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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 pp collisions with the ATLAS detector
(AMER PHYSICAL SOC, 2023/09/18) Aad G.; Abbott B.; Abbott D. C.; Abeling K.; Abidi S. H.; Aboulhorma A.; Abramowicz H.; Abreu H.; Abulaiti Y.; Abusleme Hoffman A. C.; Acharya B. S.; Achkar B.; Adam L.; Adam Bourdarios C.; Adamczyk L.; Adamek L.; Addepalli S. V.; Adelman J.; Adiguzel A.; Adorni S.; Adye T.; Affolder A. A.; Afik Y.; Agaras M. N.; Agarwala J.; Aggarwal A.; Agheorghiesei C.; Aguilar-Saavedra J. A.; Ahmad A.; Ahmadov F.; Ahmed W. S.; Ai X.; Aielli G.; Aizenberg I.; Akbiyik M.; Akesson T. P. A.; Akimov A. V.; Al Khoury K.; Alberghi G. L.; Albert J.
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 root 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 bb, 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 sigma(pp -> Y -> XH -> qqbb) for signals with m(Y) between 1.5 and 6 TeV and m(X) between 65 and 3000 GeV.