Efficient Identification of Boosted Semileptonic Top Quarks at the LHC

Abstract: Top quarks produced in multi-TeV processes will have large Lorentz boosts, and their decay products will be highly collimated. In semileptonic decay modes, this often leads to the merging of the b-jet and the hard lepton according to standard event reconstructions, which can complicate new physics searches. Here we explore ways of efficiently recovering this signal in the muon channel at the LHC. We perform a particle-level study of events with muons produced inside of boosted tops, as well as in generic QCD jets and from W-strahlung off of hard quarks. We characterize the discriminating power of cuts previously explored in the literature, as well two new ones. We find a particularly powerful isolation variable which can potentially reject light QCD jets with hard embedded muons at the 10^3 level while retaining 80~90% of the tops. This can also be fruitfully combined with other cuts for O(1) greater discrimination. For W-strahlung, a simple pT-scaled maximum \Delta R cut performs comparably to a highly idealized top-mass reconstruction, rejecting an O(1) fraction of the background with percent-scale loss of signal. Using these results, we suggest a set of well-motivated baseline cuts for any physics analysis involving semileptonic top quarks at TeV-scale momenta, using neither b-tagging nor missing energy as discriminators. We demonstrate the utility of our cuts in searching for resonances in the top-antitop invariant mass spectrum. For example, our results suggest that 100 fb^{-1} of data from a 14 TeV LHC could be used to discover a warped KK gluon up to 4.5 TeV or higher.

Identification of Boosted Semileptonic Top Quarks at the LHC

The study conducted by Rehermann and Tweedie focuses on the detection of highly Lorentz-boosted top quarks within the complex environment of the Large Hadron Collider (LHC). When top quarks are produced at multi-TeV energies, their decay products become significantly collimated, posing notable challenges in event reconstruction. Specifically, in semileptonic decay modes, the convergence of the $b$-jet and hard lepton, such as a muon, complicates the identification process using traditional methods. The paper addresses these challenges through a particle-level analysis targeting scenarios where muons are yielded from boosted top decays, as well as from QCD jets and $W$-strahlung processes.

In the pursuit of refining the detection process, two novel cuts are introduced alongside existing discriminators referenced in prior literature. A particularly promising isolation variable is explored, with the capability of rejecting background light QCD jets containing hard embedded muons with a rejection factor of up to $10^3$, while maintaining a top quark retention efficiency of 80-90%. This approach can be augmented by other cuts for improved discriminatory power, approximately $O(1)$. The research suggests a robust set of baseline cuts tailored for physics analyses involving semileptonic top quarks at TeV-scale momenta. These cuts do not rely on $b$-tagging nor missing energy as discriminators.

Additionally, the authors examine $W$-strahlung—the emission of $W$ bosons by energetic quarks—and introduce a $p_T$-scaled maximum cut that offers comparable discrimination power to idealized top-mass reconstruction metrics, effectively filtering portions of the background whilst minimally impacting signal integrity. Their findings on discriminating semileptonic boosted tops, particularly focusing on the utility of the $\mu$ channel, set the foreground for suggested baseline cuts.

From a practical perspective, the proposed techniques, once validated through detailed detector simulations at CMS or ATLAS, enable the LHC to enhance its sensitivity to new physics scenarios. Specifically, the paper notes that LHC data, even with existing detector technologies, could potentially facilitate the discovery of a warped KK gluon with masses as high as 4.5 TeV or greater at an integrated luminosity of 100 fb$^{-1}$.

The theoretical implications extend into the realm of electroweak symmetry breaking, where highly boosted tops might serve as probes for new multi-TeV-scale physics, potentially indicating underlying strong dynamics or composite resonances. The methods extend the analytical capabilities of current collider experiments without necessitating major instrumentation advancements.

Future investigations could incorporate detector effects, refine $b$-tagging strategies in the high-$p_T$ regime, and explore analogous frameworks for identifying electrons from top decays. Exploring these directions could further improve the accuracy and reliability of boosted top quark identification, thereby broadening the scope of viable experimental searches at the LHC.