Discovering heavy new physics in boosted $Z$ channels: $Z\to l^{+}{l}^{-}$ vs $Z\to \nu \overline{\nu }$

We propose a strategy for new physics searches in channels which contain a boosted $Z$ boson and a boosted massive jet in the final state. Our proposal exploits the previously overlooked advantages of boosted $Z\to \nu \overline{\nu }$ 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 ($T^{\prime }$) which decay to $tZ$ final states. A comparison with the di-leptonic channel reveals that, despite the large $t\overline{t}$ background, signals with missing energy combined with jet substructure techniques offer superior probes of new physics at TeV scales. The effect can be attributed to a factor of $\sim 3$ enhancement in the signal cross section, coming from the branching ratio of $Z\to \nu \overline{\nu }$. We exploit the unique event topology of singly produced top partners to suppress the $t\overline{t}$ background, as well as further improve on the existing proposals to detect $T^{\prime }$ in the boosted di-lepton channel. Our conclusions on advantages of $Z\to \nu \overline{\nu }$ can be extended to most resonance searches which utilize a boosted $Z$ boson in the final state.

Figure 1

Single production channel for a $T^{\prime }$ decaying into $tZ$.

Figure 2

Single production cross section for $T^{\prime }$ or its conjugate decaying into $tZ$ (or $\overline{t}Z$) in the sample model described by Eq. (1), for various values of ${\lambda }_L$ with $f=780\mathrm{GeV}$.

Figure 3

Reach of Run II for discovering $T^{\prime }$ states of $M_{T^{\prime }}=1\mathrm{TeV}$ (left) and $M_{T^{\prime }}=1.5\mathrm{TeV}$ (right). Shaded regions, signifying $S/\sqrt{B}>5$ and number of signal events $N_{\mathrm{ev}}>10$, illustrate the discovery reach of LHC Run II. The reference model points from Table 1 are marked with a star. The horizontal dashed line in each plot marks the benchmark integrated luminosity of $100{\mathrm{fb}}^{-1}$. The reference production cross sections on the $x$ axis assume a lower cut of $p_T^j>15\mathrm{GeV}$, for the spectator quarks.