Identifying Boosted Objects with N-subjettiness (1011.2268v3)

Abstract: We introduce a new jet shape -- N-subjettiness -- designed to identify boosted hadronically-decaying objects like electroweak bosons and top quarks. Combined with a jet invariant mass cut, N-subjettiness is an effective discriminating variable for tagging boosted objects and rejecting the background of QCD jets with large invariant mass. In efficiency studies of boosted W bosons and top quarks, we find tagging efficiencies of 30% are achievable with fake rates of 1%. We also consider the discovery potential for new heavy resonances that decay to pairs of boosted objects, and find significant improvements are possible using N-subjettiness. In this way, N-subjettiness combines the advantages of jet shapes with the discriminating power seen in previous jet substructure algorithms.

• The paper introduces N-subjettiness as a novel metric that probes jet substructure to effectively differentiate boosted electroweak bosons and top quarks from QCD backgrounds.
• It demonstrates that using N-subjettiness achieves around 30% tagging efficiency with only a 1% fake rate, outperforming traditional jet clustering methods.
• The technique is computationally efficient and theoretically robust, providing a promising framework for enhancing search strategies in all-hadronic decay channels for new resonances.

Overview of "Identifying Boosted Objects with N-subjettiness"

The paper "Identifying Boosted Objects with N-subjettiness" by Jesse Thaler and Ken Van Tilburg introduces a new approach for identifying boosted hadronically-decaying objects, such as electroweak bosons and top quarks, through a novel jet shape metric called $N$-subjettiness. This method is tailored to distinguish boosted physics signals from the large QCD background that appears as jets with high invariant mass at the Large Hadron Collider (LHC).

The authors propose $N$-subjettiness as an effective tool that leverages the fine-grained internal structure of jets, which is often lost in traditional jet-clustering algorithms. By doing so, they address a critical challenge in high-energy physics: efficiently tagging electroweak gauge bosons and top quarks among a sea of background jet noise.

Technical Contributions

The main contribution of the paper is the introduction and validation of $N$-subjettiness. It is designed to count the number of "subjets" within a larger jet formed from a boosted hadronically-decaying particle. This variable quantifies how tightly energy is collimated along hypothetical subjet axes and offers a discriminant power superior to existing jet shape tools.

• Definition of $N$-subjettiness: $N$-subjettiness, denoted as $\tau_N$, measures how well a jet can be resolved into $N$ subjets. A small $\tau_N$ value signifies that the jet aligns closely with $N$ exclusive subjets, indicating a more complex structure apt for capturing signals from two-prong (e.g., $W$, $Z$, and Higgs bosons) or three-prong (e.g., top quarks) decays, while larger values are typical of QCD jets.
• Efficiency Studies: The authors conduct extensive simulations of boosted $W$ and top jets, validating $N$-subjettiness against traditional jet substructure tagging techniques like the YSplitter and the Johns Hopkins Top Tagger. They demonstrate that $N$-subjettiness achieves tagging efficiencies of around 30% with a 1% fake rate for QCD jets, highlighting its efficacy.
• Case Study on New Resonances: One compelling application presented is in the context of discovering new resonances, such as a hypothetical $Z'$ decaying into pairs of boosted objects. The researchers illustrate that employing $N$-subjettiness enhances the signal discovery potential, especially when searching for all-hadronic decay channels of new massive particles—a historically challenging problem due to QCD-induced backgrounds.

Implications and Future Directions

The methodology presented has significant theoretical and practical implications:

1. Theoretical Calculability: Due to its basis in inclusive jet shapes, $N$-subjettiness maintains desirable theoretical properties such as infrared- and collinear-safety. This makes it a promising candidate for future theoretical studies and precision calculations, potentially allowing for perturbative QCD calculations or resummation techniques to be employed effectively.
2. Flexibility in Application: Experimentally, $N$-subjettiness is a computationally tractable quantity that can be adjusted post-hoc to optimize signal vs. background across different transversal momentum regimes and detector configurations.
3. Impact on Search Strategies: The results indicate a robust method for handling and reducing backgrounds in all-hadronic final-state analyses, nudging current search strategies towards exploiting energy distribution profiles, instead of purely invariant mass and other historical metrics.
4. Prospects in Multivariate Analyses: While focused on $\tau_N$ ratios, the paper recognizes the potential for multivariate analysis possibly combining several $N$-subjettiness measures to extract additional information latent in jet energy distributions.

Considering the rapid progress in techniques for boosted object identification, $N$-subjettiness stands out as a powerful tool in high luminosity collider environments like the LHC. Its introduction sets a benchmark that may pave the way for further advancements in particle tagging methodologies, synergizing with ongoing developments in machine learning and computational techniques in particle physics.