The ATLAS Roma1 group advances Top Physics along three complementary axes: quantum entanglement in tt pairs, the associated production of a Higgs boson with a single top quark (tH), and precision studies of tt production near threshold that reveal non-relativistic QCD effects.
Top quantum entanglement
ATLAS observed quantum entanglement in tt events by exploiting spin-correlation patterns—most prominent near the production threshold—and reconstructing an entanglement witness from dilepton angular observables. The result demonstrates that collider final states can encode non-classical correlations at the heaviest mass scale accessible today.
Roma1 contribution: lepton reconstruction/calibration and event selection, modelling of spin-correlated tt, unfolding and systematics on the dilepton-angle observables, validation of threshold-enhanced categories.
Nature (2024): Observation of quantum entanglement with top quarks
tH (single top + Higgs)
The tH process probes directly the top–Higgs Yukawa phase and magnitude via interference between diagrams with the Yukawa and with the W–Higgs coupling. The latest ATLAS search at 13 TeV (140 fb−1) targets multiple Higgs decays (b\bar b, WW*, ZZ*, ττ) with a lepton from the top, using multivariate discriminants and dedicated signal/control regions; it reports an observed excess and sets limits including the inverted-sign scenario.
Roma1 contribution: category design and BDTs, b-tagging and τ-ID performance inputs, background control strategy (single-top/tt), object/calibration systematics, interpretation versus the top-Yukawa sign.
ATLAS: Search for tH at 13 TeV (140 fb−1)
The quasi-bound-state called “toponium”
Close to twice the top mass, non-relativistic interactions can transiently enhance the tt production rate. ATLAS measures a cross-section excess near threshold by combining low-mtt spectra with angle-based categories sensitive to the underlying spin state, and rejects models that neglect these effects. This precision regime links collider data to NRQCD predictions and sharpens constraints on top-QCD dynamics.
Roma1 contribution: low-mtt selections and fits, modelling with NRQCD-motivated templates, validation of angle-binned categories (shared with entanglement), and cross-checks versus alternative MC/scale/PDF choices.
ATLAS-CONF-2025-008 · ATLAS Briefing · Press statement
