Observation of a pseudoscalar excess at the $t\bar{t}$ threshold

Abstract

The CMS Collaboration recently observed a pseudoscalar excess of top quark-antiquark ($t\bar{t}$) events near the production threshold. One possible interpretation involves the formation of a short-lived quasi-bound state of $t\bar{t}$. This proceeding summarizes the first presentation of the results at the 59th Rencontres de Moriond and outlines its potential implications.

Figures (3)

• Figure 1: Observed (points with statistical error bars) and predicted (stacked colored histograms) $m_{{t\xspace{}\bar{t}\xspace}\xspace}$ distribution in three out of nine ($c_\text{hel}$, $c_\text{han}$) bins. In the upper panels, the $t$$\bar{t}$ histogram shows the FO pQCD prediction after the fit to the data. The shown event rates are divided by the bin width. The lower panels display the ratio of the data to the FO pQCD + back-ground prediction, with the $\eta_{t}$ signal overlaid at its fitted cross section (red line). The gray band indicates the postfit uncertainty.
• Figure 2: Best fit value (cross) and allowed regions at one (solid line), three (dashed line), and five (dotted-dashed line) standard deviations for the cross section of $\eta_{t}$ and $\chi_{t}$ production, as observed in data (black). The FO pQCD + back-ground expectation of zero $\eta_{t}$ and $\chi_{t}$ contributions is denoted by a red star. Negative cross section values refer to a reduction of the $t$$\bar{t}$ cross sections relative to the FO pQCD prediction around the threshold.
• Figure 3: Ratios of the predictions for powheg$\texttt{hvq}$ + herwig (gray), powheg$\texttt{bb4l}$ + pythia (orange) to powheg$\texttt{hvq}$ + pythia (black) for $m_{{t\xspace{}\bar{t}\xspace}\xspace}$ (left), $c_\text{hel}$ at the $t$$\bar{t}$ threshold (center), and $c_\text{hel}$ in the $t$$\bar{t}$ continuum (right), both integrated over $c_\text{han}$. The effect of adding $\eta_{t}$ to powheg$\texttt{hvq}$ + pythia is shown in red for comparison.