content_image/334348.jpg
<img src='content_image/334348.jpg'>

where ϵ < 1 and the diagonal term is summed over generations. This model was discussed in [15] as a candidate for describing the observed 2 σ deviation from the SM prediction of forward-backward asymmetry in the top-pair signal at the CDF detector of the Tevatron collider [16]. The small diagonal couplings characterized by the parameter ϵ$_{U}$ exist only to escape bounds on like-sign top quark events from the decay of two Z $^{′}$s by forcing the dominant decay Z ′ → u ¯ u . This study found the best match to the asymmetry and to the invariant mass distribution of the top pair with M$_{Z}$ ′ = 160 GeV and α$_{X}$ = 0 . 024, with any small ϵ$_{U}$ ̸ = 0 giving comparable results. We take these values and choose ϵ$_{U}$ = 0 . 1. The dominant process for triple-top production in this model (for small ϵ$_{U}$ ) is the t-channel exchange of the Z $^{′}$, shown in Figure 4. This diagram illustrates the unique topology of these events in this Z ′ model, with the three tops produced at LO. 

## IV. RESULTS AND CONCLUSIONS 

The LO triple-top production cross sections for the two new physics models discussed are calculated with MadGraph according to the prescriptions in Section II. The Z ′ model is 

