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T: Fachverband Teilchenphysik

T 10: Axions/ALPS I

T 10.4: Talk

Monday, March 16, 2026, 17:00–17:15, KH 01.013

Search for top quark decays to long-lived axion-like particles (ALPs) with ATLAS: ALP candidate reconstruction — •Frederic Fischer, Martin Christiansen, Lucia Masetti, Hendrik Smitmanns, Jessica Höfner, and Annika Stein — Johannes Gutenberg-Universität Mainz, Institut für Physik

The Standard Model (SM), although it agrees with great precision with experimental measurements, is still insufficient to give answers to many fundamental questions. One prime example is the strong CP problem which axions could solve. Axions as well as the broader axion-like particles (ALPs) are introduced as electrically neutral scalar fields extending the SM to accomodate new physics. One way to approach ALPs is to parameterise ALP couplings to SM particles like top quarks. ALPs appear in flavour-changing exotic top decays where the top quark decays into an ALP and an up- or charm-quark. Within this decay mode, parts of the parameter space dictates ALPs to have lifetimes long enough to travel macroscopic distances before decaying. This search is dedicated to look for tt events with one SM leptonically decaying top quark and one exotically decaying top quark. In this topology the focus is on ALP decays in the hadronic calorimeter at ATLAS with a centre-of-mass energy of √s = 13.6 TeV. Being electrically neutral, they leave no signals in the ATLAS tracking system. Moreover, the ratio of signals in the electromagnetic calorimeter and hadronic calorimeter are used to suppress SM backgrounds. This talk presents the reconstruction strategy of the ALP candidate to be used in a search for long-lived ALPs from exotic top decays at ATLAS.

Keywords: long-lived axion-like particles; exotic $t\bar{t}$ decays