Discovering Heavy New Physics in Boosted $Z$ Channels: $Z \rightarrow l ^+l^- $ vs. $ Z \rightarrow \nu \bar{\nu}$

Abstract

We propose a strategy for new physics searches in channels which contain a boosted $Z$ boson in the final state. Our proposal exploits the previously overlooked advantages of boosted $Z\rightarrow \nu \bar{\nu}$ topologies, where collimated neutrinos result in signals with large missing energy. We illustrate the advantage of this channel in a case study of singly produced TeV scale charge 2/3 fermionic top partners ($T'$) which decay to $t+Z$. A comparison with the di-leptonic channel reveals that signals with missing energy offer superior probes of new physics at TeV scales, despite the large $t\bar{t}$ backgrounds. The effect can be attributed to a factor of $\sim 3$ enhancement in the signal cross section, coming from the branching ratio of $Z\rightarrow \nu \bar{\nu}$. We exploit the unique event topology of singly produced top partners to suppress the $t\bar{t}$ background, as well as further improve on the existing proposals to detect $T'$ in the boosted di-lepton channel. Our conclusions on advantages of $Z\rightarrow \nu \bar{\nu}$ can be extended to most physics searches which utilize a boosted $Z$ boson in the final state.