Anomaly detection search for new resonances decaying into a Higgs boson and a generic new particle X in hadronic final states using √𝑠=13 TeV 𝑝⁢𝑝 collisions with the ATLAS detector

Abstract

A search is presented for a heavy resonance 𝑌 decaying into a Standard Model Higgs boson 𝐻 and a new particle 𝑋 in a fully hadronic final state. The full Large Hadron Collider run 2 dataset of proton-proton collisions at √𝑠=13 TeV collected by the ATLAS detector from 2015 to 2018 is used and corresponds to an integrated luminosity of 139 fb−1. The search targets the high 𝑌-mass region, where the 𝐻 and 𝑋 have a significant Lorentz boost in the laboratory frame. A novel application of anomaly detection is used to define a general signal region, where events are selected solely because of their incompatibility with a learned background-only model. It is constructed using a jet-level tagger for signal-model-independent selection of the boosted 𝑋 particle, representing the first application of fully unsupervised machine learning to an ATLAS analysis. Two additional signal regions are implemented to target a benchmark 𝑋 decay into two quarks, covering topologies where the 𝑋 is reconstructed as either a single large-radius jet or two small-radius jets. The analysis selects Higgs boson decays into 𝑏⁢¯𝑏, and a dedicated neural-network-based tagger provides sensitivity to the boosted heavy-flavor topology. No significant excess of data over the expected background is observed, and the results are presented as upper limits on the production cross section 𝜎(𝑝⁢𝑝→𝑌→𝑋⁢𝐻→𝑞⁢𝑞⁢𝑏𝑏) for signals with 𝑚𝑌 between 1.5 and 6 TeV and 𝑚𝑋 between 65 and 3000 GeV.