Systematically Constructing the Likelihood for Boosted $H\to gg$ Decays

TL;DR

This paper analytically and empirically investigates the likelihood ratio for identifying boosted Higgs decays to gluons from QCD jets, proposing new observables and demonstrating significant improvements in signal-to-background discrimination at the LHC.

Contribution

It provides a systematic analytical framework for constructing likelihood ratios for boosted Higgs to gluon decays, introducing new observables and validation methods.

Findings

Likelihood ratio improvement is approximately inverse of the strong coupling at high boosts.

Imposing jet mass constraints and measuring softer subjet energy fraction enhances discrimination.

Signal-to-background ratio can be increased by several hundred times at high jet energies.

Abstract

We study the binary discrimination problem of identification of boosted $H\to gg$ decays from massive QCD jets in a systematic expansion in the strong coupling. Though this decay mode of the Higgs is unlikely to be discovered at the LHC, we analytically demonstrate several features of the likelihood ratio for this problem through explicit analysis of signal and background matrix elements. Through leading-order, we prove that by imposing a constraint on the jet mass and measuring the energy fraction of the softer subjet an improvement of signal to background ratio that is independent of the kinematics of the jets at high boosts can be obtained, and is approximately equal to the inverse of the strong coupling evaluated at the Higgs mass. At next-to-leading order, we construct a powerful discrimination observable through a sort of anomaly detection approach by simply inverting the next-to-leading order $H\to gg$ matrix element with soft gluon emission, which is naturally infrared and collinear safe. Our analytic conclusions are validated in simulated data from all-purpose event generators and subsequent parton showering and demonstrate that the signal-to-background ratio can be improved by a factor of several hundred at high, but accessible, jet energies at the LHC.