Top quark forward-backward asymmetry from new t-channel physics

Abstract

Motivated by recent measurements of the top quark forward-backward asymmetry at the Tevatron, we study how t-channel new physics can contribute to a large value. We concentrate on a theory with an abelian gauge boson possessing flavor changing couplings between up and top quarks, but satisfies flavor physics constraints. Collider constraints are strong, but can be consistent with the aid of small flavor diagonal couplings. We find that M_Z' ~ 160 GeV can yield a total lab-frame asymmetry of ~18% without being in conflict with other observables. There are implications for future collider searches, including exotic top quark decays, like-sign top quark production, and detailed measurements of the top production cross section. An alternate model with a gauged non-Abelian flavor symmetry would have similar phenomenology, but lacks the like-sign top signal.

Figures (4)

• Figure 1: $A_{FB}^{t}$ as a function of $\sqrt{\hat{s}}=M_{t\bar{t}}$ for $M_{Z^\prime} = 160$ GeV.
• Figure 2: $\alpha_X \equiv g_{X}^{2}/(4 \pi)$ versus $A_{FB}^{new}$ and $\sigma(t\bar{t})$ for $M_{Z^\prime}=100,200,400$ GeV (from the left). In the lower panel, shaded regions deviate by more than $2\sigma$ from $\sigma(t\bar{t})^{new}$. Corresponding disfavored regions are shown as thinned lines in the upper plot. The superscript "$new$" emphasizes that only pure $Z^\prime$ and SM contributions are included (without fake processes). These fakes leads to some subtlety in the allowed region, as discussed in the text.
• Figure 3: A contour plot of $A_{FB}^{new}$ and BR($t \rightarrow Z^\prime u$) in the $\alpha_{X}$ - $M_{Z^\prime}$ plane. In colored regions, $\sigma(t \bar{t})^{new}$ deviates $2\sigma$ from of the measurement quoted in text. Parameter space around the red star is preferred. A much larger $\alpha_{X}$ will gives too many like-sign top quarks, or a large distortion of the $M_{t\bar{t}}$ spectrum. Larger masses lead to larger distortions of the $M_{t\bar{t}}$ spectrum, and smaller masses give a large branching ratio for $t \rightarrow Z^\prime u$, which leads to tension between measurement of top cross-sections in different channels.
• Figure 4: The $M_{t\bar{t}}$ invariant mass spectrum. Data from the CDF measurement Aaltonen:2009iz is shown along with our SM simulation. Also shown are $M_{Z^\prime} = 100,200,300$ GeV, with $\alpha_X = 0.013,0.03,0.055$, respectively. Each $(\alpha_X, M_{Z^\prime})$ pair would provide an $A_{FB}^{new} \simeq 10\%$.