On the threshold behaviour of heavy top production

Abstract

The observation of an excess of ttbar production in the threshold region, by CMS and ATLAS, has been interpreted as a toponium contribution, i.e. from below-threshold ttbar virtual states. The news here is the nontrivial experimental extraction of such a signal, not its existence as such. Indeed, already 35+ years ago an NRQCD Green's function approach was used to model the above- and below-threshold production of ttbar pairs in pp/ppbar collisions. The relevant cross section equations from that study are now (re-)implemented in the Pythia 8 event generator. While the above-threshold part is straightforward, the physical interpretation and modelling of below-threshold events is nontrivial, and a final prescription is cross-checked against two simpler ones. Cross sections and some event properties are presented.

Figures (2)

• Figure 1: The threshold behavior. (a) Analytical expressions for singlet and octet cases, $E = m_{{\mathrm t}\overline{\mathrm t}} - 2m_{{\mathrm t}}$. (b) Cross section in a four-mass scenario simulation for the expected singlet/octet mix, $E = \hat{m} - m_{{\mathrm t} 1} - m_{{\mathrm t} 2}$.
• Figure 2: Example of event properties for the ${\mathrm t}\overline{\mathrm t}$ pair mass range $300 < \hat{m} < 400$ GeV. (a) Cross section $\mathrm{d}\sigma/\mathrm{d}\hat{m}$. (b) Cross section for the heavier and lighter of the ${\mathrm t}$ and $\overline{\mathrm t}$, respectively.