} \to t\bar{t}$. Absolute (left) and relative to total strength (right). Plotted versus top production angle and squared-velocity in the partonic CM frame.
• Figure 5: Total LO spin correlation strength in $gg \to t\bar{t}$. Plotted versus top production angle and squared-velocity in the partonic CM frame. (Dashed line indicates $p_T = m_t$.)