Finding top quarks with shower deconstruction (1211.3140v2)

Abstract: We develop a new method for tagging jets produced by hadronically decaying top quarks. The method is an application of shower deconstruction, a maximum information approach that was previously applied to identifying jets produced by Higgs bosons that decay to bottom quarks. We tag an observed jet as a top jet based on a cut on a calculated variable that is an approximation to the ratio of the likelihood that a top jet would have the structure of the observed jet to the likelihood that a non-top QCD jet would have this structure. We find that the shower deconstruction based tagger can perform better in discriminating boosted top quark jets from QCD jets than other publicly available tagging algorithms.

Overview of "Finding Top Quarks with Shower Deconstruction"

The paper "Finding Top Quarks with Shower Deconstruction" by Davison E. Soper and Michael Spannowsky introduces an advanced methodology for distinguishing top quark jets from background QCD jets at hadron colliders, with a specific focus on the Large Hadron Collider (LHC). The technique employs shower deconstruction, a method that maximizes information usage to improve the identification of jets originating from hadronically decaying top quarks, a significant challenge due to the large number of QCD jets in collider data.

Shower Deconstruction Method

The primary innovation presented is the extension of shower deconstruction—previously applied to Higgs boson identification—to top quark jets. This approach involves comparing the likelihood of a jet structure conforming to a top quark jet versus those arising from other QCD processes. The authors define a discriminative variable, $\chi$, approximating the likelihood ratio of these two hypotheses. High values of $\chi$ suggest a greater probability of a jet being a top jet.

Algorithmic Framework

1. Fat Jet Formation: The process begins by clustering particles into a large-radius jet, dubbed the "fat jet," using algorithms like Cambridge-Aachen to capture the top quark decay products.
2. Microjet Analysis: Within the fat jet, particles are further grouped into smaller, more granular clusters called microjets. The number of microjets is limited to the most significant contributors above a defined transverse momentum threshold.
3. Shower Histories: The core of shower deconstruction involves hypothesizing multiple possible shower histories (parton-level processes) that could lead to the observed microjet configuration. This includes emissions and decays modeled with probabilities akin to those in parton shower Monte Carlo generators.
4. Probability Computation: For each hypothesized history, likelihood densities are computed for signal and background models of jet generation, taking into account color flow effects and kinematic constraints.

Results and Performance

The paper reports extensive comparisons with several other established top tagging algorithms, demonstrating superior performance in discriminating top jets from backgrounds across varying conditions:

• Moderate Boost Regimes: Particularly around 500 GeV transverse momentum, shower deconstruction consistently yields lower background fake rates for a fixed signal acceptance compared to other methods like the Johns Hopkins and CMS top taggers.
• Low Boost Scenarios: Even in challenging low transverse momentum settings (around 200 GeV), which are pertinent for numerous physics searches, shower deconstruction outperforms conventional taggers, maintaining better discrimination despite increased contamination from non-top jets.

Implications and Future Directions

The adaptability and precision of shower deconstruction imply broad applications for nuanced signal extraction in high-energy physics. By effectively utilizing the detailed substructure of jets, it offers a robust tool for both current and potential studies at the LHC. Furthermore, the paper outlines the potential for parameter estimation within signal hypotheses, exemplified by the capability to refine measurements such as the W boson mass within the top quark decay chain.

Looking forward, further development could integrate this methodology into full-event analyses, enabling comprehensive reconstructions beyond singular jet assessments. The advancements in particle identification and event categorization through improved algorithms like shower deconstruction pave the way for more accurate searches for new physics phenomena.